CN103441276A - Preparation method of carbon-coated porous lithium iron phosphate powder - Google Patents

Abstract

A method for preparing carbon-coated porous lithium iron phosphate powder. Dissolve ferric salt in water to form a solution, and use a precipitating agent to adjust pH=7.0-13.0 until Fe ₂ O ₃ .nH ₂ O is completely precipitated; The positively charged Fe ₂ O ₃ .nH ₂ O colloidal particles obtained after separation, washing and acidification; the colloidal particles were added with deionized water and surfactant, and then vigorously stirred to obtain Fe ₂ O ₃ .nH ₂ O colloidal particles; Add water-soluble lithium source, phosphorus source, carbon source and doping ion compound to Fe ₂ O ₃ .nH ₂ O colloid, stir vigorously to form a colloidal mixed slurry uniformly mixed at the molecular level; the slurry is spray-dried to obtain spherical iron phosphate Lithium precursor; the precursor is calcined under the protection of an inert gas in a microwave oven to obtain a carbon-coated spherical porous lithium iron phosphate powder.

Description

A kind of preparation method of carbon-coated porous lithium iron phosphate powder

technical field

The invention relates to a preparation method of a positive electrode material of a lithium ion battery.

Background technique

Lithium iron phosphate (LiFePO ₄ ), the cathode material of lithium-ion batteries, has a theoretical capacity of 170mAh/g, high reversible charge specific capacity, and has the advantages of wide source of raw materials, low pollution, good safety, and long cycle life. Cathode materials for power and energy storage lithium-ion batteries. However, the ionic conductivity and electronic conductivity of lithium iron phosphate are both low, which is only suitable for charging and discharging at low current density, and the specific capacity decreases when charging and discharging at a high rate, which limits the application of this material. At home and abroad, a lot of research has been done on the modification of lithium iron phosphate to improve the conductivity of lithium iron phosphate, mainly including the preparation of nano-scale LiFePO ₄ , the preparation of porous LiFePO ₄ , carbon coating, metal ion doping and other methods, among which carbon coating Modification research mainly focuses on the use of different carbon sources and coating technologies to achieve various forms of carbon coating.

At present, the preparation methods of lithium iron phosphate powder mainly include solid phase method, carbothermal reduction method, sol-gel method, hydrothermal method and microwave method. The sol-gel method is an effective method to prepare high-performance LiFePO ₄ materials, especially the addition of a small amount of dopant ions, and the use of the sol-gel method that can precisely control the composition is an important technical means. Chinese patent (CN102005564A) discloses a method for preparing nanocrystalline lithium iron phosphate powder using iron hydroxide colloid, using Fe(OH) ₃ colloid as raw material, adding lithium source, phosphorus source and organic carbon source to the colloid, strong Stir evenly and dry in vacuum at low temperature to form a uniform nanoscale precursor containing lithium iron phosphorus carbon, put it into a crucible and heat it up to 500-800°C in a muffle furnace protected by an inert atmosphere, keep it warm for 2-24 hours, and organic carbon The source is cracked into carbon under an inert atmosphere, and ferric iron is reduced to ferrous iron by carbon to form carbon-coated lithium iron phosphate, which is naturally cooled to room temperature and then ground or pulverized to obtain nanocrystalline lithium iron phosphate powder. Chinese patent (CN102881903A) discloses a method for porous lithium iron phosphate powder, dissolving ferric salt in water to form a solution, adding lye to obtain Fe(OH) ₃ nanoparticles in a boiling state, Fe(OH) ₃ Nanoparticles are dispersed in water after washing, and ferric oxide colloid is formed under strong stirring; water-soluble lithium source, phosphorus source, carbon source and doped ion compound are added to ferric oxide colloid, and strong stirring is formed to form a molecular level uniform mixture The colloidal mixed slurry; the slurry is spray-dried to obtain a lithium iron phosphate precursor with uniform particle size; the precursor is obtained under an inert atmosphere in a muffle furnace to obtain a carbon-coated spherical porous lithium iron phosphate powder.

However, in the above patents, glucose and the like are used as carbon sources to prepare carbon-coated lithium iron phosphate. During the carbonization of these traditional carbon sources, polymer chains often need to be destroyed and decomposed to produce gases such as H ₂ O and CO ₂ and small molecular organic compounds. This leads to the formation of defects such as cracks and pores in the carbon film. In the literature J. Mater. Chem, 2012, 22 (11): 4611-4614, the author used ionic liquid 1-ethyl-3-methyl-imidazole bistrifluoromethanesulfonimide salt (EMIm-TFSI) as a carbon source , after pyrolysis at high temperature, a uniform and dense nitrogen-doped carbon film with a thickness of about 15nm is formed on the surface of the electrode material LiFePO ₄ particles. He believes that this uniform and dense nitrogen-doped carbon film obviously reduces the polarization inside the electrode, so its rate performance and The cycle performance is significantly improved compared with the carbon-coated LiFePO ₄ material with glucose as the carbon source. On the other hand, the above-mentioned patents all adopt traditional heating methods, that is, heating from the surface to the inside through radiation, convection, and conduction. This heating method is likely to cause uneven carbon content of lithium iron phosphate particles from the surface to the inside. Inhomogeneous and unstable carbon films can easily cause polarization during charge and discharge, resulting in poor rate performance and cycle performance of lithium iron phosphate materials.

Contents of the invention

The object of the present invention is to provide a method for preparing carbon-coated porous lithium iron phosphate powder.

The invention is a preparation method of carbon-coated porous lithium iron phosphate powder, the steps of which are:

(1) Preparation of Fe ₂ O ₃ colloid: Weigh the iron source, dissolve it in deionized water to prepare a solution with a Fe ³⁺ concentration of 0.1-1 mol/L, use ammonia water or urea solution with a mass concentration of 10% as a precipitant, and The organic small molecule dispersant is dissolved in deionized water as the bottom liquid, the molar ratio is: dispersant: Fe ³⁺ =1:10, pH=6.8-7.0;

(2) Add the Fe ³⁺ solution and the precipitating agent to the bottom solution at the same time, and control the pH=7.0-13.0 until Fe ₂ O ₃ .nH ₂ O is completely precipitated; after the precipitate is separated and washed, under stirring, the molar ratio of HCl : Fe ₂ O ₃ =1:100-1:500 Add 0.01mol/L dilute hydrochloric acid for acidification to obtain electropositive Fe ₂ O ₃ .nH ₂ O colloidal particles; after the colloidal particles are separated by centrifugation, add deionized water , add surfactant at the same time, the mass ratio is: surfactant: Fe ₂ O ₃ =1:10-1:100, stir at 30-100°C, the stirring time lasts 1-10h, and obtain the mass ratio of Fe ₂ O ₃ 5-30% Fe ₂ O ₃ .nH ₂ O colloid;

(3) Slurry preparation: according to the stoichiometric ratio Li _x Fe _y PO ₄ :M _z , where M is the dopant ion, x=0.8-1.2, y=0.8-1.2, z=0.01-0.1, weigh the lithium source , phosphorus source and doping metal compound, the addition of carbon source is the mass ratio of Li _x Fe _y PO ₄ :M _z 5-20%; Lithium source, phosphorus source, carbon source and doping metal compound are added to Fe ₂ In the O ₃ .nH ₂ O colloid, continue to stir to form a uniformly dispersed colloidal mixed slurry;

(4) Precursor preparation: The slurry is dried by spray drying with an inlet temperature of 110-130°C to obtain a spherical lithium iron phosphate precursor with uniform composition and uniform particle size distribution;

(5) Calcination: Put the precursor obtained by spray drying into a crucible, heat up to 500-800°C under the protection of an inert atmosphere, keep it warm for 10-40 min, and then cool naturally to obtain a carbon-coated spherical porous lithium iron phosphate powder.

The present invention uses Fe ₂ O ₃ .nH ₂ O colloid as the dispersion system, which can make all raw material components uniformly mixed at the molecular level, which is conducive to the uniform coating of doped ions into the lattice and carbon film; the ionic liquid is used as the carbon source , using ionic liquid cracking under microwave field for carbon film coating. The prepared carbon-coated lithium iron phosphate is spherical and porous particles, and the carbon film uniformly coated in the micropores of the particles is conducive to the infiltration of electrolyte and the transmission of electrons, so it has good electrochemical performance and rate performance.

Description of drawings

Fig. 1 is the process flow diagram of the synthesis of carbon-coated porous lithium iron phosphate of the present invention, Fig. 2 is the scanning electron microscope picture of the carbon-coated porous lithium iron phosphate synthesized by the present invention, Fig. 3 is the carbon-coated porous iron phosphate synthesized by the present invention Lithium discharge specific capacity curve.

Detailed ways

As shown in Fig. 1, the present invention is a kind of preparation method of carbon-coated porous lithium iron phosphate powder, and its steps are:

(1) Preparation of Fe ₂ O ₃ colloid: Weigh the iron source, dissolve it in deionized water to prepare a solution with a Fe ³⁺ concentration of 0.1-1 mol/L, use ammonia water or urea solution with a mass concentration of 10% as a precipitant, and The organic small molecule dispersant is dissolved in deionized water as the bottom liquid, the molar ratio is: dispersant: Fe ³⁺ =1:10, pH=6.8-7.0;

(2) Add the Fe ³⁺ solution and the precipitating agent to the bottom solution at the same time, and control the pH=7.0-13.0 until Fe ₂ O ₃ .nH ₂ O is completely precipitated; after the precipitate is separated and washed, under stirring, the molar ratio of HCl : Fe ₂ O ₃ =1:100-1:500 Add 0.01mol/L dilute hydrochloric acid for acidification to obtain electropositive Fe ₂ O ₃ .nH ₂ O colloidal particles; after the colloidal particles are separated by centrifugation, add deionized water , add surfactant at the same time, the mass ratio is: surfactant: Fe ₂ O ₃ =1:10-1:100, stir at 30-100°C, the stirring time lasts 1-10h, and obtain the mass ratio of Fe ₂ O ₃ 5-30% Fe ₂ O ₃ .nH ₂ O colloid;

(3) Slurry preparation: according to the stoichiometric ratio Li _x Fe _y PO ₄ :M _z , where M is the dopant ion, x=0.8-1.2, y=0.8-1.2, z=0.01-0.1, weigh the lithium source , phosphorus source and doping metal compound, the addition of carbon source is the mass ratio of Li _x Fe _y PO ₄ :M _z 5-20%; Lithium source, phosphorus source, carbon source and doping metal compound are added to Fe ₂ In the O ₃ .nH ₂ O colloid, continue to stir to form a uniformly dispersed colloidal mixed slurry;

(4) Precursor preparation: The slurry is dried by spray drying with an inlet temperature of 110-130°C to obtain a spherical lithium iron phosphate precursor with uniform composition and uniform particle size distribution;

(5) Calcination: Put the precursor obtained by spray drying into a crucible, heat up to 500-800°C under the protection of an inert atmosphere, keep it warm for 10-40 min, and then cool naturally to obtain a carbon-coated spherical porous lithium iron phosphate powder.

According to the preparation method described above, the iron source is specifically ferric citrate (FeC ₆ H ₅ O ₇ ), or ferric chloride (FeCl ₃ .6H ₂ O), or ferric nitrate [Fe(NO ₃ ) ₃ · 9H ₂ O], or iron sulfate [Fe ₂ (SO ₄ ) ₃ ], or a combination of the above iron sources.

According to the above-mentioned preparation method, the small organic molecule dispersant is specifically citric acid, or glutamic acid, or ascorbic acid, or tartaric acid, or salicylic acid, or a combination of the above.

According to the preparation method described above, the surfactant is specifically gelatin, or polyethylene glycol, or polyvinyl alcohol, or ammonium polymethacrylate, or ammonium oleate, or ammonium alginate, or Combinations of the above-mentioned surfactant substances.

According to the above-mentioned preparation method, the lithium source is specifically lithium carbonate (Li ₂ CO ₃ ), or lithium hydroxide (LiOH), or lithium dihydrogen phosphate (LiH ₂ PO ₄ ), or the above-mentioned lithium source combination of substances.

According to the preparation method described above, the phosphorus source is specifically diammonium hydrogen phosphate [(NH ₄ ) ₂ HPO ₄ ], or ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ), or lithium dihydrogen phosphate (LiH ₂ PO ₄ ), or phosphoric acid (H ₃ PO ₄ ), or a combination of the above phosphorus sources.

According to the preparation method described above, the carbon source is specifically 1-ethyl-3-methylimidazole nitrate [EMIm]NO ₃ , or 1-butyl-3-methylimidazole nitrate [BMIm]NO ₃ , or 1-methylimidazolium tetrafluoroborate [MIm]BF ₄ , or 1-ethylimidazolium tetrafluoroborate [EIm]BF ₄ , or 1-methylimidazolium dihydrogen phosphate [MIm] H ₂ PO ₄ , or 1-butylimidazole dihydrogen phosphate [BIm] H ₂ PO ₄ , or 1-ethyl-3-methylimidazole dinitrile amine [EMIm]N(CN) ₂ , or 1-butyl-3-methylimidazole dinitrile amine [BMIm]N(CN) ₂ , etc., or a combination of the above-mentioned ionic liquids.

According to the preparation method described above, the doping metal compound is specifically magnesium nitrate [Mg(NO ₃ ) ₂ .2H ₂ O], or magnesium oxalate [MgC ₂ O ₄ .2H ₂ O], or nickel nitrate [Ni (NO ₃ ) ₂ 6H ₂ O], or nickel acetate [C ₂ H ₃ NiO ₂ ], or cobalt nitrate [Co(NO ₃ ) ₂ 6H ₂ O], or cobalt acetate [C ₄ H ₆ O ₄ Co·4H ₂ O], or aluminum nitrate [Al(NO ₃ ) ₃ ·9H ₂ O], or a combination of the doping metal compounds mentioned above.

The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments.

Example 1:

According to the stoichiometric ratio Li _0.99 Mg _0.01 FePO ₄ , first weigh 27.03g of ferric chloride (FeCl ₃ .6H ₂ O), dissolve it in deionized water to form a solution with a Fe ³⁺ concentration of 0.1mol/L, and use a mass concentration of 10% 1.92g of citric acid was dissolved in 100mL of deionized water as the bottom liquid; the Fe ³⁺ solution and the precipitant were added to the bottom liquid at the same time, and the amount of the precipitant was adjusted to control the pH=8.0 to Fe ₂ O ₃ . 3H ₂ O is completely precipitated; after separating and washing the precipitate, add 50mL of 0.01mol/L dilute hydrochloric acid under stirring for acidification, high-speed centrifugation (4000r/min) after separation, add 100mL of deionized water, and then add 0.08g of Ammonium oleate and 0.08g gelatin were vigorously stirred at 600 rpm at 30°C for 4h to obtain Fe ₂ O ₃ .3H ₂ O colloid. Weigh 1.57g of 1-ethyl-3-methylimidazole nitrate, 2.37g of lithium hydroxide and 0.26g of magnesium nitrate. Add these materials into the Fe ₂ O ₃ .3H ₂ O colloid, and continue stirring at 600 rpm for 2 hours to obtain a uniform colloidal mixed slurry. The slurry was spray-dried (inlet temperature 130°C) to form a spherical lithium iron phosphate precursor. The precursor was put into a crucible and heated to 500 °C in a nitrogen-protected microwave oven, kept for 20 min and then cooled naturally to obtain a carbon-coated spherical porous lithium iron phosphate powder. The particle morphology is shown in Figure 2.

The battery performance tests of the obtained lithium iron phosphate cathode materials all adopt CR2025 button batteries, which are assembled in a glove box filled with an inert atmosphere. The negative electrode is metal lithium sheet, and the electrolyte is 1mol.L ^- LiPF6/EC:DMC (1:1), where EC is ethylene carbonate and DMC is dimethyl carbonate. The preparation process of the positive electrode sheet is as follows: mix the prepared positive electrode material with the conductive agent acetylene black, and the binder PVDF (polyvinylidene fluoride) at a ratio of 85:8:7, and add an appropriate amount of NMP (N-methylpyrrolidone) on the agate Grind evenly in a mortar to form a viscous gel-like mixture, and then evenly spread it on a 0.02mm thick aluminum foil, place it at 120°C for 20 hours in a vacuum, and use the blue battery test system for the assembled battery to test its charge and discharge performance and cycle Performance Testing. As shown in Figure 3, under the condition of charge and discharge rate of 2C, the initial discharge specific capacity of the material is 143.2mAh/g, and the capacity retention rate after 50 cycles is 99.8%.

Example 2:

According to the stoichiometric ratio Li _0.99 Mg _0.01 FePO ₄ , first weigh 27.03g of ferric chloride (FeCl ₃ .6H ₂ O), dissolve it in deionized water to form a solution with a Fe ³⁺ concentration of 0.1mol/L, and use a mass concentration of 10% 1.92g of citric acid was dissolved in 100mL of deionized water as the bottom liquid; the Fe ³⁺ solution and the precipitant were added to the bottom liquid at the same time, and the amount of the precipitant was adjusted to control the pH=8.0 to Fe ₂ O ₃ . 3H ₂ O is completely precipitated; after separating and washing the precipitate, add 50mL of 0.01mol/L dilute hydrochloric acid under stirring for acidification, high-speed centrifugation (4000r/min) after separation, add 100mL of deionized water, and then add 0.08g of Ammonium oleate and 0.08 g polyethylene glycol were vigorously stirred at 600 rpm at 30°C for 4 hours to obtain Fe ₂ O ₃ .3H ₂ O colloid. Weigh 1.57g of 1-ethyl-3-methylimidazole nitrate, 2.37g of lithium hydroxide and 0.26g of magnesium nitrate. Add these materials into the Fe ₂ O ₃ .3H ₂ O colloid, and continue stirring at 600 rpm for 2 hours to obtain a uniform colloidal mixed slurry. The slurry was spray-dried (inlet temperature 130°C) to form a spherical lithium iron phosphate precursor. The precursor was put into a crucible and heated to 500 °C in a nitrogen-protected microwave oven, kept for 20 min and then cooled naturally to obtain carbon-coated spherical porous lithium iron phosphate powder.

The battery was assembled according to the method of Example 1 and tested. Under the condition of charge and discharge rate of 2C, the initial discharge capacity of the material reached 140.0mAh/g, and the capacity retention rate after 50 cycles was 100%.

Example 3:

According to the stoichiometric ratio Li _0.99 Mg _0.01 FePO ₄ , first weigh 27.03g of ferric chloride (FeCl ₃ .6H ₂ O), dissolve it in deionized water to form a solution with a Fe ³⁺ concentration of 0.1mol/L, and use a mass concentration of 10% 1.92g of citric acid was dissolved in 100mL of deionized water as the bottom liquid; the Fe ³⁺ solution and the precipitant were added to the bottom liquid at the same time, and the amount of the precipitant was adjusted to control the pH=8.0 to Fe ₂ O ₃ . 3H ₂ O is completely precipitated; after separating and washing the precipitate, add 50mL of 0.01mol/L dilute hydrochloric acid under stirring for acidification, high-speed centrifugation (4000r/min) after separation, add 100mL of deionized water, and then add 0.08g of Ammonium oleate and 0.08g gelatin were vigorously stirred at 600 rpm at 30°C for 4h to obtain Fe ₂ O ₃ .3H ₂ O colloid. Weigh 1.57g of 1-ethylimidazolium tetrafluoroborate, 2.37g of lithium hydroxide and 0.26g of magnesium nitrate. Add these materials into the Fe ₂ O ₃ .3H ₂ O colloid, and continue stirring at 600 rpm for 2 hours to obtain a uniform colloidal mixed slurry. The slurry was spray-dried (inlet temperature 130°C) to form a spherical lithium iron phosphate precursor. The precursor was put into a crucible and heated to 500 °C in a nitrogen-protected microwave oven, kept for 20 min and then cooled naturally to obtain a carbon-coated spherical porous lithium iron phosphate powder.

According to the method of Example 1, the battery was assembled and tested. Under the condition of charge and discharge rate of 2C, the initial discharge capacity of the material reached 141.3mAh/g, and the capacity retention rate after 50 cycles was 98.6%.

Example 4:

According to the stoichiometric ratio Li _0.99 Mg _0.01 FePO ₄ , first weigh 27.03g of ferric chloride (FeCl ₃ .6H ₂ O), dissolve it in deionized water to form a solution with a Fe ³⁺ concentration of 0.1mol/L, and use a mass concentration of 10% 1.92g of citric acid was dissolved in 100mL of deionized water as the bottom liquid; the Fe ³⁺ solution and the precipitant were added to the bottom liquid at the same time, and the amount of the precipitant was adjusted to control the pH=8.0 to Fe ₂ O ₃ . 3H ₂ O is completely precipitated; after separating and washing the precipitate, add 50mL of 0.01mol/L dilute hydrochloric acid under stirring for acidification, high-speed centrifugation (4000r/min) after separation, add 100mL of deionized water, and then add 0.08g of Ammonium oleate and 0.08 g polyethylene glycol were vigorously stirred at 600 rpm at 30°C for 4 hours to obtain Fe ₂ O ₃ .3H ₂ O colloid. Weigh 1.57g of 1-ethylimidazolium tetrafluoroborate, 2.37g of lithium hydroxide and 0.26g of magnesium nitrate. Add these materials into the Fe ₂ O ₃ .3H ₂ O colloid, and continue stirring at 600 rpm for 2 hours to obtain a uniform colloidal mixed slurry. The slurry was spray-dried (inlet temperature 130°C) to form a spherical lithium iron phosphate precursor. Put the precursor into the crucible and raise the temperature to 500°C in a nitrogen-protected microwave oven, keep it warm for 20 minutes, and then cool it down naturally to obtain a carbon-coated spherical porous lithium iron phosphate powder.

According to the method of Example 1, the battery was assembled and tested. Under the condition of charge and discharge rate of 2C, the initial discharge capacity of the material reached 138.3mAh/g, and the capacity retention rate after 50 cycles was 99.2%.

Example 5:

According to the stoichiometric ratio Li _0.99 Mg _0.01 FePO ₄ first weigh 24.33g ferric chloride (FeCl ₃ .6H ₂ O) and 2.45g ferric citrate (FeC ₆ H ₅ O ₇ ) (molar ratio ferric chloride: ferric citrate =9:1), dissolved in deionized water to form a solution with a Fe ³⁺ concentration of 0.1mol/L, using ammonia water with a mass concentration of 10% as a precipitant, and dissolving 1.92g of glutamic acid in 100mL of deionized water as a bottom solution ;Add the Fe ³⁺ solution and the precipitant to the bottom liquid at the same time, adjust the amount of the precipitant to control the pH=8.0 until Fe ₂ O ₃ .3H ₂ O is completely precipitated; after the precipitate is separated and washed, add 50mL of 0.01mol /L dilute hydrochloric acid for acidification, high-speed centrifugation (4000r/min) and then add 100mL of deionized water, then add 0.08g of ammonium oleate and 0.08g of gelatin in the deionized water, and vigorously stir at 600 rpm at 30°C for 4h to obtain Fe ₂ O ₃ .3H ₂ O colloid. Weigh 1.57g of 1-ethyl-3-methylimidazole nitrate, 2.37g of lithium hydroxide and 0.26g of magnesium nitrate. Add these materials into the Fe ₂ O ₃ .3H ₂ O colloid, and continue stirring at 600 rpm for 2 hours to obtain a uniform colloidal mixed slurry. The slurry was spray-dried (inlet temperature 130°C) to form a spherical lithium iron phosphate precursor. The precursor was put into a crucible and heated to 500 °C in a nitrogen-protected microwave oven, kept for 20 min and then cooled naturally to obtain a carbon-coated spherical porous lithium iron phosphate powder.

According to the method of Example 1, the battery was assembled and tested. Under the condition of charge and discharge rate of 2C, the initial discharge capacity of the material reached 138.3mAh/g, and the capacity retention rate after 50 cycles was 99.8%.

Embodiment 6:

According to the stoichiometric ratio Li _0.99 Mg _0.01 FePO ₄ first weigh 24.33g ferric chloride (FeCl ₃ .6H ₂ O) and 2.45g ferric citrate (FeC ₆ H ₅ O ₇ ) (molar ratio ferric chloride: ferric citrate =9:1), dissolved in deionized water to form a solution with a Fe ³⁺ concentration of 0.1mol/L, using ammonia water with a mass concentration of 10% as a precipitant, and dissolving 1.92g of glutamic acid in 100mL of deionized water as a bottom solution ;Add the Fe ³⁺ solution and the precipitant to the bottom liquid at the same time, adjust the amount of the precipitant to control the pH=8.0 until Fe ₂ O ₃ .3H ₂ O is completely precipitated; after the precipitate is separated and washed, add 50mL of 0.01mol /L dilute hydrochloric acid for acidification, high-speed centrifugation (4000r/min) and separation, add 100mL deionized water, then add 0.08g ammonium oleate and 0.08g polyethylene glycol to the deionized water, stir vigorously at 600 rpm at 30°C for 4h Get Fe ₂ O ₃ .3H ₂ O colloid. Weigh 1.57g of 1-ethyl-3-methylimidazole nitrate, 2.37g of lithium hydroxide and 0.26g of magnesium nitrate. Add these materials into the Fe ₂ O ₃ .3H ₂ O colloid, and continue stirring at 600 rpm for 2 hours to obtain a uniform colloidal mixed slurry. The slurry was spray-dried (inlet temperature 130°C) to form a spherical lithium iron phosphate precursor. The precursor was put into a crucible and heated to 500 °C in a nitrogen-protected microwave oven, kept for 20 min and then cooled naturally to obtain a carbon-coated spherical porous lithium iron phosphate powder.

According to the method of Example 1, the battery was assembled and tested. Under the condition of charge and discharge rate of 2C, the initial discharge capacity of the material reached 143.1mAh/g, and the capacity retention rate after 50 cycles was 98.4%.

Embodiment 7:

According to the stoichiometric ratio Li _0.99 Mg _0.01 FePO ₄ first weigh 24.33g ferric chloride (FeCl ₃ .6H ₂ O) and 2.45g ferric citrate (FeC ₆ H ₅ O ₇ ) (molar ratio ferric chloride: ferric citrate =9:1), dissolved in deionized water to form a solution with a Fe ³⁺ concentration of 0.1mol/L, using ammonia water with a mass concentration of 10% as a precipitant, and dissolving 1.92g of glutamic acid in 100mL of deionized water as a bottom solution ;Add the Fe ³⁺ solution and the precipitant to the bottom liquid at the same time, adjust the amount of the precipitant to control the pH=8.0 until Fe ₂ O ₃ .3H ₂ O is completely precipitated; after the precipitate is separated and washed, add 50mL of 0.01mol /L dilute hydrochloric acid for acidification, high-speed centrifugation (4000r/min) and then add 100mL of deionized water, then add 0.08g of ammonium oleate and 0.08g of gelatin in the deionized water, and vigorously stir at 600 rpm at 30°C for 4h to obtain Fe ₂ O ₃ .3H ₂ O colloid. Weigh 1.57g of 1-ethylimidazolium tetrafluoroborate, 2.37g of lithium hydroxide and 0.26g of magnesium nitrate. Add these materials into the Fe ₂ O ₃ .3H ₂ O colloid, and continue stirring at 600 rpm for 2 hours to obtain a uniform colloidal mixed slurry. The slurry was spray-dried (inlet temperature 130°C) to form a spherical lithium iron phosphate precursor. The precursor was put into a crucible and heated to 500 °C in a nitrogen-protected microwave oven, kept for 20 min and then cooled naturally to obtain a carbon-coated spherical porous lithium iron phosphate powder.

The battery was assembled according to the method of Example 1 and tested. Under the condition of charge and discharge rate of 2C, the initial discharge capacity of the material reached 137.4mAh/g, and the capacity retention rate after 50 cycles was 97.6%.

Embodiment 8:

According to the stoichiometric ratio Li _0.99 Mg _0.01 FePO ₄ first weigh 24.33g ferric chloride (FeCl ₃ .6H ₂ O) and 2.45g ferric citrate (FeC ₆ H ₅ O ₇ ) (molar ratio ferric chloride: ferric citrate =9:1), dissolved in deionized water to form a solution with a Fe ³⁺ concentration of 0.1mol/L, using ammonia water with a mass concentration of 10% as a precipitant, and dissolving 1.92g of glutamic acid in 100mL of deionized water as a bottom solution ;Add the Fe ³⁺ solution and the precipitant to the bottom liquid at the same time, adjust the amount of the precipitant to control the pH=8.0 until Fe ₂ O ₃ .3H ₂ O is completely precipitated; after the precipitate is separated and washed, add 50mL of 0.01mol /L dilute hydrochloric acid for acidification, high-speed centrifugation (4000r/min) and separation, add 100mL deionized water, then add 0.08g ammonium oleate and 0.08g polyethylene glycol to the deionized water, stir vigorously at 600 rpm at 30°C for 4h Get Fe ₂ O ₃ .3H ₂ O colloid. Weigh 1.57g of 1-ethylimidazolium tetrafluoroborate, 2.37g of lithium hydroxide and 0.26g of magnesium nitrate. Add these materials into the Fe ₂ O ₃ .3H ₂ O colloid, and continue stirring at 600 rpm for 2 hours to obtain a uniform colloidal mixed slurry. The slurry was spray-dried (inlet temperature 130°C) to form a spherical lithium iron phosphate precursor. The precursor was put into a crucible and heated to 500 °C in a nitrogen-protected microwave oven, kept for 20 min and then cooled naturally to obtain carbon-coated spherical porous lithium iron phosphate powder.

The battery was assembled according to the method of Example 1 and tested. Under the condition of charge and discharge rate of 2C, the initial discharge capacity of the material was 139.6mAh/g, and the capacity retention rate after 50 cycles was 96.8%.

Claims (8)

1. A preparation method of carbon-coated porous lithium iron phosphate powder, the steps of which are: (1) Preparation of Fe ₂ O ₃ colloid: Weigh the iron source, dissolve it in deionized water to prepare a solution with a Fe ³⁺ concentration of 0.1-1 mol/L, use ammonia water or urea solution with a mass concentration of 10% as a precipitant, and The organic small molecule dispersant is dissolved in deionized water as the bottom liquid, the molar ratio is: dispersant: Fe ³⁺ =1:10, pH=6.8-7.0; (2) Add the Fe ³⁺ solution and the precipitating agent to the bottom solution at the same time, and control the pH=7.0-13.0 until Fe ₂ O ₃ .nH ₂ O is completely precipitated; after the precipitate is separated and washed, under stirring, the molar ratio of HCl : Fe ₂ O ₃ =1:100-1:500 Add 0.01mol/L dilute hydrochloric acid for acidification to obtain electropositive Fe ₂ O ₃ .nH ₂ O colloidal particles; after the colloidal particles are separated by centrifugation, add deionized water , add surfactant at the same time, the mass ratio is: surfactant: Fe ₂ O ₃ =1:10-1:100, stir at 30-100°C, the stirring time lasts 1-10h, and obtain the mass ratio of Fe ₂ O ₃ 5-30% Fe ₂ O ₃ .nH ₂ O colloid; (3) Slurry preparation: according to the stoichiometric ratio Li _x Fe _y PO ₄ :M _z , where M is the dopant ion, x=0.8-1.2, y=0.8-1.2, z=0.01-0.1, weigh the lithium source , phosphorus source and doping metal compound, the addition of carbon source is the mass ratio of Li _x Fe _y PO ₄ :M _z 5-20%; Lithium source, phosphorus source, carbon source and doping metal compound are added to Fe ₂ In the O ₃ .nH ₂ O colloid, continue to stir to form a uniformly dispersed colloidal mixed slurry; (4) Precursor preparation: The slurry is dried by spray drying with an inlet temperature of 110-130°C to obtain a spherical lithium iron phosphate precursor with uniform composition and uniform particle size distribution; (5) Calcination: Put the precursor obtained by spray drying into a crucible, heat up to 500-800°C under the protection of an inert atmosphere, keep it warm for 10-40 min, and then cool naturally to obtain a carbon-coated spherical porous lithium iron phosphate powder. 2. The preparation method of carbon-coated porous lithium iron phosphate powder according to claim 1, wherein the iron source is specifically ferric citrate (FeC ₆ H ₅ O ₇ ), or ferric chloride (FeCl ₃ .6H ₂ O), or iron nitrate [Fe(NO ₃ ) ₃ ·9H ₂ O], or iron sulfate [Fe ₂ (SO ₄ ) ₃ ], or a combination of the above iron sources. 3. according to the preparation method of the described carbon-coated porous lithium iron phosphate powder of claim 1, it is characterized in that described organic small molecule dispersant is specifically citric acid, or glutamic acid, or ascorbic acid, or tartaric acid , or salicylic acid, or a combination of the above. 4. according to the preparation method of the described carbon-coated porous lithium iron phosphate powder of claim 1, it is characterized in that described surfactant is specifically gelatin, or polyethylene glycol, or polyvinyl alcohol, or polyvinyl alcohol Ammonium methacrylate, or ammonium oleate, or ammonium alginate, or a combination of the above surfactant substances. 5. The preparation method of carbon-coated porous lithium iron phosphate powder according to claim 1, characterized in that said lithium source is specifically lithium carbonate (Li ₂ CO ₃ ), or lithium hydroxide (LiOH), or It is lithium dihydrogen phosphate (LiH ₂ PO ₄ ), or a combination of the lithium sources mentioned above. 6. The method for preparing carbon-coated porous lithium iron phosphate powder according to claim 1, wherein the phosphorus source is specifically diammonium hydrogen phosphate [(NH ₄ ) ₂ HPO ₄ ], or dihydrogen phosphate Ammonium (NH ₄ H ₂ PO ₄ ), or lithium dihydrogen phosphate (LiH ₂ PO ₄ ), or phosphoric acid (H ₃ PO ₄ ), or a combination of the above phosphorus sources. 7. The method for preparing carbon-coated porous lithium iron phosphate powder according to claim 1, wherein the carbon source is specifically 1-ethyl-3-methylimidazolium nitrate [EMIm]NO ₃ , or is 1-butyl-3-methylimidazolium nitrate [BMIm]NO ₃ , or 1-methylimidazolium tetrafluoroborate [MIm]BF ₄ , or 1-ethylimidazolium tetrafluoroborate [ EIm]BF ₄ , or 1-methylimidazole dihydrogen phosphate [MIm] H ₂ PO ₄ , or 1-butylimidazolium dihydrogen phosphate [BIm] H ₂ PO ₄ , or 1-ethyl- 3-methylimidazole dinitrile amine [EMIm]N(CN) ₂ , or 1-butyl-3-methylimidazole dinitrile amine [BMIm]N(CN) ₂ , etc., or the ionic liquid mentioned above combination. 8. The method for preparing carbon-coated porous lithium iron phosphate powder according to claim 1, characterized in that the doped metal compound is specifically magnesium nitrate [Mg(NO ₃ ) ₂ .2H ₂ O], or Magnesium oxalate [MgC ₂ O ₄ .2H ₂ O], or nickel nitrate [Ni(NO ₃ ) ₂ 6H ₂ O], or nickel acetate [C ₂ H ₃ NiO ₂ ], or cobalt nitrate [Co( NO ₃ ) ₂ 6H ₂ O], or cobalt acetate [C ₄ H ₆ O ₄ Co 4H ₂ O], or aluminum nitrate [Al(NO ₃ ) ₃ 9H ₂ O], or the above Combinations of doped metal compounds.

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