CN108172813B - Composite cathode material and preparation method thereof - Google Patents
Composite cathode material and preparation method thereof Download PDFInfo
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Abstract
The invention provides a preparation method of a composite anode material, which comprises the following steps: lithium source, aluminum source, organic carbon source and carbon-containing LiFePO4Mixing to obtain a mixed solution; heating the mixed solution to obtain gel; drying the gel and crushing to obtain powder; and sintering the powder to obtain the composite cathode material. The invention adopts lithium source, aluminum source and organic carbon source to prepare LiFePO containing carbon4The surface modification is carried out, so that the carbon source material can greatly increase the conductivity of the composite cathode material in the invention, and further improve the electron transmission speed of the surface of the cathode material; the non-carbon source material can effectively prevent the direct contact between the anode material and the electrolyte, and the interface stability of the anode material is enhanced. The invention uses lithium source, aluminum source and organic carbon source to react with carbon-containing LiFePO4The modification can increase the diffusion rate and the electron transmission rate of lithium ions, and effectively improve the rate capability and the cycling stability of the composite cathode material.
Description
Technical Field
The invention relates to the technical field of lithium ion batteries, in particular to a composite cathode material and a preparation method thereof.
Background
With the increasing shortage of energy and the increasing severity of environmental pollution, the development and application of new energy sources are imminent. The lithium ion battery has the advantages of environmental friendliness, high energy density, abundant reserves, long cycle life and the like, and therefore, the lithium ion battery is considered to be a new energy product with the most potential. Up to now, lithium ion batteries have been widely used in the fields of the current market of electronic devices, hybrid vehicles, energy storage systems, new energy electric vehicles, and the like. However, with the increasing development speed of the electronic market, the energy density of the conventional lithium ion battery has not been able to meet the demand of the electronic device. Therefore, it is important to develop a high energy density, low cost, environmentally friendly lithium ion battery.
The positive electrode material plays a very important role in a lithium ion battery system as a core component of the lithium ion battery. The positive electrode material in the lithium ion battery commercialized at present is mainly LiMn2O4、LiCoO2、LiFePO4And ternary materials. However, LiCoO2The battery is easy to react with electrolyte violently to release a large amount of heat and generate a large amount of gas, and the safety performance of the battery is seriously influenced. LiMn2O4LiMn along with the progress of the charging and discharging process2O4Mn in (1) decomposes into the electrolyte and Li+The intercalation and deintercalation of (a) may cause collapse of its crystal structure, resulting in poor high-temperature performance and cycle performance of the battery. Compared with other materials, the ternary material has the defects of low initial capacity, poor cycle performance and rate performance, quick discharge voltage attenuation and the like, and greatly limits the commercial market of the ternary material. LiFePO4Has high theoretical capacity (170mAh g)-1) The advantages of long cycle life, stable structure, low cost, environmental protection and the like are widely applied. However, LiFePO4Poor conductivity results in poor rate performance and insufficient cycle stability. Currently, much research effort is devoted to solving this problem.
Disclosure of Invention
In view of this, the present invention provides a composite cathode material and a preparation method thereof, and the composite cathode material provided by the present invention has good rate capability and cycle stability.
The invention provides a preparation method of a composite anode material, which comprises the following steps:
lithium source, aluminum source, organic carbon source and carbon-containing LiFePO4Mixing to obtain a mixed solution;
heating the mixed solution to obtain gel;
drying the gel and crushing to obtain powder;
and sintering the powder to obtain the composite cathode material.
The composite anode material prepared by the method provided by the invention is LiAlO2LiFePO with/C surface modified olivine structure4The preparation method is preferably a high-temperature solid phase method.
In the present invention, the method for preparing the composite cathode material preferably includes the steps of:
1) dissolving a first lithium source, an iron source and a phosphorus source in water to obtain a first mixed solution;
2) mixing the first mixed solution with a carbon source to obtain a first mixture;
3) heating the first mixture to obtain a gel;
4) drying and crushing the gel to obtain powder;
5) sintering the powder to obtain an intermediate product;
6) dissolving a second lithium source, an aluminum source and an organic carbon source in water to obtain a second mixed solution;
7) mixing the second mixed solution and the intermediate product to obtain a second mixture;
8) heating the second mixture to obtain a gel;
9) drying the gel and crushing to obtain powder;
10) and sintering the powder to obtain the composite cathode material.
In the present invention, the water is preferably deionized water.
In the invention, the total concentration of the first lithium source, the iron source and the phosphorus source in the first mixed solution is preferably 0.01-5 mol/L, more preferably 0.05-4 mol/L, more preferably 0.1-3 mol/L, more preferably 0.5-2 mol/L, and most preferably 1-1.5 mol/L.
In the invention, the ratio of the total mole number of the first lithium source, the iron source and the phosphorus source to the mole number of the carbon source is preferably 1 (1-10), more preferably 1 (2-8), and most preferably 1 (3-6).
In the invention, the first mixture is preferably heated under the condition of stirring, and the heating temperature is preferably 80-150 ℃, more preferably 100-130 ℃, and most preferably 110-120 ℃; the heating time is preferably 8 to 20 hours, more preferably 10 to 16 hours, and most preferably 12 to 14 hours. In the present invention, the heating is preferably performed by evaporating water in the first mixed solution.
In the invention, the method for drying the gel is preferably vacuum drying, and the drying temperature is preferably 80-120 ℃, more preferably 90-110 ℃, and most preferably 100 ℃; the drying time is preferably 10 to 20 hours, more preferably 12 to 18 hours, and most preferably 14 to 16 hours.
In the present invention, the method of crushing is preferably ball milling.
In the invention, the powder is sintered, preferably under the condition of inert gas; the sintering is preferably segmented sintering, and the sintering method is preferably as follows:
heating the powder to a first temperature at a first speed, and keeping the temperature for a first time to obtain a sintered product;
and raising the temperature of the sintered product to a second temperature at a second speed, keeping the temperature for a second time, and then cooling to obtain an intermediate product.
In the invention, the first speed is preferably 1-5 ℃/min, more preferably 2-4 ℃/min, and most preferably 2.5-3.5 ℃/min. In the invention, the first temperature is preferably 550-650 ℃, more preferably 580-620 ℃, and most preferably 600 ℃. In the present invention, the temperature is preferably raised from room temperature to the first temperature, and the room temperature is preferably 20 to 30 ℃, and more preferably 25 ℃. In the present invention, the first time is preferably 5 to 10 hours, more preferably 6 to 9 hours, and most preferably 7 to 8 hours.
In the present invention, the second speed is preferably 1 to 5 ℃/min, more preferably 2 to 4 ℃/min, and most preferably 2.5 to 3.5 ℃/min. In the invention, the second temperature is preferably 650-850 ℃, more preferably 660-820 ℃ and most preferably 680-800 ℃. In the present invention, the second time is preferably 8 to 24 hours, more preferably 10 to 20 hours, and most preferably 14 to 16 hours. In the present invention, the cooling is preferably natural cooling, more preferably self-coolingThen cooled to room temperature. In the present invention, the intermediate product is preferably carbon-containing LiFePO4More preferably an olivine-type structure of carbon-containing LiFePO4。
In the invention, the mass ratio of the second lithium source, the aluminum source and the organic carbon source is preferably 1 (0.5-1.5): (1-3), more preferably 1 (0.8-1.2): (1.5-2.5), and most preferably 1:1: (1.8-2.2). In the present invention, the intermediate product (i.e., carbon-containing LiFePO)4) The addition amount of (b) is preferably 70 to 99%, more preferably 75 to 95%, more preferably 80 to 90%, and most preferably 84 to 86% of the total mass of the intermediate product, the second lithium source, the aluminum source, and the organic carbon source.
In the present invention, the second mixture is heated, preferably slowly evaporated to dryness; the heating temperature is preferably 80-100 ℃, more preferably 85-95 ℃, and most preferably 90 ℃.
In the invention, the method for drying the gel is preferably vacuum drying, and the drying temperature is preferably 80-120 ℃, more preferably 90-110 ℃, and most preferably 100 ℃; the drying time is preferably 10 to 20 hours, more preferably 12 to 18 hours, and most preferably 14 to 16 hours.
In the present invention, the powder is sintered, preferably under inert gas conditions; the sintering is preferably segmented sintering, and the sintering method is preferably as follows:
heating the powder to a third temperature at a third speed, and keeping the temperature for a third time to obtain a sintered product;
and heating the sintered product to a fourth temperature at a fourth speed, preserving the heat for a fourth time, and then cooling to obtain the composite cathode material.
In the present invention, the third speed is preferably 1 to 5 ℃/min, more preferably 2 to 4 ℃/min, and most preferably 2.5 to 3.5 ℃/min. In the invention, the third temperature is preferably 550-650 ℃, more preferably 580-620 ℃, and most preferably 600 ℃. In the present invention, the temperature is preferably raised from room temperature to a third temperature, wherein the room temperature is preferably 20 to 30 ℃, and more preferably 25 ℃. In the present invention, the third time is preferably 5 to 12 hours, more preferably 6 to 10 hours, and most preferably 8 to 9 hours.
In the present invention, the fourth speed is preferably 1 to 5 ℃/min, more preferably 2 to 4 ℃/min, and most preferably 2.5 to 3.5 ℃/min. In the invention, the fourth temperature is preferably 650-850 ℃, more preferably 660-820 ℃ and most preferably 680-800 ℃. In the present invention, the fourth time is preferably 10 to 24 hours, more preferably 12 to 20 hours, and most preferably 14 to 16 hours. In the present invention, the cooling is preferably natural cooling, and more preferably natural cooling to room temperature. In the invention, the positive electrode composite material is LiAlO2LiFePO with/C surface modified olivine structure4A composite material.
In the invention, the sectional sintering can ensure that the raw materials added firstly are in a molten state and are fully and uniformly mixed with other raw materials added later.
In the invention, the first lithium source and the second lithium source are independently and preferably selected from one or more of lithium dihydrogen phosphate, lithium acetate, lithium carbonate and lithium hydroxide; the iron source is preferably one or more of ferrous oxalate, ferric sulfate, ferric nitrate and ferric hydroxide; the phosphorus source is preferably one or more of ammonium phosphate, ammonium hydrogen phosphate, sodium dihydrogen phosphate and sodium phosphate. In the invention, the molar ratio of the first lithium source, the iron source and the phosphorus source meets the target product LiFePO4The stoichiometric ratio of (A) to (B); the molar ratio of the first lithium source, the iron source and the phosphorus source is preferably 1:1: 1.
In the invention, the carbon source is preferably one or more of glycine, citric acid, sucrose, starch, glucose and cassava flour; the carbon source has a steric hindrance effect, can effectively inhibit the growth of material particles in a high-temperature carbonization process, and simultaneously forms conductive carbon to increase the electronic conductivity of the material. According to the invention, the carbon source is added in the process of preparing the intermediate product, so that the prepared composite anode material has good rate performance and cycle performance.
In the present invention, the aluminum source is preferably one or more of aluminum nitrate, aluminum sulfate and aluminum hydroxide.
In the invention, the organic carbon source is preferably one or more of glycine, citric acid, sucrose, starch, glucose and cassava flour.
In the present invention, the inert gas is preferably one or both of helium and argon.
The invention provides a composite anode material prepared by the method in the technical scheme, and the composite anode material is LiAlO2And C surface modified olivine structure carbon-containing LiFePO4A composite material. In the present invention, LiAlO2The mass content of the composite positive electrode material is preferably 0.5-15%, more preferably 1-12%, more preferably 3-10%, and most preferably 5-8%; the mass content of carbon in the composite cathode material is preferably 0.5-15%, more preferably 1-12%, more preferably 3-10%, and most preferably 5-8%; LiFePO4The mass content in the composite positive electrode material is preferably 70-99%, more preferably 75-95%, more preferably 80-90%, and most preferably 84-86%.
The invention adopts lithium source, aluminum source and organic carbon source to prepare LiFePO containing carbon4The surface modification is carried out, so that the carbon source material can greatly increase the conductivity of the composite cathode material in the invention, and further improve the electron transmission speed of the surface of the cathode material; the non-carbon source material can effectively prevent the direct contact between the anode material and the electrolyte, and the interface stability of the anode material is enhanced. Firstly adding a carbon source in the process of preparing the lithium iron phosphate, and then carrying out the treatment on the carbon-containing LiFePO by a lithium source, an aluminum source and an organic carbon source together4The modification can increase the diffusion rate and the electron transmission rate of lithium ions, and effectively improve the rate capability and the cycling stability of the composite cathode material.
Compared with the prior art, the method provided by the invention adopts a lithium source, an aluminum source and a carbon source to modify the carbon-containing lithium iron phosphate, and the carbon-based material is coated to greatly increase the conductivity of the composite cathode material, so that the electron transmission speed on the surface of the cathode material is increased; the non-carbon-based material coating layer can effectively prevent the anode material from being in direct contact with electrolyte, and the interface stability of the anode material is enhanced. The present invention providesThe method can prepare LiAlO2/C surface modified olivine structure carbon-containing LiFePO4The composite positive electrode material of (2), LiAlO2the/C surface modification can increase the diffusion rate and the electron transmission rate of lithium ions and can effectively improve the rate capability and the cycling stability of the lithium iron phosphate material. In addition, the preparation method of the composite cathode material provided by the invention is simple and easy to operate.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the provided drawings without creative efforts.
FIG. 1 shows a composite cathode material and carbon-containing LiFePO prepared in example 1 of the present invention4An XRD spectrum of the anode material;
FIG. 2 is an SEM image of a composite cathode material prepared in example 1 of the present invention;
FIG. 3 shows LiFePO containing carbon used in example 1 of the present invention4SEM image of the positive electrode material;
FIG. 4 is a graph showing the charge and discharge curves of a button cell prepared in example 2 of the present invention;
fig. 5 is a charge-discharge curve diagram of the button cell prepared in example 2 of the present invention in 100 cycles.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
Example 1
According to the chemical formula of lithium iron phosphate anode material (LiFePO)4) Weighing lithium carbonate, iron oxyhydroxide and ammonium phosphate according to the molar ratio shown in the specification, dissolving the lithium carbonate, the iron oxyhydroxide and the ammonium phosphate in deionized water, respectively preparing aqueous solutions with the concentration of 0.01mol/L, and mixing the aqueous solutions to obtain a mixed solution.
According to the molar ratio of the total metal ions in the mixed solution to the citric acid being 1:1 to the above mixed solution, citric acid was added to obtain a mixture.
And stirring the mixture at 80 ℃ for 20h, evaporating all deionized water to dryness to obtain a gel, drying the gel in a vacuum drying oven at 80 ℃ for 20h in vacuum, taking out the dried product, and performing ball milling and crushing to obtain powder.
Sintering the powder in nitrogen atmosphere at 1 ℃/min from 25 ℃ to 550 ℃ for 10h, then heating in nitrogen atmosphere at 1 ℃/min to 650 ℃ for 24h, and naturally cooling to room temperature to obtain the olivine-structure carbon-containing LiFePO4。
Lithium hydroxide, aluminum hydroxide and starch are mixed according to a molar ratio of 1:1:1 are completely dissolved in deionized water, and then the carbon-containing LiFePO is added4Anode material, carbon-containing LiFePO4The mass of the anode material is LiFePO485% of the total mass of the positive electrode material, the lithium hydroxide, the aluminum hydroxide and the starch; slowly evaporating to dryness at 80 deg.C to form gel, vacuum drying the gel in a vacuum drying oven at 80 deg.C for 20 hr, taking out the dried product, and ball-milling and crushing to obtain powder.
Sintering the powder at 1 ℃/min from 25 ℃ to 550 ℃ for 12h in nitrogen atmosphere, then heating at 1 ℃/min to 650 ℃ in nitrogen atmosphere, preserving heat for 24h, and naturally cooling to room temperature to obtain LiAlO2LiFePO with/C surface modified olivine structure4Composite materials, i.e. composite positive electrode materials.
LiFePO containing carbon prepared in the embodiment 1 of the invention4Anode material and carbon-containing LiFePO4@ LiAlO2XRD and SEM characterization is carried out on the microstructure and the phase structure of the/C composite material respectively, the detection results are shown in figures 1 and 2, A in figure 1 represents the LiFePO containing carbon prepared in example 14@LiAlO2XRD pattern of/C composite material, B represents the pattern obtained in example 1To carbon-containing LiFePO4XRD pattern of the anode material. As known from XRD (X-ray diffraction) pattern, carbon-containing LiFePO4@LiAlO2Composite material/C and carbon-containing LiFePO4Has the same diffraction peak and no hetero-phase peak appears, which indicates that LiAlO2the/C coating does not influence LiFePO4Phase structure of carbon-containing LiFePO4@LiAlO2the/C composite material has an olivine-type structure.
From the SEM image, LiFePO containing carbon4@LiAlO2Composite material/C and carbon-containing LiFePO4SEM images of the cathode material show that LiFePO containing carbon4Through LiAlO2After coating modification, the microstructure of the/C coating is not changed.
Example 2
Accurately weighing the composite positive electrode material prepared in the example 1, SuperP (conductive carbon black) and PVDF (polyvinylidene fluoride, binder) according to the mass ratio of 90:5:5, and then sequentially adding the PVDF, SP (conductive carbon black) and the composite positive electrode material into NMP (N-methylpyrrolidone) to stir for 12 hours to obtain uniform mixed slurry;
and uniformly coating the obtained slurry on an aluminum foil, transferring the aluminum foil to a vacuum drying oven at 120 ℃, and performing vacuum drying for 12 hours to obtain the positive plate.
The positive electrode is the positive plate, the negative electrode is a metal lithium plate, the diaphragm is PE (polypropylene), and LiPF of 0.1mol/L6And (4) carrying out battery assembly in a glove box filled with argon as an electrolyte to obtain the button battery.
And (3) wiping off the electrolyte on the button cell by using absolute ethyl alcohol, standing for 12 hours, and then carrying out electrochemical performance test at 25 ℃ and 2.5-4.2V. As shown in fig. 4 and 5, it can be seen from fig. 4 that the composite positive electrode material prepared in example 1 exhibits excellent rate capability, and the specific discharge capacities at 0.1C and 5C rates were 168.3mAh/g and 128.9mAh/g, respectively. As can be seen from fig. 5, the composite cathode material prepared in example 1 exhibits better cycling stability, and the capacity retention rate can reach 95.2% after cycling for 100 times at a rate of 1C.
Comparative example 1
A positive electrode was prepared in accordance with the procedure of example 2Sheet, unlike example 2, the carbon-containing LiFePO prepared in example 1 was used4The material replaced the composite positive electrode material in example 2.
A coin cell was prepared according to the method of example 2, which is different from example 2 in that the positive electrode sheet prepared in comparative example 1 was used instead of the positive electrode sheet in example 2.
The electrochemical performance test of the button battery is carried out at 25 ℃ and 2.5-4.2V, the discharge specific capacities of the button battery at 0.1C and 5C multiplying power respectively reach 166mAh/g and 123mAh/g, and the capacity retention rate can reach 90.3% after 100 times of circulation at 1C multiplying power.
Example 3
According to the chemical formula of lithium iron phosphate anode material (LiFePO)4) Weighing lithium hydroxide, ferric sulfate and ammonium hydrogen phosphate according to the molar ratio shown in the specification, dissolving the lithium hydroxide, ferric sulfate and ammonium hydrogen phosphate in deionized water to prepare an aqueous solution with the concentration of 1mol/L, and mixing the aqueous solution to obtain a mixed solution.
According to the molar ratio of the total amount of metal ions to the glycine in the mixed solution of 1: 2 to the above mixed solution, glycine was added to obtain a mixture.
And stirring the mixture at 100 ℃ for 10h, evaporating all deionized water to dryness to obtain a gel substance, then carrying out vacuum drying on the gel substance in a vacuum drying oven at 100 ℃ for 12h, taking out the dried substance, and carrying out ball milling and crushing to obtain powder.
Sintering the powder in argon atmosphere at 2 ℃/min from 25 ℃ to 600 ℃ for 6h, then heating in argon atmosphere at 2 ℃/min to 700 ℃ for 10h, and naturally cooling to room temperature to obtain the olivine-structure carbon-containing LiFePO4。
Lithium carbonate, aluminum nitrate and citric acid are mixed according to a molar ratio of 1:1: 1.2 dissolving the mixture in deionized water, and adding the olivine-structured carbon-containing LiFePO4Positive electrode material, said carbon-containing LiFePO4The anode material is carbon-containing LiFePO486% of the total mass of the positive electrode material, the lithium carbonate, the aluminum nitrate and the citric acid; slowly evaporating to dryness at 85 deg.C to form gel, and vacuum drying at 90 deg.C for 15 hrAnd taking out the dried substance, and performing ball milling and crushing to obtain powder.
Sintering the powder at 2 ℃/min from 25 ℃ to 600 ℃ for 6h in argon atmosphere, then heating at 2 ℃/min to 700 ℃ in argon atmosphere, preserving heat for 12h, and naturally cooling to room temperature to obtain LiAlO2/C surface modified olivine structure carbon-containing LiFePO4Composite materials, i.e. composite positive electrode materials.
A coin cell was prepared according to the method of example 2, except that the composite cathode material prepared in example 3 was used instead of the composite cathode material prepared in example 1.
The electrochemical performance of the button cell prepared in the embodiment 3 is tested at 25 ℃ and 2.5-4.2V, the detection result is similar to that of the button cell prepared in the embodiment 2, and the composite cathode material prepared in the embodiment 3 of the invention has good cycling stability and rate capability.
Example 4
According to the chemical formula of lithium iron phosphate anode material (LiFePO)4) Weighing lithium dihydrogen phosphate, ferrous oxalate and sodium phosphate according to the molar ratio shown in the specification, dissolving the lithium dihydrogen phosphate, the ferrous oxalate and the sodium phosphate in deionized water to prepare a water solution with the concentration of 2.5mol/L, and mixing the water solution to obtain a mixed solution;
according to the molar ratio of the total metal ions in the mixed solution to the starch of 1: 5.5 to the above mixed solution, glucose was added to obtain a mixture.
And stirring the mixture at 115 ℃ for 14h, evaporating all deionized water to dryness to obtain a gel substance, drying the gel substance in a vacuum drying oven at 100 ℃ for 15h in vacuum, taking out the dried substance, and performing ball milling and crushing to obtain powder.
Sintering the powder in nitrogen atmosphere at 3 ℃/min from 25 ℃ to 600 ℃ for 7.5h, then heating in nitrogen atmosphere at 3 ℃/min to 750 ℃ for 16h, and naturally cooling to room temperature to obtain the carbon-containing LiFePO with the olivine structure4。
Lithium acetate, aluminum sulfate and glycine are mixed according to a molar ratio of 1:1: 2 dissolving in deionized water, adding the obtained olivine-type structureCarbon LiFePO4Positive electrode material, said carbon-containing LiFePO4The anode material is carbon-containing LiFePO488% of the total mass of the positive electrode material, the lithium acetate, the aluminum sulfate and the glycine; slowly evaporating to dryness at 90 ℃ to form gel, then drying the gel in a vacuum drying oven at 100 ℃ for 15h in vacuum, taking out the dried product, and performing ball milling and crushing to obtain powder.
Sintering the powder at the temperature of 25-600 ℃ for 7.5h at the speed of 3 ℃/min in nitrogen atmosphere, then heating at the speed of 3 ℃/min in nitrogen atmosphere to 700 ℃ for heat preservation for 17h, and naturally cooling to room temperature to obtain LiAlO2/C surface modified olivine structure carbon-containing LiFePO4Composite materials, i.e. composite positive electrode materials.
A coin cell was prepared according to the method of example 2, which is different from example 2 in that the composite cathode material prepared in example 4 was used instead of the composite cathode material prepared in example 1.
The electrochemical performance of the button cell prepared in the embodiment 4 is tested at 25 ℃ and 2.5-4.2V, the detection result is similar to that of the button cell prepared in the embodiment 2, and the composite cathode material prepared in the embodiment 4 of the invention has good cycling stability and rate capability.
Example 5
According to the chemical formula of lithium iron phosphate anode material (LiFePO)4) Weighing lithium carbonate, ferrous nitrate and sodium dihydrogen sulfate according to the molar ratio shown in the specification, dissolving the lithium carbonate, the ferrous nitrate and the sodium dihydrogen sulfate in deionized water to prepare a water solution with the concentration of 4mol/L, and mixing the water solution to obtain a mixed solution.
According to the molar ratio of the total metal ions in the mixed solution to glucose of 1: 6 to the above mixed solution, glucose was added to obtain a mixture.
And stirring the mixture at 120 ℃ for 12h, evaporating all deionized water to dryness to obtain a gel substance, then carrying out vacuum drying on the gel substance in a vacuum drying oven at 110 ℃ for 18h, taking out the dried substance, and carrying out ball milling and crushing to obtain powder.
Sintering the powder in nitrogen atmosphere at 4 deg.C/min from 25 deg.C to 500 deg.C for 8h, and then adding nitrogenHeating to 800 deg.C at a rate of 4 deg.C/min in gas atmosphere, maintaining for 20h, and naturally cooling to room temperature to obtain olivine-structure carbon-containing LiFePO4。
Lithium hydroxide, aluminum hydroxide and cassava powder are mixed according to a molar ratio of 1:1: 2.5 dissolving in deionized water, adding the above-prepared olivine-structured carbon-containing LiFePO4Positive electrode material, said carbon-containing LiFePO4The anode material is carbon-containing LiFePO 490% of the total mass of the positive electrode material, the lithium hydroxide, the aluminum hydroxide and the cassava powder; slowly evaporating to dryness at 95 deg.C to form gel, vacuum drying at 110 deg.C for 18 hr, taking out, and ball milling to obtain powder.
Sintering the powder at 4 ℃/min from 25 ℃ to 500 ℃ for 10h in nitrogen atmosphere, then heating at 4 ℃/min to 800 ℃ in nitrogen atmosphere, preserving heat for 20h, and naturally cooling to room temperature to obtain LiAlO2/C surface modified olivine structure carbon-containing LiFePO4Composite materials, i.e. composite positive electrode materials.
A coin cell was prepared according to the method of example 2, which is different from example 2 in that the composite cathode material prepared in example 5 was used instead of the composite cathode material prepared in example 1.
The electrochemical performance of the button cell prepared in the embodiment 5 is tested at 25 ℃ and 2.5-4.2V, the detection result is similar to that of the button cell prepared in the embodiment 2, and the composite cathode material prepared in the embodiment 5 of the invention has good cycling stability and rate capability.
Example 6
According to the chemical formula of lithium iron phosphate anode material (LiFePO)4) Lithium acetate, ferric sulfate and ammonium dihydrogen phosphate are weighed according to the molar ratio shown in the specification and dissolved in deionized water to prepare a water solution with the concentration of 5mol/L, and the water solution is mixed to obtain a mixed solution.
According to the molar ratio of the total metal ions in the mixed solution to the citric acid being 1: 10 to the above mixed solution, citric acid was added to obtain a mixture.
Stirring the obtained mixture at 150 ℃ for 8h, evaporating all deionized water to dryness to obtain a gel substance, vacuum-drying the gel substance in a vacuum drying oven at 80 ℃ for 10h, taking out the dried substance, and performing ball milling and crushing to obtain powder.
Sintering the powder in argon atmosphere at 5 ℃/min from 25 ℃ to 650 ℃ for 5h, then heating in argon atmosphere at 5 ℃/min to 850 ℃ for heat preservation for 8h, and naturally cooling to room temperature to obtain the olivine-structure carbon-containing LiFePO4。
Lithium carbonate, aluminum sulfate and citric acid are mixed according to a molar ratio of 1:1: 3, dissolving in deionized water, and adding the above-prepared olivine-structure carbon-containing LiFePO4Positive electrode material, said carbon-containing LiFePO4The anode material is carbon-containing LiFePO492% of the total mass of the positive electrode material, the lithium carbonate, the aluminum sulfate and the citric acid; slowly evaporating to dryness at 100 ℃ to form gel, then drying the gel in a vacuum drying oven at 120 ℃ for 10h in vacuum, taking out the dried product, and carrying out ball milling and crushing to obtain powder.
Sintering the powder at 5 ℃/min from 25 ℃ to 650 ℃ for 5h in argon atmosphere, then heating at 5 ℃/min to 850 ℃ in argon atmosphere, preserving heat for 10h, and naturally cooling to room temperature to obtain LiAlO2/C surface modified olivine structure carbon-containing LiFePO4Composite materials, i.e. composite positive electrode materials.
A coin cell was prepared according to the method of example 2, except that the composite cathode material prepared in example 6 was used instead of the composite cathode material prepared in example 1.
Electrochemical performance tests are carried out on the button cell prepared in the embodiment 6 at 25 ℃ and 2.5-4.2V, the detection result is similar to that of the button cell prepared in the embodiment 2, and the composite cathode material prepared in the embodiment 6 of the invention has good circulation stability and rate capability.
According to the invention, firstly, a carbon source is added in the process of preparing the lithium iron phosphate, then the lithium source, the aluminum source and the carbon source are adopted again to modify the carbon-containing lithium iron phosphate simultaneously to obtain the composite anode material, and the carbon source is added twice in the process of preparing the composite anode material so that the prepared composite anode material can be obtainedThe material has better rate capability and cycling stability. According to the invention, a lithium source, an aluminum source and a carbon source are adopted to simultaneously modify the carbon-containing lithium iron phosphate, the three components simultaneously modify the lithium iron phosphate to have a synergistic effect, and under the condition that the total amount of the modified components is the same, the cycling stability and the rate capability of the anode material obtained by simultaneously modifying the lithium iron phosphate by the three components are superior to those of the anode material obtained by modifying the lithium iron phosphate by one or two of the components. In addition, the performance of the anode material obtained by modifying the carbon-containing lithium iron phosphate by simultaneously adopting the lithium source, the aluminum source and the carbon source in the preparation process is superior to that of the anode material obtained by sequentially modifying the lithium iron phosphate or the carbon-containing lithium iron phosphate by three components, namely, the carbon-containing lithium iron phosphate only contains one layer of LiAlO2And the anode material with the structure has better cycle stability and rate capability than that of the lithium iron phosphate or the carbon-containing lithium iron phosphate with the surface coated with LiAlO2The layer is coated with a positive electrode material with a multi-layer coating structure of a C layer.
From the above embodiments, the present invention provides a method for preparing a composite cathode material, including: lithium source, aluminum source, organic carbon source and carbon-containing LiFePO4Mixing to obtain a mixed solution; heating the mixed solution to obtain gel; drying the gel and crushing to obtain powder; and sintering the powder to obtain the composite cathode material. The invention adopts lithium source, aluminum source and organic carbon source to prepare LiFePO containing carbon4The surface modification is carried out, so that the carbon source material can greatly increase the conductivity of the composite cathode material in the invention, and further improve the electron transmission speed of the surface of the cathode material; the non-carbon source material can effectively prevent the direct contact between the anode material and the electrolyte, and the interface stability of the anode material is enhanced. The invention uses lithium source, aluminum source and organic carbon source to react with carbon-containing LiFePO4The modification can increase the diffusion rate and the electron transmission rate of lithium ions, and effectively improve the rate capability and the cycling stability of the composite cathode material.
Claims (4)
1. A preparation method of the composite cathode material comprises the following steps:
1) dissolving a first lithium source, an iron source and a phosphorus source in water to obtain a first mixed solution;
2) mixing the first mixed solution with a carbon source to obtain a first mixture;
3) heating the first mixture to obtain a gel;
4) drying and crushing the gel to obtain powder;
5) sintering the powder to obtain an intermediate product;
6) dissolving a second lithium source, an aluminum source and an organic carbon source in water to obtain a second mixed solution;
7) mixing the second mixed solution and the intermediate product to obtain a second mixture;
8) heating the second mixture to obtain a gel;
9) drying the gel and crushing to obtain powder;
10) sintering the powder to obtain a composite anode material, wherein the finally prepared composite anode material only contains a layer of LiAlO2And carbon-containing lithium iron phosphate of the C coating layer;
the aluminum source is selected from one or more of aluminum nitrate, aluminum sulfate and aluminum hydroxide;
in the step 8), the heating temperature is 80-100 ℃;
in the step 9), the drying temperature is 80-120 ℃;
in the step 9), the drying time is 10-20 hours;
in step 10), the sintering method comprises the following steps:
(A) heating the powder to 550-650 ℃, and preserving heat to obtain a sintered product;
(B) heating the sintered product to 660-850 ℃, preserving heat and cooling to obtain a composite anode material;
the heating rate in the step (A) is 1-5 ℃/min;
the heating rate in the step (B) is 1-5 ℃/min;
the heat preservation time in the step (A) is 5-12 hours, and the heat preservation time in the step (B) is 10-24 hours.
2. The method of claim 1, wherein the first and second lithium sources are independently selected from one or more of lithium dihydrogen phosphate, lithium acetate, lithium carbonate, and lithium hydroxide.
3. The method according to claim 1, wherein the carbon source in step 2) and the organic carbon source in step 6) are independently selected from one or more of glycine, citric acid, sucrose, starch, glucose and tapioca.
4. A composite positive electrode material produced by the method according to any one of claims 1 to 3.
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