CN113651304A - Organic carbon-coated lithium iron phosphate cathode material and preparation method thereof - Google Patents
Organic carbon-coated lithium iron phosphate cathode material and preparation method thereof Download PDFInfo
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Abstract
The invention relates to an organic carbon-coated lithium iron phosphate positive electrode material and a preparation method thereof, belonging to the technical field of preparation of lithium ion battery electrode materials. And (2) performing ball milling and mixing in a high-speed ball milling tank by using one or two of iron phosphate, lithium carbonate, citric acid, glucose and asphalt as a carbon source and alcohol as a solvent, drying the mixed solution, and sintering at a high temperature to obtain the carbon-coated lithium iron phosphate cathode material. The method utilizes common organic carbon in life as a carbon-coated carbon source, is easy to obtain and low in cost, the carbon coating is favorable for enhancing the conductivity of the material, the surface impedance of the material is reduced, the agglomeration phenomenon of the material can be reduced by coating, the diffusion path of ions is reduced, and the electrochemical performance of the material is improved. The lithium iron phosphate cathode material prepared by the method shows high capacity of 168.4mAh/g under the current density of 0.2C, almost no capacity attenuation after 100 cycles, and has good cycle stability and rate capability.
Description
Technical Field
The invention belongs to the field of preparation of lithium ion battery electrode materials, and particularly relates to an organic carbon source coated lithium iron phosphate anode material and a preparation method thereof.
Background
Lithium ion batteries are widely used because of their advantages of high operating voltage, high specific capacity, safety, stability, good cycle performance, wide operating temperature range, etc. With the popularization of new energy automobiles, high-performance lithium ion batteries are most hopeful to become one of power supplies of electric automobiles. The anode material is the core part of the lithium battery, and the performance of the anode material directly determines the performance index of the lithium ion battery product. The olivine-structured lithium iron phosphate has high theoretical specific capacity (170 mAh/g), stable structure, long cycle life, low price and environmental friendliness, is considered as the most promising positive electrode material for lithium ion power batteries, but has the defects of poor electronic conductivity and low ion diffusion coefficient, and seriously limits the electrochemical capacity of the lithium ion battery, so that the development of a lithium iron phosphate preparation process and performance enhancement research has important significance for improving the performance of the lithium iron phosphate.
At present, the modification method for the lithium iron phosphate anode material mainly comprises carbon coating, heteroatom doping, particle nanocrystallization, preparation process improvement and the like, and the carbon coating is a simple, direct and effective method for improving and enhancing the performance of olivine phosphate. The coating of carbon can form a conductive network between particles, an effective electron and ion transmission channel is provided, the growth of phosphate particles in the reaction process can be inhibited to a certain extent, so that the agglomeration is avoided, the transmission rate of electrons is accelerated, the porous structure of carbon can absorb electrolyte, the contact area of the electrolyte and an active substance is increased, the electronic conductivity of the material is obviously improved, the polarization phenomenon of a battery is weakened, and the circulation stability and the rate capability of the material are improved.
On carbon materialIn the material coating research report, the organic carbon material is considered to be an effective method for effectively improving the electrochemical performance of the lithium ion battery anode material. Zhang et al, university of Henan, takes sucrose as a carbon source, and synthesizes LiFePO with irregular spherical particles of 1-2 μm in size coated with active carbon by a sol-gel method4When the discharge current density is 0.1C, the specific capacity of the anode material reaches 163.5 mAh/g, the specific capacity retention rate is 91.1% after 110 cycles, and the anode material has good cycle stability and rate capability; chen et al of Huazhong science and technology university prepare Polyaniline (PANI) coated anode composite material LiFePO by in-situ polymerization method4(iii)/PANI. PANI has excellent conductivity and reversible redox property, and the use of PANI as a coating material can promote electron conduction and participate in electrode reaction to improve the electrochemical performance of the electrode material. LiFePO4the/PANI shows excellent specific discharge capacity and rate capability, and the initial discharge capacity respectively reaches 165mAh/g and 123mAh/g under 0.2C and 10C; the energetic person of the King of the university of Shenyang chemical industry uses LiH2PO4、Fe2O3And glucose as raw materials, preparing the carbon-coated lithium iron phosphate anode material with excellent performance by adopting a high-temperature high-energy ball milling method, and obtaining LiFePO4the/C material is of a ball-like olivine structure, the average particle size is 0.5 mu m, the first discharge specific capacity is 152.5 mAh/g under the charge-discharge rate of 0.1C, and the discharge specific capacity is basically unchanged after 60 cycles under different charge-discharge rates, so that the excellent electrochemical performance is shown.
Disclosure of Invention
The invention aims to solve the technical problem of providing an organic carbon-coated lithium iron phosphate positive electrode material and a preparation method thereof, and the organic carbon-coated lithium iron phosphate positive electrode material further effectively improves the electronic conductivity of the lithium iron phosphate battery and refines crystal grains by improving the existing lithium iron phosphate battery preparation process, thereby improving the capacity and the cycling stability of the lithium iron phosphate battery.
In order to solve the above technical problems, according to an aspect of the present invention, there is provided a method for preparing an organic carbon-coated lithium iron phosphate positive electrode material, including
The method comprises the following steps: weighing raw materials, placing the raw materials in a ball milling tank, adding alcohol as a solvent, and carrying out ball milling to obtain a mixed solution;
the raw materials are ferric phosphate, lithium carbonate and organic carbon, and the organic carbon is one or two of citric acid, glucose and asphalt; lithium: the molar ratio of iron is 1.02-1.1, and the addition amount of a carbon source is 7% -16% of the total mass of the iron phosphate and the lithium carbonate; the raw materials of the LiFePO are directly selected from iron phosphate and lithium carbonate to directly utilize the same structure, the types of the raw materials and the introduction of impurities are reduced, the excessive doping of lithium is to prevent the phenomenon of lithium deficiency caused by the loss of lithium in the firing process, and the LiFePO prepared by organic carbon4the/C material is in a regular porous spherical structure and has high specific surface area and specific discharge capacity.
Step two: transferring the obtained mixed solution into a forced air drying oven, and drying to obtain gray powder;
step three: putting the gray powder into a ceramic boat, putting the ceramic boat into a high-temperature tube furnace, sintering at high temperature under the protection of inert gas, and naturally cooling to obtain a precursor;
the temperature rise rate of the high-temperature tube furnace is 3-5 ℃/min, the temperature rises to 350 ℃ for 250-. The slow heating rate and the proper heat preservation time can fully complete the reaction, complete the crystal form development of the particles, and improve the crystallinity, thereby improving the electrochemical performance of the material.
Step four: and placing the obtained precursor in a vacuum drying box, and drying to obtain the final material.
Further, the carbon source is a mixture of glucose and citric acid, and the ratio of glucose: the mass ratio of the citric acid is (1.2-1.8): 1 small molecule organic citric acid can be firstly complexed with ferric iron to form C6H5O7Fe.5H2O, evenly distributed in the precursor, and subjected to carbon heat to obtain the precursor with even aperture and specific surface area of 32m2Multi-path hole spherical LiFePO of/g4C, glucose produces mesoporous LiFePO with a much higher specific surface area and smaller crystallite size4C, when two carbon sources are added in a matching way, mesoporous materials with uniform distribution are favorably formed, and the total ratio tableThe area is one order of magnitude larger than the specific surface area when the material is added independently, and the electronic conductivity of the material is greatly improved.
Further, in the first step, the material in the ball milling tank: ball: alcohol = 1: 3: and 6, the ball milling rotating speed is 1200r/min, and the ball milling time is 18 h.
Further, in the second step, the temperature of the air drying oven is 80 ℃, and the drying time is 12 h.
Further, in the fourth step, the temperature of the vacuum drying oven is 120 ℃, and the heat preservation time is 12 hours.
According to another aspect of the invention, an organic carbon-coated lithium iron phosphate cathode material is provided, which is obtained by the preparation method.
According to another aspect of the invention, the lithium ion battery anode is prepared by uniformly grinding the organic carbon-coated lithium iron phosphate anode material, ketjen black and polyvinylidene fluoride in proportion, adding N-methyl pyrrolidone to prepare a homogenate, uniformly coating the homogenate on an aluminum foil by using a coating machine, placing the aluminum foil on a vacuum drying oven for drying, and then preparing the wafer electrode by using a sheet punching machine.
Further, the mass ratio of the organic carbon-coated lithium iron phosphate positive electrode material to the ketjen black and the polyvinylidene fluoride is 8: 1: 1.
according to another aspect of the present invention, there is provided a lithium ion battery comprising the above-described lithium ion battery positive electrode.
The method utilizes common organic carbon in life as a carbon-coated carbon source, is easy to obtain and low in cost, the carbon coating is favorable for enhancing the conductivity of the material, the surface impedance of the material is reduced, the agglomeration phenomenon of the material can be reduced by coating, the diffusion path of ions is reduced, and the electrochemical performance of the material is improved. The lithium iron phosphate cathode material prepared by the method shows high capacity of 168.4mAh/g under the current density of 0.2C, almost no capacity attenuation after 100 cycles, and has good cycle stability and rate capability.
Drawings
Fig. 1 is an XRD pattern of the glucose and citric acid coated lithium iron phosphate positive electrode material prepared in example 4;
fig. 2 is a TG diagram of the glucose and citric acid coated lithium iron phosphate positive electrode material prepared in example 4;
fig. 3 is a CV curve graph of the glucose and citric acid coated lithium iron phosphate positive electrode material prepared in example 4;
fig. 4 is a graph of the cycle performance of the glucose and citric acid coated lithium iron phosphate positive electrode material prepared in example 4;
fig. 5 is an ac impedance plot of the glucose and citric acid coated lithium iron phosphate positive electrode material prepared in example 4.
Detailed Description
The claimed solution is further illustrated by the following examples. However, the examples and comparative examples are intended to illustrate the embodiments of the present invention without departing from the scope of the subject matter of the present invention, and the scope of the present invention is not limited by the examples. Unless otherwise specifically indicated, the materials and reagents used in the present invention are available from commercial products in the art.
Example 1
The embodiment provides a preparation method of organic carbon-coated lithium iron phosphate, which comprises the following steps:
(1) the method comprises the following steps of selecting iron phosphate and lithium carbonate as raw materials (the molar ratio of iron to lithium is 1: 1.06), selecting glucose as an organic carbon material, putting the weighed materials into a planetary ball milling tank, adding alcohol as a solvent, and performing ball milling: ball: alcohol = 1: 3: carrying out ball milling for 18h at the rotating speed of 6,1200 r/min to obtain a mixed solution;
(2) transferring the mixed solution obtained in the step (1) into a forced air drying oven, and drying at 80 ℃ for 12h to obtain gray powder;
(3) taking out a proper amount of the white powder obtained in the step (2), placing the white powder in a ceramic boat, moving the ceramic boat to a high-temperature tube furnace, heating the ceramic boat to 300 ℃ for 3 hours at a heating rate of 5 ℃/min in a nitrogen atmosphere, continuing to heat the ceramic boat to 700 ℃, and keeping the temperature for 8 hours to obtain a precursor;
（4）grinding the carbon-coated lithium iron phosphate precursor obtained in the step (3) in a mortar until no obvious particles exist, and drying in a vacuum drying oven at 120 ℃ for 12h to obtain 1-LiFePO4/C。
Example 2
The embodiment provides a preparation method of organic carbon-coated lithium iron phosphate, which comprises the following steps:
(1) the method comprises the following steps of selecting iron phosphate and lithium carbonate as raw materials (the molar ratio of iron to lithium is 1: 1.06), selecting asphalt as an organic carbon material, wherein the addition amount of the asphalt is 12% of the total mass of the iron phosphate and the lithium carbonate, putting the weighed materials into a planetary ball milling tank, adding alcohol as a solvent, and feeding in the ball milling tank: ball: alcohol = 1: 3: carrying out ball milling for 18h at the rotating speed of 6,1200 r/min to obtain a mixed solution;
(2) transferring the mixed solution obtained in the step (1) into a forced air drying oven, and drying at 80 ℃ for 12h to obtain gray powder;
(3) taking out a proper amount of the white powder obtained in the step (2), placing the white powder in a ceramic boat, moving the ceramic boat to a high-temperature tube furnace, heating the ceramic boat to 300 ℃ for 3 hours at a heating rate of 5 ℃/min in a nitrogen atmosphere, continuing to heat the ceramic boat to 700 ℃, and keeping the temperature for 8 hours to obtain a precursor;
(4) grinding the lithium iron phosphate precursor obtained in the step (3) in a mortar until no obvious particles exist, and drying in a vacuum drying oven at 120 ℃ for 12h to obtain 2-LiFePO4/C。
Example 3
(1) the method comprises the following steps of selecting iron phosphate and lithium carbonate as raw materials (the molar ratio of iron to lithium is 1: 1.06), selecting citric acid as an organic carbon material, putting the weighed materials into a planetary ball milling tank, adding alcohol as a solvent, and performing ball milling: ball: alcohol = 1: 3: carrying out ball milling for 18h at the rotating speed of 6,1200 r/min to obtain a mixed solution;
(2) transferring the mixed solution obtained in the step (1) into a forced air drying oven, and drying at 80 ℃ for 12h to obtain gray powder;
(3) taking out a proper amount of the white powder obtained in the step (2), placing the white powder in a ceramic boat, moving the ceramic boat to a high-temperature tube furnace, heating the ceramic boat to 300 ℃ for 3 hours at a heating rate of 5 ℃/min in a nitrogen atmosphere, continuing to heat the ceramic boat to 700 ℃, and keeping the temperature for 8 hours to obtain a precursor;
(4) grinding the lithium iron phosphate precursor obtained in the step (3) in a mortar until no obvious particles exist, and drying in a vacuum drying oven at 120 ℃ for 12h to obtain 3-LiFePO4/C。
Example 4
The embodiment provides a preparation method of mixed organic carbon-coated lithium iron phosphate, which comprises the following steps:
(1) the raw materials are ferric phosphate and lithium carbonate (the molar ratio of iron to lithium is 1: 1.06), the organic carbon material is glucose and citric acid, and the molar ratio of glucose: the mass ratio of the citric acid is 1.6: 1, adding 12% of carbon by mass based on the total mass of iron phosphate and lithium carbonate, putting the weighed materials into a planetary ball milling tank, adding alcohol as a solvent, and feeding in the ball milling tank: ball: alcohol = 1: 3: carrying out ball milling for 18h at the rotating speed of 6,1200 r/min to obtain a mixed solution;
(2) transferring the mixed solution obtained in the step (1) into a forced air drying oven, and drying at 80 ℃ for 12h to obtain gray powder;
(3) taking out a proper amount of the white powder obtained in the step (2), placing the white powder in a ceramic boat, moving the ceramic boat to a high-temperature tube furnace, heating the ceramic boat to 300 ℃ for 3 hours at a heating rate of 5 ℃/min in a nitrogen atmosphere, continuing to heat the ceramic boat to 700 ℃, and keeping the temperature for 8 hours to obtain a precursor;
(4) grinding the carbon-coated lithium iron phosphate precursor obtained in the step (3) in a mortar until no obvious particles exist, and drying in a vacuum drying oven at 120 ℃ for 12h to obtain 4-LiFePO4/C。
Example 5
The embodiment provides a preparation method of mixed organic carbon-coated lithium iron phosphate, which comprises the following steps:
(1) the selected raw materials are ferric phosphate and lithium carbonate (the molar ratio of iron to lithium is 1: 1.06), the organic carbon material is glucose and asphalt, and the molar ratio of asphalt: the mass ratio of glucose is 1: 1.6, adding carbon in an amount which is 12 percent of the total mass of the iron phosphate and the lithium carbonate, putting the weighed materials into a planetary ball milling tank, adding alcohol as a solvent, and feeding materials in the ball milling tank: ball: alcohol = 1: 3: carrying out ball milling for 18h at the rotating speed of 6,1200 r/min to obtain a mixed solution;
(2) transferring the mixed solution obtained in the step (1) into a forced air drying oven, and drying at 80 ℃ for 12h to obtain gray powder;
(3) taking out a proper amount of the white powder obtained in the step (2), placing the white powder in a ceramic boat, moving the ceramic boat to a high-temperature tube furnace, heating the ceramic boat to 300 ℃ for 3 hours at a heating rate of 5 ℃/min in a nitrogen atmosphere, continuing to heat the ceramic boat to 700 ℃, and keeping the temperature for 8 hours to obtain a precursor;
(4) grinding the carbon-coated lithium iron phosphate precursor obtained in the step (3) in a mortar until no obvious particles exist, and drying in a vacuum drying oven at 120 ℃ for 12h to obtain 5-LiFePO4/C。
Example 6
The embodiment provides a preparation method of mixed organic carbon-coated lithium iron phosphate, which comprises the following steps:
(1) the raw materials are ferric phosphate and lithium carbonate (the molar ratio of iron to lithium is 1: 1.06), the organic carbon material is citric acid and asphalt, and the weight ratio of citric acid: the mass ratio of the asphalt is 1: 1.6, adding carbon in an amount which is 12 percent of the total mass of the iron phosphate and the lithium carbonate, putting the weighed materials into a planetary ball milling tank, adding alcohol as a solvent, and feeding materials in the ball milling tank: ball: alcohol = 1: 3: carrying out ball milling for 18h at the rotating speed of 6,1200 r/min to obtain a mixed solution;
(2) transferring the mixed solution obtained in the step (1) into a forced air drying oven, and drying at 80 ℃ for 12h to obtain gray powder;
(3) taking out a proper amount of the white powder obtained in the step (2), placing the white powder in a ceramic boat, moving the ceramic boat to a high-temperature tube furnace, heating the ceramic boat to 300 ℃ for 3 hours at a heating rate of 5 ℃/min in a nitrogen atmosphere, continuing to heat the ceramic boat to 700 ℃, and keeping the temperature for 8 hours to obtain a precursor;
(4) grinding the carbon-coated lithium iron phosphate precursor obtained in the step (3) in a mortar until no obvious particles exist, and drying in a vacuum drying oven at 120 ℃ for 12h to obtain 6-LiFePO4/C。
Example 7
The embodiment provides a preparation method of mixed organic carbon-coated lithium iron phosphate, which comprises the following steps:
(1) the method comprises the following steps of selecting iron phosphate as a raw material, selecting lithium carbonate (the molar ratio of iron to lithium is 1: 1.02, selecting glucose and citric acid as an organic carbon material, the mass ratio of glucose to citric acid is 1.6: 1, the adding amount of carbon is 8% of the total mass of the iron phosphate and the lithium carbonate, putting the weighed materials into a planetary ball milling tank, adding alcohol as a solvent, and carrying out ball milling for 18 hours at the rotating speed of 1200r/min to obtain a mixed solution;
(2) transferring the mixed solution obtained in the step (1) into a forced air drying oven, and drying at 80 ℃ for 12h to obtain gray powder;
(3) taking out a proper amount of the white powder obtained in the step (2), placing the white powder in a ceramic boat, moving the ceramic boat to a high-temperature tube furnace, heating the ceramic boat to 300 ℃ for 3 hours at a heating rate of 5 ℃/min in a nitrogen atmosphere, continuing to heat the ceramic boat to 700 ℃, and keeping the temperature for 8 hours to obtain a precursor;
(4) grinding the carbon-coated lithium iron phosphate precursor obtained in the step (3) in a mortar until no obvious particles exist, and drying in a vacuum drying oven at 120 ℃ for 12h to obtain 7-LiFePO4/C。
Example 8
The embodiment provides a preparation method of mixed organic carbon-coated lithium iron phosphate, which comprises the following steps:
(1) the raw materials are ferric phosphate and lithium carbonate (the molar ratio of iron to lithium is 1: 1.02), the organic carbon material is glucose and citric acid, and the molar ratio of glucose: the mass ratio of the citric acid is 1.8: 1, adding the carbon in an amount which is 7% of the total mass of the iron phosphate and the lithium carbonate, putting the weighed materials into a planetary ball milling tank, adding alcohol as a solvent, and feeding in the ball milling tank: ball: alcohol = 1: 3: carrying out ball milling for 18h at the rotating speed of 6,1200 r/min to obtain a mixed solution;
(2) transferring the mixed solution obtained in the step (1) into a forced air drying oven, and drying at 80 ℃ for 12h to obtain gray powder;
(3) taking out a proper amount of the white powder obtained in the step (2), placing the white powder in a ceramic boat, moving the ceramic boat to a high-temperature tubular furnace, heating the ceramic boat to 250 ℃ for the first time at a heating rate of 5 ℃/min under a nitrogen atmosphere, preserving heat for 2 hours, continuing heating the ceramic boat to 600 ℃, and preserving heat for 9 hours to obtain a precursor;
(4) grinding the carbon-coated lithium iron phosphate precursor obtained in the step (3) in a mortar until no obvious particles exist, and drying in a vacuum drying oven at 120 ℃ for 12h to obtain 8-LiFePO4/C。
Example 9
The embodiment provides a preparation method of mixed organic carbon-coated lithium iron phosphate, which comprises the following steps:
(1) the raw materials are ferric phosphate and lithium carbonate (the molar ratio of iron to lithium is 1: 1.10), the organic carbon material is glucose and citric acid, and the molar ratio of glucose: the mass ratio of the citric acid is 1.2: 1, adding 16% of carbon by mass based on the total mass of iron phosphate and lithium carbonate, putting the weighed materials into a planetary ball milling tank, adding alcohol as a solvent, and feeding in the ball milling tank: ball: alcohol = 1: 3: carrying out ball milling for 18h at the rotating speed of 6,1200 r/min to obtain a mixed solution;
(2) transferring the mixed solution obtained in the step (1) into a forced air drying oven, and drying at 80 ℃ for 12h to obtain gray powder;
(3) taking out a proper amount of the white powder obtained in the step (2), placing the white powder in a ceramic boat, moving the ceramic boat to a high-temperature tube furnace, heating the ceramic boat to 350 ℃ for the first time at a heating rate of 3 ℃/min under a nitrogen atmosphere, preserving heat for 4 hours, continuing heating the ceramic boat to 800 ℃, and preserving heat for 6 hours to obtain a precursor;
(4) grinding the carbon-coated lithium iron phosphate precursor obtained in the step (3) in a mortar until no obvious particles exist, and drying in a vacuum drying oven at 120 ℃ for 12h to obtain 9-LiFePO4/C。
Comparative example 1
The comparative example 1 provides a preparation method of pure-phase lithium iron phosphate, which comprises the following steps:
(1) the method comprises the following steps of selecting iron phosphate and lithium carbonate as raw materials (the molar ratio of iron to lithium is 1: 1.06), putting the weighed materials into a planetary ball milling tank, adding alcohol as a solvent, and performing ball milling in the tank: ball: alcohol = 1: 3: carrying out ball milling for 18h at the rotating speed of 6,1200 r/min to obtain a mixed solution;
(2) transferring the mixed solution obtained in the step (1) into a forced air drying oven, and drying at 80 ℃ for 12h to obtain gray powder;
(3) taking out a proper amount of the white powder obtained in the step (2), placing the white powder in a ceramic boat, moving the ceramic boat to a high-temperature tube furnace, heating the ceramic boat to 300 ℃ for 3 hours at a heating rate of 5 ℃/min in a nitrogen atmosphere, continuing to heat the ceramic boat to 700 ℃, and keeping the temperature for 8 hours to obtain a precursor;
(4) and (4) grinding the lithium iron phosphate precursor obtained in the step (3) in a mortar until no obvious particles exist, and drying in a vacuum drying oven at 120 ℃ for 12h to obtain a pure phase of lithium iron phosphate.
Comparative example 2
The comparative example provides a preparation method of a carbon-coated lithium iron phosphate pure phase sintered at a lower temperature, which comprises the following steps:
(1) the method comprises the following steps of selecting iron phosphate and lithium carbonate as raw materials (the molar ratio of iron to lithium is 1: 1.06), selecting citric acid and glucose as carbon materials, putting the weighed materials into a planetary ball milling tank, adding alcohol as a solvent, and performing ball milling: ball: alcohol = 1: 3: carrying out ball milling for 18h at the rotating speed of 6,1200 r/min to obtain a mixed solution;
(2) transferring the mixed solution obtained in the step (1) into a forced air drying oven, and drying at 80 ℃ for 12h to obtain gray powder;
(3) taking out a proper amount of the white powder obtained in the step (2), placing the white powder in a ceramic boat, moving the ceramic boat to a high-temperature tube furnace, heating the ceramic boat to 300 ℃ for 3 hours at a heating rate of 5 ℃/min in a nitrogen atmosphere, continuously heating the ceramic boat to 650 ℃, and keeping the temperature for 8 hours to obtain a precursor;
(4) grinding the lithium iron phosphate precursor obtained in the step (3) in a mortar until no obvious particles exist, and drying in a vacuum drying oven at 120 ℃ for 12h to obtain 7-LiFePO4/C。
Comparative example 3
The comparative example provides a preparation method of a carbon-coated lithium iron phosphate pure phase sintered at a higher temperature, which comprises the following steps:
(1) the method comprises the following steps of selecting iron phosphate and lithium carbonate as raw materials (the molar ratio of iron to lithium is 1: 1.06), selecting citric acid and glucose as carbon materials, putting the weighed materials into a planetary ball milling tank, adding alcohol as a solvent, and performing ball milling: ball: alcohol = 1: 3: carrying out ball milling for 18h at the rotating speed of 6,1200 r/min to obtain a mixed solution;
(2) transferring the mixed solution obtained in the step (1) into a forced air drying oven, and drying at 80 ℃ for 12h to obtain gray powder;
(3) taking out a proper amount of the white powder obtained in the step (2), placing the white powder in a ceramic boat, moving the ceramic boat to a high-temperature tube furnace, heating the ceramic boat to 300 ℃ for 3 hours at a heating rate of 5 ℃/min in a nitrogen atmosphere, continuously heating the ceramic boat to 750 ℃ and keeping the temperature for 8 hours to obtain a precursor;
(4) grinding the lithium iron phosphate precursor obtained in the step (3) in a mortar until no obvious particles exist, and drying in a vacuum drying oven at 120 ℃ for 12h to obtain 8-LiFePO4/C。
Mixing the lithium iron phosphate pure phase and carbon-coated lithium iron phosphate obtained in each embodiment with Ketjen black and polyvinylidene fluoride (PVDF) according to the mass ratio of 8: 1: 1, adding a proper amount of N-methyl pyrrolidone (NMP) to prepare homogenate, uniformly coating the homogenate on an aluminum foil by using a coating machine, placing the aluminum foil in a vacuum drying oven to carry out vacuum drying for 12 hours at 120 ℃, and then preparing the aluminum foil into a wafer electrode with the diameter of 12mm by using a sheet punching machine.
The electrode sheet obtained above was used as a positive electrode, a polypropylene film wafer with a diameter of 19mm was used as a separator, a lithium metal sheet with a diameter of 12mm and a thickness of 0.2mm was used as a counter electrode and a reference electrode, and an electrolyte was a 1mol/L lithium hexafluorophosphate/ethylene carbonate/diethyl carbonate solution, a lithium ion battery was assembled in a glove box filled with high purity argon gas in accordance with the structure of a CR2016 standard button cell, and the battery was subjected to a charge and discharge test with a current density of 1C (1C =170 mA/g) on a blue cell test platform.
TABLE 1 main parameters and electrochemical Properties of examples 1-6, comparative examples 1-3
The XRD pattern shown in fig. 1 can show that the peaks of the carbon-coated lithium iron phosphate correspond to the peaks of the standard card, and there are no other obvious peaks, which indicates that pure lithium iron phosphate is generated in the examples, the structure of lithium iron phosphate is not changed by carbon coating, and the sharp peak of the product proves that the material has higher crystallinity.
The thermogravimetric graph shown in fig. 2 shows that the precursor has an obvious weight loss phenomenon at 200 ℃, which corresponds to the removal of crystal water, and the curve has no obvious change after 600 ℃, which indicates that a stable structure has been formed, and also proves that the sintering temperature of 700 ℃ is favorable for the formation of the lithium iron phosphate material.
The CV curve diagram shown in fig. 3 shows that the carbon-coated lithium iron phosphate material has symmetrical and sharp redox peaks, and three-cycle charge-discharge curves almost coincide, which indicates that the material has high electrochemical reversibility, the material does not show obvious polarization, the lithium ion diffusion rate is high, and the impedance is small, which indicates that the conductivity of the lithium iron phosphate material is greatly improved by the compact conductive layer formed by carbonization, and the reaction kinetics of releasing and lithium intercalation are enhanced.
The cycle performance diagram shown in fig. 4 shows that the material of example 4 shows higher specific capacity and cycle stability in a voltage range of 2.4-4.2V at a current density of 0.2C, the specific capacity of the first turn is 168.4mA/g, 167mA/g is still retained after 100 turns, the capacity retention rate is close to 100%, and compared with the capacity of 80.9mA/g of the first turn of comparative example 1, the carbon coating obviously improves the specific capacity and cycle stability of the lithium iron phosphate material.
The cross-axis intercept display material with a small high-frequency region in the impedance spectrogram shown in FIG. 5 has small charge transfer impedance and impedance transferred between the electrode and the electrolyte surface, and the larger straight-line tangent slope of the high-frequency region indicates the larger ion diffusion capacity of the material.
Claims (9)
1. The preparation method of the organic carbon-coated lithium iron phosphate positive electrode material is characterized by comprising the following steps
The method comprises the following steps: weighing raw materials, placing the raw materials in a ball milling tank, adding alcohol as a solvent, and carrying out ball milling to obtain a mixed solution;
the raw materials are ferric phosphate, lithium carbonate and organic carbon, the organic carbon is one or two of citric acid, glucose and asphalt, and the ratio of lithium: the molar ratio of iron is 1.02-1.1, and the addition amount of a carbon source is 7% -16% of the total mass of the iron phosphate and the lithium carbonate;
step two: transferring the obtained mixed solution into a forced air drying oven, and drying to obtain gray powder;
step three: putting the gray powder into a ceramic boat, putting the ceramic boat into a high-temperature tube furnace, sintering at high temperature under the protection of inert gas, and naturally cooling to obtain a precursor;
the heating rate of the high-temperature tube furnace is 3-5 ℃/min, the temperature is raised to 350 ℃ for 250-;
step four: and placing the obtained precursor in a vacuum drying box, and drying to obtain the final material.
2. The method of claim 1, wherein: the carbon source is a mixture of glucose and citric acid, and the carbon source is glucose: the mass ratio of the citric acid is (1.2-1.8): 1.
3. the method according to claim 1 or 2, characterized in that: in the first step, material in a ball milling tank: ball: alcohol = 1: 3: and 6, the ball milling rotating speed is 1200r/min, and the ball milling time is 18 h.
4. The method of claim 3, wherein: in the second step, the temperature of the air-blast drying oven is 80 ℃, and the drying time is 12 h.
5. The method of claim 4, wherein: in the fourth step, the temperature of the vacuum drying oven is 120 ℃, and the heat preservation time is 12 hours.
6. An organic carbon-coated lithium iron phosphate positive electrode material, which is characterized by being obtained by the preparation method of any one of claims 1 to 5.
7. A lithium ion battery positive electrode, characterized in that: the organic carbon-coated lithium iron phosphate positive electrode material of claim 6, ketjen black and polyvinylidene fluoride are uniformly ground in proportion, then N-methyl pyrrolidone is added to be blended into homogenate, the homogenate is uniformly coated on an aluminum foil by a coating machine, the aluminum foil is placed in a vacuum drying oven to be dried, and then a piece punching machine is used for preparing the wafer electrode.
8. The lithium ion battery positive electrode of claim 7, wherein: the mass ratio of the organic carbon-coated lithium iron phosphate positive electrode material to the Ketjen black and the polyvinylidene fluoride is 8: 1: 1.
9. a lithium ion battery, characterized by: comprising the lithium ion battery positive electrode of claim 7 or 8.
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