CN111342018A - Carbon-coated lithium-containing transition metal phosphate positive electrode material and preparation method thereof - Google Patents

Carbon-coated lithium-containing transition metal phosphate positive electrode material and preparation method thereof Download PDF

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CN111342018A
CN111342018A CN202010160239.0A CN202010160239A CN111342018A CN 111342018 A CN111342018 A CN 111342018A CN 202010160239 A CN202010160239 A CN 202010160239A CN 111342018 A CN111342018 A CN 111342018A
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carbon
positive electrode
selenium
transition metal
lithium
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CN111342018B (en
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花春秀
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Sichuan Lianwu New Energy Technology Co ltd
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
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    • H01M4/02—Electrodes composed of, or comprising, active material
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    • H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
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    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
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    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/58—Selection of substances as active materials, active masses, active liquids of inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy; of polyanionic structures, e.g. phosphates, silicates or borates
    • H01M4/5825—Oxygenated metallic salts or polyanionic structures, e.g. borates, phosphates, silicates, olivines
    • H—ELECTRICITY
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    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M2004/026—Electrodes composed of, or comprising, active material characterised by the polarity
    • H01M2004/028—Positive electrodes
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Abstract

The invention belongs to the technical field of lithium ion batteries, and particularly relates to a carbon-coated lithium-containing transition metal phosphate positive electrode material which comprises a lithium-containing transition metal phosphate positive electrode material and a carbon coating layer coated on the surface of the lithium-containing transition metal phosphate positive electrode material, wherein the carbon coating layer is doped with selenium element, and the structural formula of the lithium-containing transition metal phosphate positive electrode material is LiMPO4And M is at least one of Fe, Mn, Co and Ni. The carbon coating layer can better play a role in conducting electricity, so that the rate capability of the material is improved; strong acting force between the coating layer and the main body material ensures that the material can still keep good contact between the carbon material and the main body material under long circulation, and the cyclicity is obtainedLifting; the bond between the cladding and the main material is tight, and the compaction density is higher with the same carbon content.

Description

Carbon-coated lithium-containing transition metal phosphate positive electrode material and preparation method thereof
Technical Field
The invention belongs to the field of lithium ion batteries, and particularly relates to a carbon-coated lithium-containing transition metal phosphate positive electrode material and a preparation method thereof.
Background
The lithium ion battery has the advantages of high energy density, long cycle life, no memory effect, low self-discharge rate and the like, and can store and supply energyThe aspect gradually takes the leading position. Lithium ion batteries are composed mainly of positive electrodes, negative electrodes, separators, electrolytes and other accessories, wherein the positive electrode active material plays a crucial role in the overall performance of the battery. Currently, lithium cobaltate (LiCoO) is the main material of the positive electrode of the commercial lithium ion battery2) Ternary material, spinel LiMn2O4And olivine-structured lithium iron phosphate (LiFePO)4) And the like.
LiFePO4The lithium ion battery anode material is a new generation lithium ion battery anode material proposed by professor Goodenough in 1997, and has the advantages of proper discharge voltage (3.5V), high theoretical specific capacity (170mAh/g), good thermal stability and electrochemical stability, compatibility with most electrolytes, rich raw material sources, environmental friendliness and the like. However, as a positive electrode material for lithium ion batteries, LiFePO4Has very significant disadvantages of electron conductivity and Li at room temperature+The mobilities therein were all low, 10 respectively-9S/cm and 10-14～10-11cm2S, this is still a constraint on LiFePO4The biggest application problem.
In the prior art, the intrinsic electronic conductivity and ionic conductivity of the material are improved to a certain extent by adopting a carbon-coated technical means, and carbon coating is generally carried out on LiFePO4During the generation process, a layer of thin carbon material is coated on the surface in situ, and LiFePO is also prepared firstly4The material is then carbon coated. No matter what method is adopted to coat carbon, LiFePO is used4And carbon are two very different species, the crystal lattices are severely mismatched, and the interface connection is not tight enough. Although LiFePO4The material has small volume change in the charge and discharge process, and has no obvious problem in short-term circulation, but the problems of falling off of the coating material, poor conductivity and material performance decline easily occur in the long-term circulation process.
Lithium-containing transition metal phosphates having the same structure as lithium iron phosphate also have problems encountered with lithium iron phosphate.
Disclosure of Invention
One of the objects of the present invention is: aiming at the defects of the prior art, the carbon-coated lithium-containing transition metal phosphate anode material is provided, and the problems of falling off of a coating material, poor conductivity, material performance degradation and the like in the circulating process can be solved.
In order to achieve the purpose, the invention adopts the following technical scheme:
a carbon-coated lithium-containing transition metal phosphate positive electrode material comprises a lithium-containing transition metal phosphate positive electrode material and a carbon coating layer coated on the surface of the lithium-containing transition metal phosphate positive electrode material, wherein selenium is doped in the carbon coating layer, and the structural formula of the lithium-containing transition metal phosphate positive electrode material is LiMPO4And M is at least one of Fe, Mn, Co and Ni. It is to be noted that lithium iron phosphate, lithium cobalt phosphate, lithium manganese phosphate, and lithium nickel phosphate are all olivine structures, and Fe, Co, Mn, and Ni may form a mutual solid solution. Selenium and oxygen are elements of the same group and can replace the oxygen position, but since the atomic radius of selenium is much larger than that of oxygen, the selenium is mainly present in LiMPO4And forms a chemical bond with M and P, and selenium and carbon can also form a relatively strong chemical bond, which is the carbon coating and LiMPO4A firmer connection is established between the two.
As an improvement of the carbon-coated lithium-containing transition metal phosphate cathode material, the source of the selenium element comprises at least one of selenium oxide, elemental selenium and selenium-containing organic matters. Preferably, selenium-containing organic materials are used as the selenium source, so that the selenium is generated in situ, is uniformly distributed and has good combination with carbon. The selenium-containing organic substance preferably contains only C, N, H, O, Se, and more preferably contains only C, H, O, Se.
As an improvement of the carbon-coated lithium-containing transition metal phosphate positive electrode material, the selenium-containing organic matter comprises at least one of selenol, dimethyl selenium, benzoselenol, polyselenol, 2, 5-dicarboxyl selenol, selenium ether and tetramethyl tetraseleno fulvalene.
As an improvement of the carbon-coated lithium-containing transition metal phosphate positive electrode material, the mass of the selenium element accounts for 0.01-40% of the total mass of the carbon coating layer, and the mass of the carbon coating layer accounts for 0.2-8% of the total mass of the positive electrode material. The carbon coating layer occupies too large mass of the anode material, so that the formed carbon coating layer is too thick, the lithium ion is not easy to be de-embedded, and the mass specific capacity of the material is reduced; the carbon coating layer occupies too small mass of the cathode material, and the formed carbon coating layer is not continuous and complete enough. The selenium content is too small to show the coating effect; the selenium element accounts for too much, on one hand, the selenium element can replace too many oxygen atoms to influence the overall structural stability of the anode material, and on the other hand, the material cost can be greatly increased.
As an improvement of the carbon-coated lithium-containing transition metal phosphate positive electrode material, the mass of the selenium element accounts for 0.7-10% of the total mass of the carbon coating layer, and the mass of the carbon coating layer accounts for 0.5-5% of the total mass of the positive electrode material.
As an improvement of the carbon-coated lithium-containing transition metal phosphate cathode material of the present invention, the content of the selenium element gradually decreases from the position close to the lithium-containing transition metal phosphate cathode material to the position far from the lithium-containing transition metal phosphate cathode material. The carbon coating layer coated on the surface of the lithium-containing transition metal phosphate cathode material can be continuous or discontinuous, and the distribution of the selenium element in the carbon coating layer can be uniform or non-uniform. Preferably, the carbon coating layer coated on the surface of the lithium-containing transition metal phosphate cathode material is continuous, and selenium in the carbon coating layer is uniformly distributed along the surface of the cathode material particles. More preferably, the content of the selenium element gradually decreases from the position close to the lithium-containing transition metal phosphate cathode material to the position far away from the lithium-containing transition metal phosphate cathode material, and a gradient distribution of selenium is formed. Since selenium is mostly present in LiMPO4The surface layer of the lithium-containing transition metal phosphate anode material forms a chemical bond with M and P, a relatively strong chemical bond can be formed between selenium and carbon, and the higher the selenium content close to the lithium-containing transition metal phosphate anode material is, the firmer the selenium carbon bond is, so that the firmer the connection between the carbon coating layer and the lithium-containing transition metal phosphate anode material is.
As an improvement of the carbon-coated lithium-containing transition metal phosphate positive electrode material, the average particle size of the lithium-containing transition metal phosphate positive electrode material primary particles is 20-300 nm, the particle size of the lithium-containing transition metal phosphate positive electrode material secondary particles D50 is 1-10 mu m, and D100 is less than 30 mu m. Preferably, the average particle size of the lithium-containing transition metal phosphate positive electrode material primary particles is 50-200 nm, the particle size of the lithium-containing transition metal phosphate positive electrode material secondary particles D50 is 1-7 μm, and D100 is less than 20 um. The lithium-containing transition metal phosphate anode material has too large particles, which can cause too long diffusion path of lithium ions and influence the dynamic performance of the material; and if the particles of the lithium-containing transition metal phosphate cathode material are too small, the compaction density of the material is too low, and the volume energy density of the material is influenced.
In the lithium-containing transition metal phosphate positive electrode material, the M-site doping element is at least one of Cr, V, Nb, Mo, Zr, W, Y, Sc, Ru, Rh, Pd, Cu and Zn, the Li-site doping element is at least one of Na, K, Mg, Ni and Sc, and the P-site doping element is at least one of S, Se, Si, Mo and Ge. Preferably, the M-site doping element is at least one of Cr, V, Nb, Mo, Zr, and W, the Li-site doping element is at least one of Na, K, Mg, Ni, and Sc, and the P-site doping element is at least one of S, Se and Si. Further preferably, the M-site doping element is at least one of Nb, Mo, Zr, and W, the Li-site doping element is at least one of Na, K, and Mg, and the P-site doping element is at least one of S, Se and Si. The preferred doping element needs to form a stable chemical bond with selenium element, so that a firmer connection between the carbon coating layer and the lithium-containing transition metal phosphate anode material is obtained.
Another object of the present invention is to provide a method for preparing a carbon-coated lithium-containing transition metal phosphate positive electrode material described in any one of the above descriptions, comprising the steps of: s1, selecting a lithium source, an M source, a phosphorus source, a carbon source and a selenium source, mixing and sintering at high temperature; s2, adding a carbon source and a selenium source, mixing and then carbonizing at high temperature; and S3, repeating S2 for several times to obtain the product. By the multi-step coating method, the content of the selenium element can be gradually reduced from the position close to the lithium-containing transition metal phosphate cathode material to the position far away from the lithium-containing transition metal phosphate cathode material. Wherein the carbon source mainly comprises an organic carbon source, and preferably cheap glucose, sucrose and the like. In steps S2 and S3, the carbonization temperature is 400-800 ℃, and the carbonization time is 1-8 h. The carbonization temperature is too high or too low, or the carbonization time is too long or too short, which is not favorable for the performance of the selenium-doped carbon-coated lithium transition metal phosphate anode material, the material particles are easy to obviously grow up and the dynamic performance of the material is influenced due to the too high temperature or too long time, and the carbonization is incomplete due to the too low temperature or too short time, which influences the first coulomb efficiency of the material on one hand and influences the selenium-carbon coating effect on the other hand. The preferable carbonization temperature is 500-650 ℃, and the preferable carbonization time is 1-3 h.
It should be noted that the carbon-coated lithium-containing transition metal phosphate positive electrode material can be obtained by performing only step S1, which is called a one-step synthesis method.
Another object of the present invention is to provide a method for preparing a carbon-coated lithium-containing transition metal phosphate positive electrode material described in any one of the above descriptions, comprising the steps of: s1, preparing a lithium-containing transition metal phosphate positive electrode material; s2, adding a carbon source and a selenium source, mixing and then carbonizing at high temperature; and S3, repeating S2 for several times to obtain the product. The lithium-containing transition metal phosphate cathode material can be synthesized by one of high-temperature solid-phase sintering, sol-gel method or hydrothermal method. Synthesis of LiMPO4The raw material (A) is preferably MPO4And Li2CO3Or LiOH, the sintering temperature is preferably 500-800 ℃, and the sintering time is 8-24 h. By the multi-step coating method, the content of the selenium element can be gradually reduced from the position close to the lithium-containing transition metal phosphate cathode material to the position far away from the lithium-containing transition metal phosphate cathode material. In steps S2 and S3, the carbonization temperature is 400-800 ℃, and the carbonization time is 1-8 h. The carbonization temperature is too high or too low, or the carbonization time is too long or too short, which is not favorable for the performance of the carbon-coated lithium-containing transition metal lithium phosphate anode material doped with selenium, and the too high temperature or the too long time easily causes the material particles to grow significantly, thereby affecting the dynamics of the materialHowever, if the temperature is too low or the time is too short, incomplete carbonization will result, which may affect the first coulombic efficiency of the material, and may also affect the selenium-carbon coating effect. The preferable carbonization temperature is 500-650 ℃, and the preferable carbonization time is 1-3 h.
It should be noted that the carbon-coated lithium-containing transition metal phosphate positive electrode material can be obtained by performing only step S1 and step S2, which is referred to as a two-step coating method.
The beneficial effects of the invention include but are not limited to: since selenium and oxygen are elements of the same group and can replace the oxygen position, but the atomic radius of selenium is much larger than that of oxygen, which is mainly present in LiMPO4And forms a chemical bond with M and P, and selenium and carbon can also form a relatively strong chemical bond, which is the carbon coating and LiMPO4Establish comparatively firm connection between them, and then bring following beneficial effect: the carbon coating layer can better play a role in conducting electricity, so that the rate capability of the material is improved; the strong acting force between the carbon coating layer and the main body material ensures that the material can still keep the carbon material in good contact with the main body material under long circulation, and the circulation is improved; the carbon coating layer and the main body material are tightly combined, and the compacted density is higher when the carbon content is the same; selenium in LiMPO4The surface layer can also reduce the surface potential energy of the material, is more favorable for the entering and exiting of lithium ions, and plays a role in improving the rate capability of the material.
Detailed Description
As used in this specification and the appended claims, certain terms are used to refer to particular components, and it will be appreciated by those skilled in the art that a manufacturer of hardware may refer to a component by different names. This specification and claims do not intend to distinguish between components that differ in name but not function. In the following description and in the claims, the terms "include" and "comprise" are used in an open-ended fashion, and thus should be interpreted to mean "include, but not limited to. "substantially" means within an acceptable error range, within which a person skilled in the art can solve the technical problem to substantially achieve the technical result.
In the present invention, unless otherwise expressly specified or limited, the terms "mounted," "connected," "secured," and the like are to be construed broadly and can, for example, be fixedly connected, detachably connected, or integrally connected; can be mechanically or electrically connected; they may be connected directly or indirectly through intervening media, or they may be interconnected between two elements. The specific meanings of the above terms in the present invention can be understood by those skilled in the art according to specific situations.
The carbon content or selenium content of selenium mentioned in the invention refers to the mass percentage content.
Example 1
The embodiment provides a carbon-coated lithium iron phosphate cathode material, and the preparation method comprises the following steps of preparing the carbon-coated lithium iron phosphate cathode material by adopting a one-step synthesis method: FePO is reacted with4、Li2CO3According to the mol ratio of 1:1, preparing materials, simultaneously adding glucose and elemental selenium as a carbon source and a selenium source according to the selenium-carbon content of 0.5 percent and the selenium content of 10 percent in the selenium-carbon coating, mixing the materials in a sand mill for 6 hours, taking out the materials, sintering the materials in high-purity nitrogen at 700 ℃ for 12 hours, cooling the materials to room temperature, and taking out the materials to obtain the carbon-coated lithium iron phosphate cathode material.
Example 2
The difference from the embodiment 1 is that the selenium source is selenium ether instead of elemental selenium, and the rest is completely the same as the embodiment 1.
Example 3
The difference from the example 2 is that the content of selenium and carbon is 1.5%, the content of selenium in the selenium and carbon coating layer is 5%, and the rest is the same as the example 2.
Example 4
The embodiment provides a carbon-coated lithium iron phosphate cathode material, and the preparation method comprises the steps of firstly preparing LiFePO by adopting a sol-gel method4And then carrying out selenium-carbon coating on the prepared lithium iron phosphate anode material.
LiFePO4The preparation steps of the material are as follows: FeC is added2O4·H2O、LiOH·H2O and NH4H2PO4Dispersing in deionized water according to the molar ratio of 1:1:1, mixing for 4h in a sand mill, taking out, sintering at 680 ℃ in high-purity nitrogen for 15h, cooling to room temperature, taking out to obtain LiFePO4。
The selenium carbon coating method comprises the following steps: the obtained LiFePO4The raw materials, the selenol and the sucrose are mixed according to the selenium carbon content of 1.5 percent and the selenium content of 5 percent in the selenium carbon coating layer, the accurately weighed raw materials are fully and uniformly mixed, carbonized for 1.5 hours at 550 ℃ in high-purity argon, and naturally cooled to obtain the carbon-coated lithium iron phosphate cathode material.
Example 5
The embodiment provides a carbon-coated lithium iron phosphate cathode material, and the preparation method comprises the step of preparing LiFePO by a hydrothermal method4And then carrying out selenium-carbon coating on the prepared lithium iron phosphate anode material.
LiFePO4The preparation steps of the material are as follows: FeSO (ferric oxide) is added4、LiOH·H2O and H3PO4Dissolving the mixture in deionized water according to the molar ratio of 1:3:1, adding a small amount of ascorbic acid as a reducing agent, sealing the mixture in a hydrothermal kettle for reaction at 180 ℃ for 5 hours, cooling the mixture to room temperature, opening the kettle, filtering, washing and drying the mixture to obtain LiFePO4。
The selenium carbon coating method comprises the following steps: the obtained LiFePO4And mixing the raw materials with selenium oxide and glucose according to the selenium-carbon content of 5% and the selenium content of 10% in the selenium-carbon coating layer, fully and uniformly mixing the accurately weighed raw materials, carbonizing the mixture for 2 hours at the temperature of 600 ℃ in high-purity argon, and naturally cooling to obtain the carbon-coated lithium iron phosphate cathode material.
Example 6
The difference from the embodiment 5 is that dimethyl selenium replaces selenium oxide to be used as a selenium source, and the rest is completely the same as the embodiment 5.
Example 7
The embodiment provides a carbon-coated lithium iron phosphate cathode material, and the preparation method comprises the steps of preparing LiFePO by adopting a high-temperature solid-phase sintering method4And then carrying out selenium-carbon coating on the prepared lithium iron phosphate anode material.
LiFePO4Of materialsThe preparation steps are as follows: nano FePO4With Li2CO3Mixing the mixture for 4 hours in a sand mill according to the molar ratio of 2:1, taking out, sintering the mixture for 24 hours at the temperature of 750 ℃ in 10% hydrogen, cooling the mixture to room temperature, and taking out the mixture to obtain the nano LiFePO4。
The selenium carbon coating method comprises the following steps: the obtained LiFePO4And the materials, benzoselenol and glucose are mixed according to the selenium-carbon content of 3 percent and the selenium content of 0.7 percent in the selenium-carbon coating layer, the accurately weighed raw materials are fully and uniformly mixed, carbonized for 2 hours at 500 ℃ in high-purity argon, and naturally cooled to obtain the carbon-coated lithium iron phosphate cathode material.
Example 8
The embodiment provides a carbon-coated lithium iron phosphate cathode material, and the preparation method comprises the steps of preparing LiFePO by adopting a high-temperature solid-phase sintering method4And performing carbon coating treatment with different selenium contents on the prepared lithium iron phosphate anode material for two times.
LiFePO4The procedure for the preparation of the material was exactly the same as in example 6.
The selenium carbon coating step is as follows: the obtained LiFePO4Mixing with selenol and sucrose according to the selenium carbon content of 1 percent and the selenium content of 1.5 percent in the selenium carbon coating layer, carbonizing for 2 hours at 500 ℃ in high-purity argon, naturally cooling, mixing with selenol and sucrose according to the selenium carbon content of 2 percent and the selenium content of 0.3 percent in the selenium carbon coating layer, fully and uniformly mixing, and carbonizing for 2 hours at 500 ℃ in high-purity argon to obtain the carbon-coated lithium iron phosphate cathode material, wherein the selenium carbon accounts for 3 percent in total, and the total selenium content in the selenium carbon coating layer is 0.7 percent.
Example 9
The embodiment provides a carbon-coated lithium iron phosphate cathode material, and the preparation method comprises the steps of firstly preparing a carbon-coated lithium iron phosphate cathode material matrix by adopting a one-step synthesis method, and then carrying out carbon coating treatment with different selenium contents twice.
The synthesis of the carbon-coated lithium iron phosphate anode material substrate comprises the following steps: FePO is reacted with4、Li2CO3According to the mol ratio of 1:1, mixing materials, and adding glucose and selenol according to the content of 0.5 percent of selenium and carbon and the content of 10 percent of selenium in the selenium and carbon coating layerMixing the carbon source and the selenium source in a sand mill for 6h, taking out, sintering in high-purity nitrogen at 700 ℃ for 12h, cooling to room temperature, and taking out to obtain the carbon-coated lithium iron phosphate cathode material.
And performing carbon coating on the prepared carbon-coated lithium iron phosphate cathode material matrix material twice: the prepared carbon-coated lithium iron phosphate cathode material is prepared by mixing a matrix material of the carbon-coated lithium iron phosphate cathode material, selenophenol and cane sugar according to the selenium-carbon content of 0.5 percent and the selenium content of 5 percent in a selenium-carbon coating layer, fully mixing the accurately weighed raw materials, carbonizing the raw materials for 1.5 hours at 500 ℃ in high-purity argon gas, naturally cooling the raw materials, mixing the raw materials with the selenophenol and cane sugar according to the selenium-carbon content of 0.5 percent and the selenium content of 0 percent in the selenium-carbon coating layer, fully mixing the raw materials uniformly, and carbonizing the raw materials for 1.5 hours at 500 ℃ in the high-purity argon gas to obtain the carbon-coated lithium iron phosphate cathode material, wherein the total selenium-carbon proportion is 1.5 percent, and the.
Example 10
This example provides a carbon-coated lithium manganese phosphate cathode material, which is prepared by a one-step synthesis method to prepare carbon-coated lithium manganese phosphate (LiMnPO)4) A positive electrode material: mixing MnCO3、 NH4H2PO4、Li2CO3Mixing the materials according to a molar ratio of 2:2:1, adding glucose and selenol as a carbon source and a selenium source according to the proportion that the content of selenium and carbon is 1.5 percent and the content of selenium in a selenium and carbon coating layer is 5 percent, mixing the materials in a sand mill for 6 hours, taking out the materials, sintering the materials in high-purity nitrogen at 700 ℃ for 12 hours, cooling the materials to room temperature, and taking out the materials to obtain the carbon-coated lithium manganese phosphate anode material.
Example 11
This embodiment provides a carbon-coated lithium manganese iron phosphate positive electrode material, and a preparation method thereof, in which a carbon-coated lithium manganese iron phosphate (LiFe) is prepared by a one-step synthesis method0.3Mn0.7PO4) And (3) a positive electrode material. The specific process is as follows: FeC is added2O4·H2O、MnCO3、NH4H2PO4、Li2CO3Mixing the raw materials according to a molar ratio of 0.6:1.4:2:1, adding glucose and selenol as a carbon source and a selenium source according to the content of 1.5% of selenium and carbon and the content of 5% of selenium in the selenium carbon coating layer, and mixing the raw materials in a sand millAnd taking out after 6h, sintering for 10h at 720 ℃ in high-purity nitrogen, cooling to room temperature, and taking out to obtain the carbon-coated lithium manganese iron phosphate cathode material.
Example 12
This example provides a carbon-coated lithium manganese iron cobalt phosphate cathode material, which is prepared by a one-step synthesis method to prepare carbon-coated lithium manganese iron cobalt phosphate (LiFe)0.3Mn0.2Co0.5PO4) And (3) a positive electrode material. The specific process is as follows: FeC is added2O4·H2O、MnCO3、CoCO3、NH4H2PO4、 Li2CO3Mixing the materials according to a molar ratio of 0.6:0.4:1:2:1, adding glucose and selenol as a carbon source and a selenium source according to the content of 1.5% of selenium and carbon and the content of 5% of selenium in the selenium and carbon coating layer, mixing the materials in a sand mill for 6 hours, taking out the materials, sintering the materials in high-purity nitrogen at 720 ℃ for 12 hours, cooling the materials to room temperature, and taking out the materials to obtain the carbon-coated manganese, iron, cobalt and lithium phosphate cathode material.
Example 13
This embodiment provides a carbon-coated lithium manganese iron nickel phosphate cathode material, which is prepared by a one-step synthesis method to prepare carbon-coated lithium manganese iron nickel phosphate (LiFe)0.3Mn0.6Ni0.1PO4) And (3) a positive electrode material. The specific process is as follows: FeC is added2O4·H2O、MnCO3、NiO、NH4H2PO4、 Li2CO3Mixing the materials according to a molar ratio of 0.6:1.2:0.2:2:1, adding glucose and selenol as a carbon source and a selenium source according to the content of 1.5% of selenium and carbon and the content of 5% of selenium in the selenium and carbon coating layer, mixing the materials in a sand mill for 6 hours, taking out the materials, sintering the materials in high-purity nitrogen at 700 ℃ for 12 hours, cooling the materials to room temperature, and taking out the materials to obtain the carbon-coated manganese iron nickel lithium phosphate cathode material.
Comparative example 1
Unlike examples 1 and 2, any selenium source such as elemental selenium or selenium ether is not included. The rest is the same as the embodiment 1 and the embodiment 2, and the description is omitted.
Comparative example 2
Unlike example 3, no selenide is included, and the rest is the same as example 3.
Comparative example 3
Unlike example 4, no selenophenol was included, and the rest was the same as example 4.
Comparative example 4
Unlike examples 5 and 6, any selenium source such as selenium oxide or selenophenol is not included. The rest is the same as the embodiment 5 and the embodiment 6, and the description is omitted.
Comparative example 5
Unlike example 7, no benzoselenol is included. The rest is the same as in example 7.
Comparative example 6
Unlike example 8, no selenol was included. The rest is the same as in example 8.
Comparative example 7
Unlike example 9, no selenol was included. The rest is the same as in example 9.
Comparative example 8
Unlike example 10, no selenol was included. The rest is the same as in example 10.
Comparative example 9
Unlike example 11, no selenol was included. The rest is the same as in example 11.
Comparative example 10
Unlike example 12, no selenol was included. The rest is the same as in example 12.
Comparative example 11
Unlike example 13, no selenol was included. The rest is the same as in example 13.
Each of the above examples and comparative examples was tested for powder compaction density before the material was applied to a battery and the samples were examined for electrochemical performance.
Powder compaction density test: a5 g sample was placed in a pellet mold of phi 20 (diameter 2.0cm) and the mold was set on a shaker for 10 minutes. After the oscillation, the mold was placed on an oil press and pressed at 10MPa, the thickness of the sheet was measured and recorded as D cm, and the material compaction density was calculated according to the following formula: the compaction density is 5/(2.0/2)2πD g/cm3
Preparing a battery: mixing the positive electrode materials in the examples and the comparative examples with conductive carbon black and PVDF serving as a binder according to a mass ratio of 95:3:2, dispersing the mixture in N-methyl pyrrolidone (NMP) to form slurry, and stirring, coating, drying, rolling and slitting the slurry to obtain the positive electrode piece. The method comprises the steps of taking artificial graphite as a negative electrode active material, mixing the artificial graphite with conductive agent conductive carbon black and binder PVDF according to the mass ratio of 94:3:3, dispersing the mixture in NMP to form slurry, and stirring, coating, drying, rolling and slitting to obtain the negative electrode piece. Winding the positive pole piece, the negative pole piece and the PE isolating film, and then welding the terminals, packaging the aluminum foil, and injecting liquid (1 mol/LLIPF)6The solvent is EC/DMC/DEC with the volume ratio of 1:1: 1), packaging and forming, and air-extracting and forming are carried out, finally the soft package lithium ion battery is prepared, and the design capacity of the battery is 2500 mAh.
Capacity and cycle performance testing: the discharge capacity at the first cycle and the discharge capacity at the 1000 th cycle were measured by charging the battery at 25 ℃ to a charge cut-off voltage U at a constant current of 0.5C (1250mA), then charging the battery at a constant voltage to 0.05C (125mA), and then discharging the battery at 0.5C (1250mA) to 2.0V, and repeating 1000 cycles of the charge and discharge.
Capacity retention rate after cycling was (N-th cycle discharge capacity)/(1-th cycle discharge capacity) × 100%.
And (3) rate testing: charging to U at 25 deg.C with 0.5C (1250mA) constant current, constant voltage to 0.05C (125mA), discharging to 2.0V at 0.5C (1250mA), and cycling for 10 times to obtain average discharge energy, which is recorded as 0.5C cycle discharge energy; charging to U at constant current of 0.5C (1250mA), constant voltage to 0.05C (125mA), then discharging to 2.0V at 10C (25000mA), and obtaining the average value of the discharge energy after 10 times of circulation, and recording the average value as the discharge energy of 10C circulation.
The rate discharge energy retention rate is 10C cycle discharge energy/0.5C cycle discharge energy × 100%
The charge cut-off voltage U differs depending on the positive electrode material: the charge cut-off voltage of lithium iron phosphate was 3.7V, the charge cut-off voltage of lithium manganese iron phosphate was 4.5V, and the charge cut-off voltage of cobalt-or nickel-containing phosphate was 5.0V.
The test results are shown in Table 1.
TABLE 1
Figure RE-GDA0002460512550000151
Figure RE-GDA0002460512550000161
It can be seen from the examples and the comparative examples corresponding thereto that, compared with the common carbon-coated material, the lithium-containing transition metal phosphate cathode material with the selenium-doped carbon coating layer of the present invention has a higher powder compaction density, and the lithium ion battery manufactured by using the lithium-containing transition metal phosphate with the selenium-doped carbon coating layer of the present invention as the cathode material has a higher specific capacity, a better rate capability and a long cycle capability.
As can be seen from examples 1 to 2 and examples 5 to 6, the selenium-containing organic substance has a better effect than selenium oxide or elemental selenium as a selenium source. The selenium oxide and the elemental selenium are usually granular, solid-phase mixing is difficult to disperse uniformly, and organic selenium is liquid, and can be dissolved or dispersed into a liquid-phase solvent together with an organic carbon source to form a solution or suspension, so that uniform dispersion is easier to realize. In addition, the organic selenium contains a large amount of carbon besides selenium, so that selenium and carbon can be simultaneously provided in situ, and a stable carbon-selenium bond is more easily formed.
It can be seen from examples 7 to 8 and examples 3 and 9 that, under the condition of the same carbon coating amount and the same selenium content, the selenium content gradient material obtained by the multi-step coating method has higher powder compaction density compared with the selenium content uniform material obtained by the one-step synthesis method and the two-step coating method, and the lithium ion battery has higher specific capacity, better rate capability and long cycle performance. The reason is that the selenium in the selenium content gradient type material is more positioned at the interface of the transition metal phosphate material and the selenium carbon coating layer, so that the function of interface connection can be better enhanced.
In conclusion, the selenium-doped carbon coating layer can better play a role in conducting electricity and bring the rate capability of the materialIs promoted; the strong acting force between the selenium-doped carbon coating layer and the main body material enables the material to still keep the carbon material in good contact with the main body material under long circulation, and the cyclicity is improved; the selenium-doped carbon coating layer and the main body material are tightly combined, the compacted density is higher under the condition of the same carbon content, and the compacted density of the material is increased; selenium in LiMPO4The surface layer can also reduce the surface potential energy of the material, is more favorable for the entering and exiting of lithium ions, and plays a role in improving the rate capability of the material.
The foregoing description shows and describes several preferred embodiments of the invention, but as aforementioned, it is to be understood that the invention is not limited to the forms disclosed herein, but is not to be construed as excluding other embodiments and is capable of use in various other combinations, modifications, and environments and is capable of changes within the scope of the inventive concept as expressed herein, commensurate with the above teachings, or the skill or knowledge of the relevant art. And that modifications and variations may be effected by those skilled in the art without departing from the spirit and scope of the invention as defined by the appended claims.

Claims (10)

1. The carbon-coated lithium-containing transition metal phosphate positive electrode material is characterized by comprising a lithium-containing transition metal phosphate positive electrode material and a carbon coating layer coated on the surface of the lithium-containing transition metal phosphate positive electrode material, wherein selenium is doped in the carbon coating layer, and the structural formula of the lithium-containing transition metal phosphate positive electrode material is LiMPO4And M is at least one of Fe, Mn, Co and Ni.
2. The carbon-coated lithium-containing transition metal phosphate positive electrode material of claim 1, wherein the source of elemental selenium comprises at least one of selenium oxide, elemental selenium, and selenium-containing organics.
3. The carbon-coated lithium-containing transition metal phosphate positive electrode material according to claim 2, wherein the selenium-containing organic substance comprises at least one of selenol, dimethylselenium, benzoselenol, polyselenol, 2, 5-dicarboxylselenol, selenoether, and tetramethyltetraseleno fulvalene.
4. The carbon-coated lithium-containing transition metal phosphate positive electrode material according to claim 1, wherein the selenium element accounts for 0.01-40% of the total mass of the carbon coating layer, and the carbon coating layer accounts for 0.2-8% of the total mass of the positive electrode material.
5. The carbon-coated lithium-containing transition metal phosphate positive electrode material according to claim 4, wherein the mass of the selenium element accounts for 0.7-10% of the total mass of the carbon coating layer, and the mass of the carbon coating layer accounts for 0.5-5% of the total mass of the positive electrode material.
6. The carbon-coated lithium-containing transition metal phosphate positive electrode material according to claim 1, wherein the content of elemental selenium gradually decreases from near the lithium-containing transition metal phosphate positive electrode material to far from the lithium-containing transition metal phosphate positive electrode material.
7. The carbon-coated lithium-containing transition metal phosphate positive electrode material according to claim 1, wherein the average particle size of the primary particles of the lithium-containing transition metal phosphate positive electrode material is 20 to 300nm, and the particle size of the secondary particles of the lithium-containing transition metal phosphate positive electrode material D50 is 1 to 10 μm.
8. The carbon-coated lithium-containing transition metal phosphate positive electrode material according to claim 1, wherein in the lithium-containing transition metal phosphate positive electrode material, the M-site doping element is at least one of Cr, V, Nb, Mo, Zr, W, Y, Sc, Ru, Rh, Pd, Cu, and Zn, the Li-site doping element is at least one of Na, K, Mg, Ni, and Sc, and the P-site doping element is at least one of S, Se, Si, Mo, and Ge.
9. The method for preparing the carbon-coated lithium-containing transition metal phosphate positive electrode material according to any one of claims 1 to 8, comprising the following steps: s1, selecting a lithium source, an M source, a phosphorus source, a carbon source and a selenium source, mixing and sintering at high temperature; s2, adding a carbon source and a selenium source, mixing and then carbonizing at high temperature; and S3, repeating S2 for several times to obtain the product.
10. The method for preparing the carbon-coated lithium-containing transition metal phosphate positive electrode material according to any one of claims 1 to 8, comprising the following steps: s1, preparing a lithium-containing transition metal phosphate positive electrode material; s2, adding a carbon source and a selenium source, mixing and then carbonizing at high temperature; and S3, repeating S2 for several times to obtain the product.
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