CN112510199A - Nitrogen-doped carbon multi-layer coated graphite negative electrode material and preparation method thereof - Google Patents
Nitrogen-doped carbon multi-layer coated graphite negative electrode material and preparation method thereof Download PDFInfo
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Abstract
The invention discloses a nitrogen-doped carbon multilayer coated graphite cathode material and a preparation method thereof, wherein the cathode material comprises the following raw materials in percentage by mass: 70% -75% of graphite; 15% -20% of first ionic liquid and 10% -15% of second ionic liquid; wherein the first ionic liquid is phosphorus-containing ionic liquid and uniformly coats the outer surface of the graphite; the second ionic liquid is non-phosphorus-containing ionic liquid and is uniformly coated on the outer surface of the first ionic liquid. The graphite negative electrode material has higher compaction density, specific capacity and first efficiency, and effectively solves the problems of large irreversible capacity loss and lower specific capacity of the artificial graphite material in the practical application of preparing the negative electrode of the lithium battery.
Description
Technical Field
The invention relates to the technical field of negative electrode materials, in particular to a nitrogen-doped carbon multi-layer coated graphite negative electrode material and a preparation method thereof.
Background
With the continuous deepening of global energy crisis, the gradual depletion of petroleum resources, the aggravation of atmospheric pollution and global temperature rise, and the development of clean energy is imperative. In the development of new energy, the lithium ion battery with the advantages of high working voltage, high energy density, stable discharge voltage, long cycle life, environmental friendliness and the like is widely applied. The cathode material is one of the key factors for evaluating the comprehensive performance of the lithium ion battery. The lithium battery cathode material which is commercially used at present is mainly graphite, and the graphite has the characteristics of low lithium extraction/insertion potential (0-0.25V vs Li/Li +), high capacity, rich resources, low price and the like. However, the graphite material as the lithium ion negative electrode material has some disadvantages, which are mainly shown in that: the graphite layers have weak acting force and poor compatibility with electrolyte, and solvent molecules are inserted into the graphite layers in the charging and discharging processes to cause the graphite sheets to peel off, so that the cycle performance is poor.
Chinese patent CN 103682350B discloses a lithium battery negative electrode material of asphalt liquid phase coating modified artificial graphite, the first efficiency specific capacity of the modified artificial graphite negative electrode material is improved, but the first efficiency which is not 94% and the specific capacity which is not 360% still have improved space.
Chinese patent CN101604743A discloses that heteroatom-doped resin type high molecular pyrolytic carbon is used to coat graphite material, which significantly improves the electrical properties of graphite, but has some problems, such as poor affinity between resin type material and graphite, more volatile components in the resin pyrolysis process, and high specific surface area, which may affect tap density and compacted density.
Therefore, the chinese patent CN 104282896a adopts the ionic liquid to coat the graphite, which can solve the above problems well, however, only one layer of coating structure is adopted, and the performance still needs to be improved.
Disclosure of Invention
In view of the above, the present invention provides a multilayered nitrogen-doped carbon-coated graphite negative electrode material and a preparation method thereof, wherein different ionic liquids are introduced as carbon sources to uniformly coat two layers of carbon materials on the surface of the graphite material.
In order to achieve the purpose, the invention adopts the following technical scheme:
the nitrogen-doped carbon multilayer coated graphite cathode material comprises the following raw materials in percentage by mass: 70% -75% of graphite; 15% -20% of first ionic liquid and 10% -15% of second ionic liquid; wherein the first ionic liquid is phosphorus-containing ionic liquid and uniformly coats the outer surface of the graphite; the second ionic liquid is non-phosphorus-containing ionic liquid and is uniformly coated on the outer surface of the first ionic liquid.
Preferably, the graphite is at least one of natural graphite and artificial graphite.
Preferably, the cation of the first ionic liquid is a quaternary phosphonium salt cation, and the purity of the first ionic liquid is more than or equal to 99.5 percent.
Preferably, the second ionic liquid is at least one of N-methylbutylpyrrolidine bistrifluoromethanesulfonimide salt, N-methylbutylpiperidinbistrifluoromethanesulfonimide salt, 1-ethyl-3-methylimidazolium bistrifluoromethanesulfonimide salt, 1-ethyl-3-methylimidazolium tetrafluoroborate, trimethylpropylammonium bistrifluoromethanesulfonimide salt and 1-hexyl-3-methylimidazolium bistrifluoromethanesulfonimide salt, the purity of the second ionic liquid is more than or equal to 99.5%, wherein the molar ratio of nitrogen to carbon is 7: 9-8: 9.
a preparation method of the nitrogen-doped carbon multilayer coated graphite negative electrode material comprises the following steps:
(1) primary coating: weighing graphite and first ionic liquid, adding the graphite and the first ionic liquid into a high-speed stirrer, dispersing for 30-40 minutes at a rotating speed of 6000-7000 r/min, and obtaining the graphite coated with the first ionic liquid after treatment;
(2) primary carbonization: placing the graphite coated with the first ionic liquid in an atmosphere protection furnace for sintering, raising the temperature to 1100-1200 ℃ at the temperature rise rate of 30-35 ℃/min, and preserving the heat for 2-3 hours;
(3) secondary coating: weighing the product obtained in the step (2) and a second ionic liquid, adding the product and the second ionic liquid into a high-speed stirrer, dispersing for 50-60 minutes at a rotating speed of 2000-3000 r/min, and obtaining a secondary coating after treatment;
(4) secondary carbonization: and (3) placing the secondary coating in an atmosphere protection furnace for sintering, raising the temperature to 800-1000 ℃ at the heating rate of 10-25 ℃/min, and preserving the heat for 3-4 hours to obtain a finished product.
Preferably, the protective atmosphere used in the atmosphere protection furnace is at least one selected from helium, nitrogen, argon and carbon dioxide.
As a preferred scheme, the outer surface of the graphite is uniformly coated with a layer of nitrogen-doped phosphorus carbon material and a layer of nitrogen-doped carbon material from inside to outside in sequence.
Compared with the prior art, the invention has obvious advantages and beneficial effects, and specifically, the technical scheme includes that:
1. by adopting two different ionic liquids, compared with the traditional solid carbon source precursor, the two different ionic liquids are easier to penetrate into the graphite surface holes due to the flowing property, so that the specific surface area is favorably and greatly reduced, the cracking temperature range is wider (400-1000 ℃) under lower steam pressure, and rapid solvent evaporation is not accompanied, so that uniform coating thin layers are favorably formed on the surfaces of graphite particles, the performances such as compaction density, primary efficiency and the like are favorably and greatly improved.
2. The two different ionic liquids are not only carbon sources but also nitrogen sources, carbon is coated on the surface of graphite, and simultaneously nitrogen is doped, and the surface of the graphite is coated by an N-doped carbon layer, so that the electronic conductivity of the material is improved, and the surface stability of the material is also improved, so that the material has excellent rate capability and cycle performance, and can be better applied to power batteries with high performance requirements.
3. The ionic liquid has higher viscosity and has a wetting effect on the surface of graphite, so that the graphite material is not easy to agglomerate in the carbonization process, the carbonization and scattering processes are omitted, the coating process is simplified, and the preparation cost is reduced.
4. The invention has simple process, convenient operation and less production equipment, thereby further reducing the cost, being convenient for popularization and application and being suitable for large-scale production.
Drawings
FIG. 1 is a schematic view of the surface topography of a material tested by a scanning electron microscope according to the present invention.
Detailed Description
The invention discloses a nitrogen-doped carbon multilayer coated graphite cathode material which comprises the following raw materials in percentage by mass: 70% -75% of graphite; 15% -20% of first ionic liquid and 10% -15% of second ionic liquid; wherein the first ionic liquid is phosphorus-containing ionic liquid and uniformly coats the outer surface of the graphite; the second ionic liquid is non-phosphorus-containing ionic liquid and is uniformly coated on the outer surface of the first ionic liquid. The graphite is at least one of natural graphite and artificial graphite. The cation of the first ionic liquid is a quaternary phosphonium salt cation, and the purity of the first ionic liquid is more than or equal to 99.5 percent. The second ionic liquid is at least one of N-methylbutyl pyrrolidine bistrifluoromethane sulfimide salt, N-methylbutyl piperidine bistrifluoromethane sulfimide salt, 1-ethyl-3-methylimidazole tetrafluoroborate, trimethyl propyl ammonium bistrifluoromethane sulfimide salt and 1-hexyl-3-methylimidazole bistrifluoromethane sulfimide salt, the purity of the second ionic liquid is more than or equal to 99.5%, wherein the molar ratio of nitrogen to carbon is 7: 9-8: 9.
the invention also discloses a preparation method of the nitrogen-doped carbon multilayer coated graphite cathode material, which comprises the following steps:
(1) primary coating: weighing graphite and first ionic liquid, adding the graphite and the first ionic liquid into a high-speed stirrer, dispersing for 30-40 minutes at a rotating speed of 6000-7000 r/min, and obtaining the graphite coated with the first ionic liquid after treatment.
(2) Primary carbonization: the graphite coated with the first ionic liquid is placed in an atmosphere protection furnace for sintering, the temperature is raised to 1100-1200 ℃ at the heating rate of 30-35 ℃/min, and the temperature is kept for 2-3 hours. The protective atmosphere used in the atmosphere protective furnace is at least one selected from helium, nitrogen, argon and carbon dioxide.
(3) Secondary coating: and (3) weighing the product obtained in the step (2) and the second ionic liquid, adding the product and the second ionic liquid into a high-speed stirrer, dispersing for 50-60 minutes at a rotating speed of 2000-3000 r/min, and obtaining a secondary coating after treatment.
(4) Secondary carbonization: and (3) placing the secondary coating in an atmosphere protection furnace for sintering, raising the temperature to 800-1000 ℃ at the heating rate of 10-25 ℃/min, and preserving the heat for 3-4 hours to obtain a finished product, wherein the outer surface of the graphite is uniformly coated with a layer of nitrogen-doped phosphorus carbon material and a layer of nitrogen-doped carbon material from inside to outside in sequence. The protective atmosphere used in the atmosphere protective furnace is at least one selected from helium, nitrogen, argon and carbon dioxide.
The invention is explained in more detail below in a number of examples and comparative examples:
example 1:
the nitrogen-doped carbon multilayer coated graphite cathode material comprises the following raw materials in percentage by mass: 70% of graphite; 20% of a first ionic liquid and 10% of a second ionic liquid; wherein the first ionic liquid is phosphorus-containing ionic liquid and uniformly coats the outer surface of the graphite; the second ionic liquid is non-phosphorus-containing ionic liquid and is uniformly coated on the outer surface of the first ionic liquid. The graphite is natural graphite. The cation of the first ionic liquid is a quaternary phosphonium salt cation, and the purity of the first ionic liquid is 99.5%. The second ionic liquid is N-methylbutyl pyrrolidine bistrifluoromethanesulfonimide salt, the purity of the second ionic liquid is 99.5%, wherein the molar ratio of nitrogen to carbon is 7: 9.
the invention also discloses a preparation method of the nitrogen-doped carbon multilayer coated graphite cathode material, which comprises the following steps:
(1) primary coating: weighing graphite and the first ionic liquid, adding the graphite and the first ionic liquid into a high-speed stirrer, dispersing for 30 minutes at a rotating speed of 6000 r/min, and obtaining the graphite coated with the first ionic liquid after treatment.
(2) Primary carbonization: and (3) placing the graphite coated with the first ionic liquid in an atmosphere protection furnace for sintering, raising the temperature to 1100 ℃ at a heating rate of 33 ℃/min, and preserving the temperature for 2.5 hours. The protective atmosphere used in the atmosphere protective furnace is selected from helium.
(3) Secondary coating: and (3) weighing the product obtained in the step (2) and the second ionic liquid, adding into a high-speed stirrer, dispersing for 50 minutes at a rotating speed of 2000 r/min, and obtaining a secondary coating after treatment.
(4) Secondary carbonization: and (3) placing the secondary coating in an atmosphere protection furnace for sintering, raising the temperature to 1000 ℃ at the heating rate of 15 ℃/min, and preserving the heat for 3 hours to obtain a finished product, wherein the outer surface of the graphite is uniformly coated with a layer of nitrogen-doped phosphorus carbon material and a layer of nitrogen-doped carbon material from inside to outside in sequence. The protective atmosphere used in the atmosphere protecting furnace is selected from nitrogen.
Example 2:
the nitrogen-doped carbon multilayer coated graphite cathode material comprises the following raw materials in percentage by mass: 75% of graphite; 15% of a first ionic liquid and 15% of a second ionic liquid; wherein the first ionic liquid is phosphorus-containing ionic liquid and uniformly coats the outer surface of the graphite; the second ionic liquid is non-phosphorus-containing ionic liquid and is uniformly coated on the outer surface of the first ionic liquid. The graphite is artificial graphite. The cation of the first ionic liquid is a quaternary phosphonium salt cation, and the purity of the first ionic liquid is 99.9%. The second ionic liquid is N-methylbutylpiperidine bistrifluoromethanesulfonylimide salt, the purity of the second ionic liquid is 99.8%, wherein the molar ratio of nitrogen to carbon is 8: 9.
the invention also discloses a preparation method of the nitrogen-doped carbon multilayer coated graphite cathode material, which comprises the following steps:
(1) primary coating: weighing graphite and first ionic liquid, adding the graphite and the first ionic liquid into a high-speed stirrer, dispersing for 40 minutes at the rotating speed of 7000 r/min, and obtaining the graphite coated with the first ionic liquid after treatment.
(2) Primary carbonization: and (3) placing the graphite coated with the first ionic liquid in an atmosphere protection furnace for sintering, raising the temperature to 1200 ℃ at a temperature rise rate of 30 ℃/min, and preserving the temperature for 2 hours. The protective atmosphere used in the atmosphere protecting furnace is selected from nitrogen.
(3) Secondary coating: and (3) weighing the product obtained in the step (2) and the second ionic liquid, adding into a high-speed stirrer, dispersing for 55 minutes at a rotating speed of 3000 r/min, and obtaining a secondary coating after treatment.
(4) Secondary carbonization: and (3) placing the secondary coating in an atmosphere protection furnace for sintering, raising the temperature to 800 ℃ at the heating rate of 25 ℃/min, and preserving the heat for 3.2 hours to obtain a finished product, wherein the outer surface of the graphite is uniformly coated with a layer of nitrogen-doped phosphorus carbon material and a layer of nitrogen-doped carbon material from inside to outside in sequence. The protective atmosphere used in the atmosphere protective furnace is selected from argon.
Example 3:
the nitrogen-doped carbon multilayer coated graphite cathode material comprises the following raw materials in percentage by mass: 72% of graphite; 180% of a first ionic liquid and 10% of a second ionic liquid; wherein the first ionic liquid is phosphorus-containing ionic liquid and uniformly coats the outer surface of the graphite; the second ionic liquid is non-phosphorus-containing ionic liquid and is uniformly coated on the outer surface of the first ionic liquid. The graphite is a mixture of natural graphite and artificial graphite. The cation of the first ionic liquid is a quaternary phosphonium salt cation, and the purity of the first ionic liquid is 99.9%. The second ionic liquid is 1-ethyl-3-methylimidazole bistrifluoromethanesulfonylimide salt, the purity of the second ionic liquid is 99.9%, and the molar ratio of nitrogen to carbon is 7: 9.
the invention also discloses a preparation method of the nitrogen-doped carbon multilayer coated graphite cathode material, which comprises the following steps:
(1) primary coating: weighing graphite and the first ionic liquid, adding the graphite and the first ionic liquid into a high-speed stirrer, dispersing for 35 minutes at the rotating speed of 6500 r/min, and obtaining the graphite coated with the first ionic liquid after treatment.
(2) Primary carbonization: and (3) placing the graphite coated with the first ionic liquid in an atmosphere protection furnace for sintering, raising the temperature to 1150 ℃ at a temperature rise rate of 35 ℃/min, and preserving the temperature for 3 hours. The protective atmosphere used in the atmosphere protective furnace is selected from argon.
(3) Secondary coating: and (3) weighing the product obtained in the step (2) and the second ionic liquid, adding into a high-speed stirrer, dispersing for 60 minutes at a rotation speed of 2100 r/min, and obtaining a secondary coating after treatment.
(4) Secondary carbonization: and (3) placing the secondary coating in an atmosphere protection furnace for sintering, raising the temperature to 900 ℃ at the heating rate of 10 ℃/min, and preserving the heat for 3.5 hours to obtain a finished product, wherein the outer surface of the graphite is uniformly coated with a layer of nitrogen-doped phosphorus carbon material and a layer of nitrogen-doped carbon material from inside to outside in sequence. The protective atmosphere used in the atmosphere protecting furnace is selected from carbon dioxide.
Example 4:
the nitrogen-doped carbon multilayer coated graphite cathode material comprises the following raw materials in percentage by mass: 735% of graphite; 16% of first ionic liquid and 11% of second ionic liquid; wherein the first ionic liquid is phosphorus-containing ionic liquid and uniformly coats the outer surface of the graphite; the second ionic liquid is non-phosphorus-containing ionic liquid and is uniformly coated on the outer surface of the first ionic liquid. The graphite is natural graphite. The cation of the first ionic liquid is a quaternary phosphonium salt cation, and the purity of the first ionic liquid is 99.9%. The second ionic liquid is 1-ethyl-3-methylimidazolium tetrafluoroborate, the purity of the second ionic liquid is 99.99%, and the molar ratio of nitrogen to carbon is 8: 9.
the invention also discloses a preparation method of the nitrogen-doped carbon multilayer coated graphite cathode material, which comprises the following steps:
(1) primary coating: weighing graphite and the first ionic liquid, adding the graphite and the first ionic liquid into a high-speed stirrer, dispersing for 33 minutes at a rotating speed of 6200 r/min, and obtaining the graphite coated with the first ionic liquid after treatment.
(2) Primary carbonization: and (3) placing the graphite coated with the first ionic liquid in an atmosphere protection furnace for sintering, raising the temperature to 1180 ℃ at a heating rate of 34 ℃/min, and preserving the temperature for 2.4 hours. The protective atmosphere used in the atmosphere protecting furnace is selected from carbon dioxide.
(3) Secondary coating: and (3) weighing the product obtained in the step (2) and the second ionic liquid, adding into a high-speed stirrer, dispersing for 53 minutes at the rotating speed of 2400 r/min, and obtaining a secondary coating after treatment.
(4) Secondary carbonization: and (3) placing the secondary coating in an atmosphere protection furnace for sintering, raising the temperature to 800 ℃ at the heating rate of 18 ℃/min, and preserving the heat for 3.8 hours to obtain a finished product, wherein the outer surface of the graphite is uniformly coated with a layer of nitrogen-doped phosphorus carbon material and a layer of nitrogen-doped carbon material from inside to outside in sequence. The protective atmosphere used in the atmosphere protective furnace is selected from helium.
Example 5:
the nitrogen-doped carbon multilayer coated graphite cathode material comprises the following raw materials in percentage by mass: 74% of graphite; 15% of first ionic liquid and 11% of second ionic liquid; wherein the first ionic liquid is phosphorus-containing ionic liquid and uniformly coats the outer surface of the graphite; the second ionic liquid is non-phosphorus-containing ionic liquid and is uniformly coated on the outer surface of the first ionic liquid. The graphite is artificial graphite. The cation of the first ionic liquid is a quaternary phosphonium salt cation, and the purity of the first ionic liquid is 99.8%. The second ionic liquid is trimethyl propyl ammonium bis (trifluoromethanesulfonimide) salt, the purity of the second ionic liquid is 99.6%, and the molar ratio of nitrogen to carbon is 7: 9.
the invention also discloses a preparation method of the nitrogen-doped carbon multilayer coated graphite cathode material, which comprises the following steps:
(1) primary coating: weighing graphite and the first ionic liquid, adding the graphite and the first ionic liquid into a high-speed stirrer, dispersing for 38 minutes at the rotating speed of 6700 r/min, and obtaining the graphite coated with the first ionic liquid after treatment.
(2) Primary carbonization: and (3) placing the graphite coated with the first ionic liquid into an atmosphere protection furnace for sintering, raising the temperature to 1130 ℃ at a heating rate of 31 ℃/min, and preserving the temperature for 2.6 hours. The protective atmosphere used in the atmosphere protective furnace is selected from helium and nitrogen mixture.
(3) Secondary coating: and (3) weighing the product obtained in the step (2) and the second ionic liquid, adding into a high-speed stirrer, dispersing for 54 minutes at the rotating speed of 2800 r/min, and obtaining a secondary coating after treatment.
(4) Secondary carbonization: and (3) placing the secondary coating in an atmosphere protection furnace for sintering, raising the temperature to 950 ℃ at the heating rate of 14 ℃/min, and preserving the heat for 3.4 hours to obtain a finished product, wherein the outer surface of the graphite is uniformly coated with a layer of nitrogen-doped phosphorus carbon material and a layer of nitrogen-doped carbon material from inside to outside in sequence. The protective atmosphere used in the atmosphere protective furnace is selected from helium, nitrogen and argon mixture.
Example 6:
the nitrogen-doped carbon multilayer coated graphite cathode material comprises the following raw materials in percentage by mass: 71% of graphite; 19% of a first ionic liquid and 10% of a second ionic liquid; wherein the first ionic liquid is phosphorus-containing ionic liquid and uniformly coats the outer surface of the graphite; the second ionic liquid is non-phosphorus-containing ionic liquid and is uniformly coated on the outer surface of the first ionic liquid. The graphite is a mixture of natural graphite and artificial graphite. The cation of the first ionic liquid is a quaternary phosphonium salt cation, and the purity of the first ionic liquid is 99.7%. The second ionic liquid is 1-hexyl-3-methylimidazole bistrifluoromethanesulfonylimide salt, the purity of the second ionic liquid is more than or equal to 99.8%, wherein the molar ratio of nitrogen to carbon is 8: 9.
the invention also discloses a preparation method of the nitrogen-doped carbon multilayer coated graphite cathode material, which comprises the following steps:
(1) primary coating: weighing graphite and the first ionic liquid, adding the graphite and the first ionic liquid into a high-speed stirrer, dispersing for 37 minutes at the rotating speed of 6800 r/min, and obtaining the graphite coated with the first ionic liquid after treatment.
(2) Primary carbonization: and (3) placing the graphite coated with the first ionic liquid in an atmosphere protection furnace for sintering, raising the temperature to 1140 ℃ at a heating rate of 32 ℃/min, and preserving the temperature for 2.8 hours. The protective atmosphere used in the atmosphere protective furnace is selected from helium, nitrogen, argon and carbon dioxide mixture.
(3) Secondary coating: and (3) weighing the product obtained in the step (2) and the second ionic liquid, adding into a high-speed stirrer, dispersing for 58 minutes at the rotation speed of 2600 r/min, and obtaining a secondary coating after the treatment is finished.
(4) Secondary carbonization: and (3) placing the secondary coating in an atmosphere protection furnace for sintering, raising the temperature to 880 ℃ at the heating rate of 22 ℃/min, and preserving the heat for 4 hours to obtain a finished product, wherein the outer surface of the graphite is uniformly coated with a layer of nitrogen-doped phosphorus carbon material and a layer of nitrogen-doped carbon material from inside to outside in sequence. The protective atmosphere used in the atmosphere protective furnace is selected from helium, nitrogen, argon and carbon dioxide mixture.
Comparative example 1
Graphite material obtained using pitch as a coating material.
Comparative example 2
A graphite material obtained by using a phenol resin as a coating material.
Electrochemical performance tests were performed on the foregoing examples and comparative examples as follows:
in order to test the performance of the lithium ion battery cathode material coated with the ionic liquid graphite cathode material, a half-cell test method is used for testing, the cathode material of the above embodiment and comparative example, SBR (solid content is 50%), CMC: Super-p is 95.5: 2: 1.5: 1 (weight ratio), a proper amount of deionized water is added to be blended into slurry, the slurry is coated on a copper foil and dried in a vacuum drying oven for 12 hours to prepare a cathode sheet, the electrolyte is 1M LiPF6/EC + DEC + DMC is 1: 1, a polypropylene microporous membrane is a diaphragm, a counter electrode is a lithium sheet, and the battery is assembled. A constant-current charge and discharge experiment is carried out in a LAND battery test system, the charge and discharge voltage is limited to 0.01-3.0V, data collection and control are carried out by a charge and discharge cabinet controlled by a computer, and the obtained data are shown in the following table 1.
Table 1 shows the performance comparison of the negative electrode materials of the different examples and comparative examples.
As can be seen from Table 1, the specific surface area of the graphite coated twice with the first ionic liquid and the second ionic liquid and carbonized twice is fully reduced, the tap density is effectively improved, and various electrical properties are remarkably improved. The reason is that different ionic liquids are introduced as carbon sources, and the nitrogen-doped carbon-coated graphite cathode material with uniform surface coating is obtained. The graphite cathode material has a spherical shape, the surface of the graphite cathode material is uniformly coated with two layers of different carbon materials, the electronic conductivity of the graphite cathode material is effectively improved, the surface stability of the graphite cathode material is enhanced, and the obtained graphite cathode material has excellent rate capability and cycle performance.
In addition, as shown in fig. 1, the graphite material is coated with two layers of carbon materials uniformly, the surface is smooth, no obvious holes exist, and therefore the specific surface area can be well controlled, and the compaction density of the graphite is improved.
The above description is only a preferred embodiment of the present invention, and is not intended to limit the technical scope of the present invention, so that any minor modifications, equivalent changes and modifications made to the above embodiment according to the technical spirit of the present invention are within the technical scope of the present invention.
Claims (7)
1. A nitrogen-doped carbon multilayer coated graphite cathode material is characterized in that: comprises the following raw materials in percentage by mass: 70% -75% of graphite; 15% -20% of first ionic liquid and 10% -15% of second ionic liquid; wherein the first ionic liquid is phosphorus-containing ionic liquid and uniformly coats the outer surface of the graphite; the second ionic liquid is non-phosphorus-containing ionic liquid and is uniformly coated on the outer surface of the first ionic liquid.
2. The nitrogen-doped carbon multi-layer coated graphite anode material of claim 1, wherein: the graphite is at least one of natural graphite and artificial graphite.
3. The nitrogen-doped carbon multi-layer coated graphite anode material of claim 1, wherein: the cation of the first ionic liquid is a quaternary phosphonium salt cation, and the purity of the first ionic liquid is more than or equal to 99.5 percent.
4. The nitrogen-doped carbon multi-layer coated graphite anode material of claim 1, wherein: the second ionic liquid is at least one of N-methylbutyl pyrrolidine bistrifluoromethane sulfimide salt, N-methylbutyl piperidine bistrifluoromethane sulfimide salt, 1-ethyl-3-methylimidazole tetrafluoroborate, trimethyl propyl ammonium bistrifluoromethane sulfimide salt and 1-hexyl-3-methylimidazole bistrifluoromethane sulfimide salt, the purity of the second ionic liquid is more than or equal to 99.5%, wherein the molar ratio of nitrogen to carbon is 7: 9-8: 9.
5. a method for preparing the nitrogen-doped carbon multi-layer coated graphite anode material according to any one of claims 1 to 4, wherein the method comprises the following steps: the method comprises the following steps:
(1) primary coating: weighing graphite and first ionic liquid, adding the graphite and the first ionic liquid into a high-speed stirrer, dispersing for 30-40 minutes at a rotating speed of 6000-7000 r/min, and obtaining the graphite coated with the first ionic liquid after treatment;
(2) primary carbonization: placing the graphite coated with the first ionic liquid in an atmosphere protection furnace for sintering, raising the temperature to 1100-1200 ℃ at the temperature rise rate of 30-35 ℃/min, and preserving the heat for 2-3 hours;
(3) secondary coating: weighing the product obtained in the step (2) and a second ionic liquid, adding the product and the second ionic liquid into a high-speed stirrer, dispersing for 50-60 minutes at a rotating speed of 2000-3000 r/min, and obtaining a secondary coating after treatment;
(4) secondary carbonization: and (3) placing the secondary coating in an atmosphere protection furnace for sintering, raising the temperature to 800-1000 ℃ at the heating rate of 10-25 ℃/min, and preserving the heat for 3-4 hours to obtain a finished product.
6. The nitrogen-doped carbon multi-layer coated graphite anode material of claim 5, wherein: the protective atmosphere used in the atmosphere protective furnace is at least one selected from helium, nitrogen, argon and carbon dioxide.
7. The nitrogen-doped carbon multi-layer coated graphite anode material of claim 5, wherein: the outer surface of the graphite is uniformly coated with a layer of nitrogen-doped phosphorus carbon material and a layer of nitrogen-doped carbon material from inside to outside in sequence.
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