CN112670461A - Natural graphite carbon coated negative electrode material, preparation method thereof and lithium ion battery - Google Patents
Natural graphite carbon coated negative electrode material, preparation method thereof and lithium ion battery Download PDFInfo
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Abstract
The invention discloses a natural graphite carbon-coated negative electrode material, a preparation method thereof and a lithium ion battery. The preparation method comprises the following steps: s1, performing heat treatment on a mixture of natural graphite and asphalt to obtain natural graphite with the surface coated with the asphalt; wherein the median particle diameter D50 of the natural graphite is 5.0-7.5 μm, and the D10/D90 of the natural graphite is more than 0.35; the coking value of the asphalt is 30-80%; the mass ratio of the natural graphite to the asphalt is 100: 8-100: 30; and S2, carbonizing the mixture of the natural graphite with the surface coated with the asphalt and the liquid resin. The invention can improve the power and cycle performance of the cathode material and reduce the production cost while ensuring the cathode material to have the advantages of high capacity and high compaction.
Description
Technical Field
The invention relates to the technical field of graphite cathode materials, in particular to a natural graphite carbon-coated cathode material, a preparation method thereof and a lithium ion battery.
Background
Compared with traditional batteries such as nickel-hydrogen batteries and lead-acid batteries, the lithium ion secondary battery has the advantages of high energy density, high output voltage, low self-discharge rate, environmental friendliness, no memory effect, long service life and the like, and is widely applied to the fields of portable consumer electronics, electric tools, electric vehicles, energy storage and the like. At present, the maximum consumption belongs to the new energy automobile industry.
The power performance requirements of the negative electrode material in the Hybrid Electric Vehicle (HEV) direction of electric tools and new energy vehicles are very high, and some need to meet the low-temperature charging capability of-10 ℃ or even-20 ℃. At present, most of HEV negative electrode materials are prepared from coal-based needle coke as a raw material in the market, the materials have two short plates, on one hand, the materials need graphitization processing and are too high in cost, on the other hand, the graphitization degree of the materials is usually only 92-93%, the gram capacity is only 330-340 mAh/g, and although the power performance is good, the energy density is low, so that the defects are not negligible.
The natural graphite used as the raw material of the cathode material for long-term use has the characteristics of high capacity, high compaction and high power, and has the defects of poor expansion and long cycle performance. Thus, in the HEV application direction, natural graphite has advantages in energy density, but needs to be designed and modified to achieve good power and cycle performance. In the prior art, a graphitized product compounded by natural graphite and asphalt is coated with a layer of amorphous carbon, but the process is complicated and the cost is too high, for example, patent CN106532051A discloses that the mixed natural graphite is coated with asphalt and then graphitized, and then is coated and modified with phenolic resin. Patent CN103151497A discloses that natural graphite is coated with asphalt, then cured and carbonized, and then coated with phenolic resin for modification, but it was found in experiments that the larger median particle size of the raw material used causes the problems of poor power performance, poor cycle performance and poor expansion. If proper raw material granularity and modification mode can be selected to realize reasonable particle structure, the modified natural graphite material can obtain low-temperature charging performance meeting the temperature of minus 20 ℃ and excellent expansion and cycle performance.
In view of the above understanding, research and development personnel try to develop a negative electrode applied to a high-power direction by using natural graphite as a raw material, inherit the advantages of high capacity and high compaction, improve the stability and the cycle performance of a negative electrode material, avoid a graphitization process in the technical process, and have certain cost advantage compared with artificial graphite.
Disclosure of Invention
The invention aims to solve the problems of insufficient power, poor long cycle performance and high cost when a natural graphite cathode material is applied to a Hybrid Electric Vehicle (HEV) and an electric tool, and provides a natural graphite carbon-coated cathode material, a preparation method thereof and a lithium ion battery.
The invention solves the technical problems through the following technical scheme.
The invention provides a preparation method of a natural graphite carbon-coated negative electrode material, which comprises the following steps:
s1, performing heat treatment on a mixture of natural graphite and asphalt to obtain natural graphite with the surface coated with the asphalt;
wherein the median particle diameter D50 of the natural graphite is 5.0-7.5 μm, and the D10/D90 of the natural graphite is more than 0.35; the coking value of the asphalt is 30-80%; the mass ratio of the natural graphite to the asphalt is 100: 8-100: 30;
and S2, carbonizing the mixture of the natural graphite with the surface coated with the asphalt and the liquid resin.
In S1, the natural graphite may be a natural graphite which is conventional in the art and satisfies a median particle diameter D50 of 5.0 to 7.5 μm and D10/D90 > 0.35. Wherein D10/D90 generally refers to the ratio of D10 to D90.
Preferably, in S1, the natural graphite has a median particle diameter D50 of 5.5 to 7.5 μm, for example, 5.5 μm, 6.5 μm, or 7.5 μm.
Preferably, in S1, the natural graphite has a D10/D90 of 0.35-0.40, such as 0.35 or 0.40.
Preferably, in S1, the tap density of the natural graphite powder is more than or equal to 0.50g/cm3Ash content is less than or equal to 0.10 percent.
More preferably, in S1, the natural graphite powder has a tap density of 0.5 to 0.73g/cm3For example, 0.5g/cm3、0.58g/cm3、0.65g/cm3Or 0.73g/cm3。
In S1, the asphalt may be an asphalt conventional in the art that satisfies a coking value of 30% to 80%.
Preferably, in S1, the pitch has a coking value of 30% to 70%, more preferably 30% to 60%, such as 30%, 50% or 60%.
Preferably, in S1, the pitch has a median particle diameter D50 ≦ 15.0 μm, for example 8.0 μm.
Preferably, in S1, the mass ratio of the natural graphite to the asphalt is 100:8 to 100:20, more preferably 100:8 to 100:15, such as 100:8, 100:10.8 or 100: 15.
In S1, the mixture of natural graphite and pitch may be obtained by conventional means in the art. For example, the natural graphite is mechanically mixed with the pitch to obtain a mixture of the natural graphite and the pitch.
In S1, the operation and conditions of the heat treatment may be conventional in the art and are generally carried out in an inert atmosphere. Wherein, the inert atmosphere can be an atmosphere which is conventional in the field and does not participate in the reaction of the system, such as one or more of nitrogen, argon and helium.
Preferably, in S1, the heat treatment is a heating treatment under stirring.
Preferably, in S1, the temperature of the heat treatment is 400 to 800 ℃, for example, 550 ℃.
Preferably, in S1, the heat treatment time is 2 to 3 hours, for example, 2 hours. The skilled person knows that the time of the heat treatment refers to a constant temperature time that satisfies the temperature of the heat treatment.
S1, those skilled in the art know that after the heat treatment is completed, the natural graphite with the surface coated with the asphalt is cooled and then subjected to the next operation. Preferably, the cooled natural graphite surface-coated with pitch has a temperature of < 100 ℃, e.g. < 50 ℃.
In S2, the liquid resin may be a resin that is liquid at room temperature, preferably a liquid resin with a scorch value of 10% to 30%, more preferably 15% to 25%, for example 20%.
Preferably, in S2, the liquid resin is one or more of epoxy resin, phenolic resin, polyacrylonitrile, polyvinyl alcohol, polystyrene, polypyrrolidone, polyacrylic acid, polyvinyl chloride, catalytic cracking slurry oil and asphalt. For example, the phenolic resin may be a novolac thermoplastic phenolic resin.
Preferably, in S2, the C content of the liquid resin is more than or equal to 99.5%.
Preferably, in S2, the sum of the contents of the metal elements in the liquid resin is less than or equal to 100 ppm.
In S2, the mass ratio of the natural graphite with the surface coated with the asphalt to the liquid resin may be conventional in the art, and is preferably 100:5 to 100:20, more preferably 100:8 to 100:12, for example 100:8 or 100: 12.
S2, the mixture of the natural graphite surface-coated with pitch and the liquid resin may be obtained by a conventional method in the art. For example, the natural graphite coated with the asphalt on the surface is mechanically mixed with the liquid resin to obtain a mixture of the natural graphite coated with the asphalt on the surface and the liquid resin.
Preferably, in S2, the carbonization treatment is a static heating operation performed in an inert atmosphere.
Preferably, in S2, the temperature of the carbonization treatment is 1000 ℃ to 1700 ℃.
More preferably, in S2, the temperature of the carbonization treatment is 1100 ℃ to 1300 ℃, for example 1100 ℃ or 1300 ℃.
Preferably, in S2, the carbonization time is 2 to 5 hours, for example, 3 hours.
At S2, those skilled in the art know that after the carbonization, those skilled in the art know that the natural graphite carbon coated negative electrode material is cooled and then the next operation is performed. Preferably, the temperature of the cooled natural graphite carbon coated negative electrode material is less than 100 ℃, for example less than 50 ℃.
In S2, preferably, after the carbonization treatment, the resultant product is subjected to sieving and demagnetizing treatment.
In the present invention, preferably, the heat treatment or the carbonization treatment does not add any solvent and/or additive. The solvent generally refers to a substance for dissolving the raw material. The additives are generally substances such as curing agents, dispersants, binders and the like.
In the present invention, the total mass of the coating layer formed by the asphalt and the coating layer formed by the liquid resin is preferably 5% to 10%, more preferably 8.0% to 9.5%, for example, 8.4% or 9.1% of the mass of the natural graphite carbon-coated negative electrode material.
The invention also provides the natural graphite carbon-coated negative electrode material prepared by the preparation method.
In the invention, the natural graphite carbon coated negative electrode material is of a core-shell structure and comprises an inner shell, a first coating shell and a second coating shell from inside to outside; the inner shell is made of natural spheroidized graphite, the first coating shell is a soft carbon structure formed by the asphalt, and the second coating shell is a hard carbon structure formed by the liquid resin.
The invention also provides a lithium ion battery, and the cathode material of the lithium ion battery is coated by the natural graphite carbon.
On the basis of the common knowledge in the field, the above preferred conditions can be combined randomly to obtain the preferred embodiments of the invention.
The reagents and starting materials used in the present invention are commercially available.
The positive progress effects of the invention are as follows:
compared with the prior art, the invention adopts natural graphite particles with specific particle size and distribution as raw materials, uses asphalt and liquid resin with specific coking value, and improves the size (D50) of the obtained cathode material by 30-80 percent compared with the raw materials in a mode of secondary-solid-liquid coating modification under a certain dosage, the OI value of a pole piece is small, the surface of the material obtains a uniform modification effect, and simultaneously obtains a macroscopic particle shape beneficial to high-rate charging, the cathode material has good isotropy, the expansion of the cathode material in the circulation process can be effectively reduced, and the cathode material with high power and good cycle performance can be obtained.
In the invention, the median particle size D50 of the natural graphite carbon-coated negative electrode material is 6.5-12.0 μm; D10/D90 is more than or equal to 0.35; dmax is less than 30 mu m; the BET specific surface area is 1.9 to 6.0m2(ii)/g; TAP with TAP density of 0.80-1.10 g/cm3(ii) a 2 ton powder compact density is more than 1.30g/cm3；1.60g/cm3The OI value of the pole piece under the compacted density is less than 15.0. The invention can realize the capacity of the half cell of 354-362 mAh/g; the first efficiency of the half cell is 88-92%; performing half-cell test at normal temperature, and charging the battery at 3C constant current until the SOC of the battery reaches more than 85% at the cut-off voltage of 2V; performing half-cell test at normal temperature, and charging the battery at 5C constant current until the SOC of the battery reaches more than 50% at the cut-off voltage of 2V; and (3) carrying out a half-cell test at the temperature of minus 20 ℃, and carrying out constant current charging at 0.2 ℃ until the SOC of the 2V cut-off voltage reaches over 79 percent.
In a preferred embodiment of the invention, the median particle diameter D50 of the natural graphite carbon-coated negative electrode material is 8.0-10.0 μm; D10/D90 is more than or equal to 0.40; dmax is less than 20 mu m; the BET specific surface area is 2.0 to 6.0m2(ii)/g; TAP with TAP density of 1.00-1.10 g/cm3(ii) a 2 ton powder compact density is more than 1.30g/cm3；1.60g/cm3The OI value of the pole piece under the compacted density is less than 10.0. The preferred embodiment of the invention can realize that the capacity of the half cell is 358-362 mAh/g; the first efficiency of the half cell is 90-92%; the full battery test 3C charging test meets the condition that no lithium is separated in 300 weeks; all-purposeA battery test 5C charging test is carried out, and lithium is not separated out after 50 weeks; the half-cell test is carried out at normal temperature, and the SOC of 3C which is charged to 2V cut-off voltage with constant current can reach 90 percent; the half-cell test is carried out at normal temperature, and the SOC of 5C which is charged to 2V cut-off voltage at constant current can reach 65%; the SOC of the battery which is subjected to a half-cell test at the temperature of minus 20 ℃ and is charged to 2V cut-off voltage by constant current of 0.2C can reach 90 percent.
Drawings
Fig. 1, fig. 2, and fig. 3 are respectively a 500, 1000, and 3000 times electron microscope topography of the high cycle natural graphite anode material prepared in example 4.
Fig. 4 and 5 are sectional topography views of the high-cycle natural graphite negative electrode material prepared in example 5 under a field emission scanning electron microscope.
Fig. 6, 7 and 8 are morphology diagrams of the natural graphite raw material, the clad products of the comparative example 4 and the clad product of the example 5 under a 3000-fold electron microscope, respectively.
Detailed Description
The invention is further illustrated by the following examples, which are not intended to limit the scope of the invention. The experimental methods without specifying specific conditions in the following examples were selected according to the conventional methods and conditions, or according to the commercial instructions.
Various natural graphites circulating in the market can be used in the invention; bitumen is available from companies such as Liaoning Xinde, Lutcog, Germany.
In the following examples and comparative examples:
the natural graphite is obtained from SG04, SG05, SG06-1, SG06-2, SG07 and SG08 from Qingbei carbon Co.
Wherein SG04 satisfies: spherical graphite with D50 ═ 4.7 μm, D10/D90 ═ 0.33, TAP ═ 0.48, and ash content of 0.01%;
SG05 is satisfied: spherical graphite with D50 ═ 5.5 μm, D10/D90 ═ 0.35, TAP ═ 0.50 and ash content of 0.01%;
SG06-1 satisfies: spherical graphite with D50 ═ 6.5 μm, D10/D90 ═ 0.35, TAP ═ 0.58 and ash content of 0.01%;
SG06-2 satisfies the following conditions: spherical graphite with D50 ═ 6.5 μm, D10/D90 ═ 0.40, TAP ═ 0.65 and ash content of 0.01%;
SG07 is satisfied: spherical graphite with D50 ═ 7.5 μm, D10/D90 ═ 0.40, TAP ═ 0.73, and ash content of 0.01%;
SG08 is satisfied: spherical graphite with D50 ═ 8.5 μm, D10/D90 ═ 0.40, TAP ═ 0.73, and ash content of 0.01%.
The asphalt is special asphalt for a lithium battery cathode material purchased from Liaoning Xin De chemical Co.
The liquid resin is linear thermoplastic phenolic resin with the coking value of 20 percent, and the content of C is more than or equal to 99.5 percent, and the sum of the contents of all metal elements is less than or equal to 100 ppm.
Example 1
Mechanically mixing raw materials: spherical graphite SG05 and asphalt with the coking value of 50% and D50 ≈ 8.0 μm are mechanically mixed according to the mass ratio of 100: 15.
Low-temperature heat treatment: and (3) putting the materials into a reaction kettle, heating while stirring, keeping the temperature of 550 ℃ at the final temperature for 2 hours, cooling to 50 ℃, and discharging to obtain the natural graphite with the surface coated with the asphalt.
Mechanically mixing semi-finished products: and mechanically mixing the natural graphite with the asphalt coated on the surface with liquid resin with a coking value of 20% according to a mass ratio of 100:8 to obtain a mixture of the natural graphite with the asphalt coated on the surface and the liquid resin.
And (3) medium-temperature carbonization treatment: carbonizing the mixture of natural graphite and liquid resin with the surface coated with asphalt, and adding N2Under protection, keeping the temperature of the highest final temperature of 1100 ℃ for 3h, then cooling to 50 ℃ and discharging.
And (3) processing a finished product: mixing, screening and demagnetizing to obtain the cathode material example material 1. The mass fraction of the soft carbon structure and the hard carbon structure in the natural graphite carbon-coated negative electrode material is 8.4%.
Example 2
Mechanically mixing raw materials: spherical graphite SG05 and asphalt with the coking value of 70% and D50 ≈ 8.0 μm are mechanically mixed according to the mass ratio of 100: 10.8.
Low-temperature heat treatment: and (3) putting the materials into a reaction kettle, heating while stirring, keeping the temperature of 550 ℃ at the final temperature for 2 hours, cooling to 50 ℃, and discharging to obtain the natural graphite with the surface coated with the asphalt.
Mechanically mixing semi-finished products: and mechanically mixing the natural graphite with the asphalt coated on the surface with liquid resin with a coking value of 20% according to a mass ratio of 100:8 to obtain a mixture of the natural graphite with the asphalt coated on the surface and the liquid resin.
And (3) medium-temperature carbonization treatment: carbonizing the mixture of natural graphite and liquid resin with the surface coated with asphalt, and adding N2Under protection, keeping the temperature of the highest final temperature of 1100 ℃ for 3h, then cooling to 50 ℃ and discharging.
And (3) processing a finished product: mixing, screening and demagnetizing to obtain the cathode material example material 2. The mass fraction of the soft carbon structure and the hard carbon structure in the natural graphite carbon-coated negative electrode material is 8.4%.
Example 3
Mechanically mixing raw materials: spherical graphite SG06-1 and asphalt with the coking value of 50% and D50 ≈ 8.0 μm are mechanically mixed according to the mass ratio of 100: 15.
Low-temperature heat treatment: and (3) putting the materials into a reaction kettle, heating while stirring, keeping the temperature of 550 ℃ at the final temperature for 2 hours, cooling to 50 ℃, and discharging to obtain the natural graphite with the surface coated with the asphalt.
Mechanically mixing semi-finished products: and mechanically mixing the natural graphite with the asphalt coated on the surface with liquid resin with a coking value of 20% according to a mass ratio of 100:8 to obtain a mixture of the natural graphite with the asphalt coated on the surface and the liquid resin.
And (3) medium-temperature carbonization treatment: carbonizing the mixture of natural graphite and liquid resin with the surface coated with asphalt, and adding N2Under protection, keeping the temperature of the highest final temperature of 1100 ℃ for 3h, then cooling to 50 ℃ and discharging.
And (3) processing a finished product: mixing, screening and demagnetizing to obtain the cathode material example material 3. The mass fraction of the soft carbon structure and the hard carbon structure in the natural graphite carbon-coated negative electrode material is 8.4%.
Example 4
Mechanically mixing raw materials: spherical graphite SG06-2 and asphalt with the coking value of 50% and D50 ≈ 8.0 μm are mechanically mixed according to the mass ratio of 100: 15.
Low-temperature heat treatment: and (3) putting the materials into a reaction kettle, heating while stirring, keeping the temperature of 550 ℃ at the final temperature for 2 hours, cooling to 50 ℃, and discharging to obtain the natural graphite with the surface coated with the asphalt.
Mechanically mixing semi-finished products: and mechanically mixing the natural graphite with the asphalt coated on the surface with liquid resin with a coking value of 20% according to a mass ratio of 100:15 to obtain a mixture of the natural graphite with the asphalt coated on the surface and the liquid resin.
And (3) medium-temperature carbonization treatment: carbonizing the mixture of natural graphite and liquid resin with the surface coated with asphalt, and adding N2Under protection, keeping the temperature of the highest final temperature of 1100 ℃ for 3h, then cooling to 50 ℃ and discharging.
And (3) processing a finished product: mixing, screening and demagnetizing to obtain the cathode material example material 4. The mass fraction of the soft carbon structure and the hard carbon structure in the natural graphite carbon-coated negative electrode material is 8.4%.
The topography under the electron microscope is shown in attached figures 1-3.
Example 5
Mechanically mixing raw materials: spherical graphite SG07 and asphalt with the coking value of 50% and D50 ≈ 8.0 μm are mechanically mixed according to the mass ratio of 100: 15.
Low-temperature heat treatment: and (3) putting the materials into a reaction kettle, heating while stirring, keeping the temperature of 550 ℃ at the final temperature for 2 hours, cooling to 50 ℃, and discharging to obtain the natural graphite with the surface coated with the asphalt.
Mechanically mixing semi-finished products: and mechanically mixing the natural graphite with the asphalt coated on the surface with liquid resin with a coking value of 20% according to a mass ratio of 100:8 to obtain a mixture of the natural graphite with the asphalt coated on the surface and the liquid resin.
And (3) medium-temperature carbonization treatment: carbonizing the mixture of natural graphite and liquid resin with the surface coated with asphalt, and adding N2Under protection, keeping the temperature of the highest final temperature of 1100 ℃ for 3h, then cooling to 50 ℃ and discharging.
And (3) processing a finished product: mixing, screening and demagnetizing to obtain the cathode material example material 5. The mass fraction of the soft carbon structure and the hard carbon structure in the natural graphite carbon-coated negative electrode material is 8.4%.
The cross-sectional topography under the field emission scanning electron microscope is shown in FIGS. 4-5.
Example 6
Mechanically mixing raw materials: spherical graphite SG06-2 and asphalt with the coking value of 50% and D50 ≈ 8.0 μm are mechanically mixed according to the mass ratio of 100: 15.
Low-temperature heat treatment: and (3) putting the materials into a reaction kettle, heating while stirring, keeping the temperature of 550 ℃ at the final temperature for 2 hours, cooling to 50 ℃, and discharging to obtain the natural graphite with the surface coated with the asphalt.
Mechanically mixing semi-finished products: and mechanically mixing the natural graphite with the asphalt coated on the surface with liquid resin with a coking value of 20% according to a mass ratio of 100:12 to obtain a mixture of the natural graphite with the asphalt coated on the surface and the liquid resin.
And (3) medium-temperature carbonization treatment: carbonizing the mixture of natural graphite and liquid resin with the surface coated with asphalt, and adding N2Under protection, keeping the temperature of the highest final temperature of 1100 ℃ for 3h, then cooling to 50 ℃ and discharging.
And (3) processing a finished product: mixing, screening and demagnetizing to obtain the cathode material example material 6. The mass fraction of the soft carbon structure and the hard carbon structure in the natural graphite carbon-coated negative electrode material is 9.1%.
Example 7
Mechanically mixing raw materials: spherical graphite SG06-2 and asphalt with the coking value of 50% and D50 ≈ 8.0 μm are mechanically mixed according to the mass ratio of 100: 15.
Low-temperature heat treatment: and (3) putting the materials into a reaction kettle, heating while stirring, keeping the temperature of 550 ℃ at the final temperature for 2 hours, cooling to 50 ℃, and discharging to obtain the natural graphite with the surface coated with the asphalt.
Mechanically mixing semi-finished products: and mechanically mixing the natural graphite with the asphalt coated on the surface with liquid resin with a coking value of 20% according to a mass ratio of 100:8 to obtain a mixture of the natural graphite with the asphalt coated on the surface and the liquid resin.
And (3) medium-temperature carbonization treatment: carbonizing the mixture of natural graphite and liquid resin with the surface coated with asphalt, and adding N2Keeping the temperature of 1300 ℃ with the protection for 3h, cooling to 50 ℃ and discharging.
And (3) processing a finished product: mixing, screening and demagnetizing to obtain the cathode material, namely, the material 7 in the embodiment. The mass fraction of the soft carbon structure and the hard carbon structure in the natural graphite carbon-coated negative electrode material is 8.4%.
Example 8
Mechanically mixing raw materials: spherical graphite SG06-2 and asphalt with the coking value of 50% and D50 ≈ 8.0 μm are mechanically mixed according to the mass ratio of 100: 15.
Low-temperature heat treatment: and (3) putting the materials into a reaction kettle, heating while stirring, keeping the temperature of 550 ℃ at the final temperature for 2 hours, cooling to 50 ℃, and discharging to obtain the natural graphite with the surface coated with the asphalt.
Mechanically mixing semi-finished products: and mechanically mixing the natural graphite with the asphalt coated on the surface with liquid resin with a coking value of 20% according to a mass ratio of 100:8 to obtain a mixture of the natural graphite with the asphalt coated on the surface and the liquid resin.
And (3) medium-temperature carbonization treatment: carbonizing the mixture of natural graphite and liquid resin with the surface coated with asphalt, and adding N2Keeping the temperature of 900 ℃ at the maximum final temperature for 3h under protection, then cooling to 50 ℃ and discharging.
And (3) processing a finished product: mixing, screening and demagnetizing to obtain the cathode material example material 8. The mass fraction of the soft carbon structure and the hard carbon structure in the natural graphite carbon-coated negative electrode material is 8.4%.
Example 9
Mechanically mixing raw materials: spherical graphite SG06-2 and asphalt with the coking value of 50% and D50 ≈ 8.0 μm are mechanically mixed according to the mass ratio of 100: 15.
Low-temperature heat treatment: and (3) putting the materials into a reaction kettle, heating while stirring, keeping the temperature of 550 ℃ at the final temperature for 2 hours, cooling to 50 ℃, and discharging to obtain the natural graphite with the surface coated with the asphalt.
Mechanically mixing semi-finished products: and mechanically mixing the natural graphite with the asphalt coated on the surface with liquid resin with a coking value of 20% according to a mass ratio of 100:8 to obtain a mixture of the natural graphite with the asphalt coated on the surface and the liquid resin.
And (3) medium-temperature carbonization treatment: carbonizing the mixture of natural graphite and liquid resin with the surface coated with asphalt, and adding N2Keeping the temperature of the maximum final temperature of 2000 ℃ for 3h under protection, then cooling to 50 ℃ and discharging.
And (3) processing a finished product: mixing, screening and demagnetizing to obtain the cathode material, example material 9. The mass fraction of the soft carbon structure and the hard carbon structure in the natural graphite carbon-coated negative electrode material is 8.4%.
Comparative example 1
Mechanically mixing raw materials: spherical graphite SG08 and asphalt with the coking value of 50% and D50 ≈ 8.0 μm are mechanically mixed according to the mass ratio of 100: 15.
Low-temperature heat treatment: and (3) putting the materials into a reaction kettle, heating while stirring, keeping the temperature of 550 ℃ at the final temperature for 2 hours, cooling to 50 ℃, and discharging to obtain the natural graphite with the surface coated with the asphalt.
Mechanically mixing semi-finished products: and mechanically mixing the natural graphite with the asphalt coated on the surface with liquid resin with a coking value of 20% according to a mass ratio of 100:8 to obtain a mixture of the natural graphite with the asphalt coated on the surface and the liquid resin.
And (3) medium-temperature carbonization treatment: carbonizing the mixture of natural graphite and liquid resin with the surface coated with asphalt, and adding N2Keeping the temperature of the highest final temperature at 1150 ℃ for 3h under protection, cooling to 50 ℃ and discharging.
And (3) processing a finished product: and mixing, screening and demagnetizing to obtain the negative electrode material comparative example material 1. The mass fraction of the soft carbon structure and the hard carbon structure in the natural graphite carbon-coated negative electrode material is 8.4%.
Comparative example 2
Mechanically mixing raw materials: spherical graphite SG04 and asphalt with the coking value of 50% and D50 ≈ 8.0 μm are mechanically mixed according to the mass ratio of 100: 15.
Low-temperature heat treatment: and (3) putting the materials into a reaction kettle, heating while stirring, keeping the temperature of 550 ℃ at the final temperature for 2 hours, cooling to 50 ℃, and discharging to obtain the natural graphite with the surface coated with the asphalt.
Mechanically mixing semi-finished products: and mechanically mixing the natural graphite with the asphalt coated on the surface with liquid resin with a coking value of 20% according to a mass ratio of 100:8 to obtain a mixture of the natural graphite with the asphalt coated on the surface and the liquid resin.
And (3) medium-temperature carbonization treatment: carbonizing the mixture of natural graphite and liquid resin with the surface coated with asphalt, and adding N2Under protection, the highest endKeeping the temperature at 1150 ℃ for 3h, cooling to 50 ℃ and discharging.
And (3) processing a finished product: and mixing, screening and demagnetizing to obtain the negative electrode material comparative example material 2. The mass fraction of the soft carbon structure and the hard carbon structure in the natural graphite carbon-coated negative electrode material is 8.4%.
Comparative example 3
Mechanically mixing raw materials: spherical graphite SG05 and asphalt with the coking value of 50% and D50 ≈ 8.0 μm are mechanically mixed according to the mass ratio of 100: 25.
Low-temperature heat treatment: and (3) putting the materials into a reaction kettle, heating while stirring, keeping the temperature of 550 ℃ at the final temperature for 2 hours, cooling to 50 ℃, and discharging to obtain the natural graphite with the surface coated with the asphalt.
Mechanically mixing semi-finished products: and mechanically mixing the natural graphite with the asphalt coated on the surface with liquid resin with a coking value of 20% according to a mass ratio of 100:8 to obtain a mixture of the natural graphite with the asphalt coated on the surface and the liquid resin.
And (3) medium-temperature carbonization treatment: carbonizing the mixture of natural graphite and liquid resin with the surface coated with asphalt, and adding N2Under protection, keeping the temperature of the highest final temperature of 1100 ℃ for 3h, then cooling to 50 ℃ and discharging.
And (3) processing a finished product: and mixing, screening and demagnetizing to obtain the negative electrode material comparative example material 3. The mass fraction of the soft carbon structure and the hard carbon structure in the natural graphite carbon-coated negative electrode material is 12.5%.
Comparative example 4
Mechanically mixing raw materials: spherical graphite SG07 and asphalt with the coking value of 50% and D50 ≈ 8.0 μm are mechanically mixed according to the mass ratio of 100: 15.
And (3) medium-temperature carbonization treatment: carbonizing the mixture of natural graphite and asphalt in N2Under protection, keeping the temperature of the highest final temperature of 1100 ℃ for 3h, then cooling to 50 ℃ and discharging.
And (3) processing a finished product: and mixing, screening and demagnetizing to obtain a negative electrode material comparative example material 4. The soft carbon structure accounts for 7.0 percent of the total mass fraction of the natural graphite carbon-coated cathode material.
The morphology graphs of the natural graphite raw material, the cladding products of the comparative example 4 and the cladding product of the example 5 under a 3000-fold electron microscope are shown in FIGS. 6-8.
The results of some experimental parameters of examples 1 to 9 and comparative examples 1 to 3 are shown in table 1, and the test results of the prepared negative electrode materials are shown in table 2.
Table 1 part of the experimental parameters
Effects of the embodiment
1. The test conditions of the negative electrode materials prepared in examples 1 to 9 and comparative examples 1 to 3 were as follows:
(1) the particle sizes D50, D10/D90 and Dmax were determined by laser method from Mastersize 2000 (Malvern 2000).
(2) Tap density was measured by a american kangta tap density instrument.
(3) The specific surface area is a BET specific surface area, and is measured by a nitrogen adsorption method using ASAP 2460.
(4) The OI value is the ratio of 004-face peak area to 110-face peak area of the negative electrode material of the test pole piece, and the formula of the pole piece is the obtained negative electrode material: CMC: SBR (97: 1.5: 1.5) is dried and not rolled, and is measured by an X-ray diffractometer Bruker D8 advanced instrument.
(5) The 2t powder compaction density was measured by a microcomputer controlled electronic pressure tester model LD43.305 from force testing (shanghai) scientific instruments ltd.
(6) Testing the performance of the half cell:
assembly of half cells
Preparing an electrode: at room temperature, the obtained anode material: CMC: SP: mixing SBR 95.5 wt% to 1.5 wt% in pure water to form slurry; the slurry was uniformly coated on a copper foil with a coating surface density of about 5mg/cm2Then the copper foil is put into a vacuum drying oven to be dried for 12 hours at the temperature of 80 ℃. Cutting the dried copper foil into 2cm in area2The wafer of (a) is made into a working electrode.
Assembling the half cell: under the condition of room temperature, a metal lithium sheet is taken as a counter electrode, and the product obtained in the step (1) is taken as workElectrode, PE diaphragm (polyethylene) as diaphragm, 1mol/L LiPF6DEC (volume ratio of 1:1) solution is taken as electrolyte and assembled into a CR2430 type button cell in a vacuum glove box. Wherein the compacted density of the negative pole piece is 1.50g/cm3The density of the single face is 10mg/cm2。
The assembled cell was allowed to stand at room temperature for 24 hours before electrochemical testing was initiated on a U.S. ArbinBT bt2000 cell tester.
Secondly, carrying out capacity and first efficiency tests, discharging to 0.005V at 0.1C, standing for 20min, discharging to 0.005V at 0.1C to obtain the first lithium intercalation gram capacity of the graphite, standing for 30min, charging to 2.0V at 0.1C, completing the first circulation, and obtaining the first lithium deintercalation gram capacity of the graphite, namely the material capacity. The ratio of the first lithium removal gram capacity to the lithium insertion gram capacity of the graphite is the first efficiency.
A normal temperature 3C constant current SOC test method: and (3) performing 3C constant current discharge on the button type half battery which completes the first circulation to 0.005V on an ArbinBT2000 type battery tester at normal temperature to obtain the corresponding material embedded lithium gram capacity, wherein the percentage of the gram capacity to the lithium removal gram capacity of the first circulation material is the normal-temperature 3C constant current SOC.
Fourthly, a normal temperature 5C constant current SOC test method: and (3) at normal temperature, on an ArbinBT2000 type battery tester, performing 5C constant current discharge on the button type half battery which completes the first circulation to 0.005V to obtain the corresponding material embedded lithium gram capacity, wherein the percentage of the gram capacity to the lithium removal gram capacity of the first circulation material is the normal-temperature 5C constant current SOC.
Sixthly, a 0.2C constant current SOC test method at the temperature of 20 ℃ below zero: and (3) performing 0.2C constant current discharge on the electricity withheld after the first circulation to 0.005V on an ArbinBT2000 type battery tester at the temperature of minus 20 ℃ to obtain the corresponding material lithium insertion gram capacity, wherein the percentage of the gram capacity to the lithium removal gram capacity of the material subjected to the first circulation is 0.2C constant current SOC at the temperature of minus 20 ℃.
(7) And (3) testing the performance of the full battery:
assembling soft package full battery
The negative electrode of the full cell is: the obtained negative electrode material: SP: CMC: SBR accounts for 96 percent, 1 percent, 1.5 percent and 1.5 percent (mass ratio), and the positive electrode of the full battery isLCO: PVDF: SP: KS-6 is 95.5 percent, 2.1 percent, 1.2 percent and 1.2 percent (mass ratio), and the electrolyte of the full battery is 1M-LiPF6DMC, EMC is 1:1:1 (volume ratio) solution, the designed capacity of the positive electrode is 145mAh/g, the designed capacity of the negative electrode is 360mAh/g, the designed excess coefficient is 1.10, and the compaction density of the pole piece of the negative electrode material is 1.50g/cm3The density of the single face is 10mg/cm2。
Electrochemical performance testing was performed on an arbinbbt 2000 model us battery tester.
Secondly, performing 3C constant current test, wherein the charging and discharging voltage range is 3.0V-4.2V, the discharging rate is 1C, the charging rate is 3C, and the battery is disassembled after 300 weeks to observe the surface of the negative pole piece.
And thirdly, performing 5C constant current test, wherein the charging and discharging voltage range is 3.0V-4.2V, the discharging rate is 1C, the charging rate is 5C, and after 50 weeks, disassembling the battery and observing the surface of the negative pole piece.
2. Specific test results of the negative electrode materials prepared in examples 1 to 9 and comparative examples 1 to 3 are shown in table 2:
test results of the negative electrode materials prepared in Table 2
According to the invention, the prepared natural graphite carbon coated negative electrode material can meet the requirement that the median particle diameter D50 is 6.5-12 mu m and is 1.60g/cm3The OI value of the pole piece under the compacted density is less than 15.0, which shows that the prepared natural graphite carbon coated negative electrode material has smaller particles and good isotropy, and can solve the defect of large expansion of the natural graphite negative electrode material.
The process quality analysis will be performed for examples 1 to 9 and comparative examples 1 to 3 as follows:
example 2 compared with example 1, the coke value of the asphalt is increased from 50% to 70%, the adding ratio is decreased from 100:15 to 100:10.8, and the design of the carbon residue is kept unchanged. Example 2 is higher in BET specific surface area and lower in primary efficiency than example 1, and is presumably caused by poor coating uniformity, indicating that high coking value pitch is not favorable for achieving uniform coating.
Example 3 compared with example 1, the natural graphite raw material with the grain size of SG06-1 is adopted, the tap density of the material is improved, and the specific surface is reduced, but the electrochemical test results of 3C and 5C of a half cell and a full cell are deteriorated, which shows that the size of the raw material is increased, and the rate capability of the material is not good.
Example 4 compared to examples 1 and 3, SG06-2 used narrowed the particle size distribution, increased the tap density of the material, reduced BET, improved the first efficiency and power performance, while maintaining d50 at 6.5 μm, indicating that the material prepared from the narrow distribution feedstock had better overall properties.
In example 5, compared with examples 1, 3 and 4, the SG07 natural graphite material with a larger particle size was used, and as a result, the tap density of the material was improved, but the rate capability of the material was significantly lowered.
Example 6 compared with example 4, the addition amount of the liquid resin is increased from 100:8 to 100:12, the particle size of the material is increased, the tap density, the specific surface area and the powder compaction are all reduced, the quick filling performance is obviously improved, the increase of the coating amount improves the material compounding degree, so that the power is improved, but the designable compaction density is low due to the hardening of the material, and the energy density is influenced.
In example 7, compared with example 4, the carbonization temperature is increased from 1100 ℃ to 1300 ℃, the capacity, the first efficiency and the powder pressure of the material are improved, and the quick charging performance is slightly poor.
In the above seven examples, the spherical graphite is subjected to carbon coating modification, and the particle size and distribution of the raw materials, the coking value of the asphalt, the asphalt addition ratio and the liquid resin addition ratio are simply examined. By comprehensively considering the performances of the material in processability, energy density and power performance, examples 4 and 7 are relatively excellent schemes, wherein the multiplying power and low-temperature performance of example 4 are more prominent, and the capacity and the first efficiency of example 7 are more prominent.
Compared with example 4, in example 8, the carbonization temperature is reduced from 1150 ℃ to 900 ℃, and the capacity and the primary efficiency of the final material are greatly reduced.
In example 9, compared with example 4, the carbonization temperature is increased from 1150 ℃ to 2000 ℃, the capacity and the first efficiency of the final material are slightly improved, but the multiplying power and the low-temperature performance are reduced too much.
In comparative example 1, the raw material particle diameter D50 was changed to 8.5 μm as compared with example 4, and the tap density of the material was improved, but the magnification and the low-temperature performance were excessively lowered.
Comparative example 2 compared with example 4, the particle size D50 of the raw material was changed to 4.7 μm, the particle size distribution was broader, and the tap density of the raw material was reduced to 0.48g/cm3The tap density of the final material is only 0.77g/cm3Resulting in material loss during processing and failure to test electrical properties.
Comparative example 3 compared with example 4, the addition of the asphalt is increased from 100:15 to 100:25, the sum of the mass fractions of the soft carbon structure and the hard carbon structure in the material is over 10 percent, and the material capacity and the efficiency are too low.
As can be seen from FIGS. 6 and 7, the surface morphology of the material of comparative example 4 is not significantly different from that of the uncoated natural graphite material, because the pitch is not soft and ductile at normal temperature and the coating uniformity is not good. As seen from FIG. 8, the two-step coating method of hot asphalt-liquid phase resin adopted in the present invention provides a better and uniform modification of the graphite surface, and the material surface is significantly smoother than the former two.
It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrative embodiments, and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.
Furthermore, it should be understood that although the present description refers to embodiments, not every embodiment may contain only a single embodiment, and such description is for clarity only, and those skilled in the art should integrate the description, and the embodiments may be combined as appropriate to form other embodiments understood by those skilled in the art.
Claims (10)
1. A preparation method of a natural graphite carbon-coated negative electrode material is characterized by comprising the following steps:
s1, performing heat treatment on a mixture of natural graphite and asphalt to obtain natural graphite with the surface coated with the asphalt;
wherein the median particle diameter D50 of the natural graphite is 5.0-7.5 μm, and the D10/D90 of the natural graphite is more than 0.35; the coking value of the asphalt is 30-80%; the mass ratio of the natural graphite to the asphalt is 100: 8-100: 30;
and S2, carbonizing the mixture of the natural graphite with the surface coated with the asphalt and the liquid resin.
2. The method according to claim 1, wherein in S1, the natural graphite has a median particle diameter D50 of 5.5 to 7.5 μm, for example, 5.5 μm, 6.5 μm, or 7.5 μm;
and/or in S1, the natural graphite has D10/D90 of 0.35-0.40;
and/or in S1, the tap density of the natural graphite powder is more than or equal to 0.50g/cm3Ash content is less than or equal to 0.10 percent;
preferably, the tap density of the natural graphite powder is 0.5-0.73 g/cm3For example, 0.5g/cm3、0.58g/cm3、0.65g/cm3Or 0.73g/cm3。
3. The process according to claim 1, wherein in S1, the pitch has a coking value of 30% to 70%, preferably 30% to 60%, for example 50%;
and/or, in S1, the pitch has a median particle diameter D50 ≦ 15.0 μm, for example 8.0 μm.
4. The method according to claim 1, wherein in S1, the mass ratio of the natural graphite to the asphalt is 100:8 to 100:20, preferably 100:8 to 100:15, such as 100:8, 100:10.8 or 100: 15.
5. The production method according to claim 1, wherein in S1, the heat treatment is a heat treatment under stirring;
and/or in S1, the temperature of the heat treatment is 400-800 ℃, for example 550 ℃;
and/or in S1, the heat treatment time is 2-3 h;
and/or, in S1, cooling the heat-treated product, wherein the temperature of the cooled natural graphite with the surface coated with the asphalt is less than 100 ℃, for example less than 50 ℃.
6. The process according to claim 1, wherein the liquid resin is a liquid resin having a scorch value of 10% to 30%, preferably 15% to 25%, for example 20%;
and/or in S2, the liquid resin is one or more of epoxy resin, phenolic resin, polyacrylonitrile, polyvinyl alcohol, polystyrene, polyvinylpyrrolidone, polyacrylic acid, polyvinyl chloride, catalytic cracking slurry oil and asphalt;
and/or in S2, the C content of the liquid resin is more than or equal to 99.5 percent;
and/or in S2, the sum of the contents of the metal elements in the liquid resin is less than or equal to 100 ppm.
7. The method according to claim 1, wherein the mass ratio of the natural graphite coated with the asphalt to the liquid resin is 100:5 to 100:20, preferably 100:8 to 100:12, such as 100:8 or 100: 12;
and/or, in S2, the carbonization treatment is static heating operation in an inert atmosphere;
and/or in S2, the temperature of the carbonization treatment is 1000-1700 ℃, preferably 1100-1300 ℃;
and/or in S2, the carbonization treatment time is 2-5 h, for example 3 h;
and/or in S2, cooling the product after carbonization, wherein the temperature of the cooled natural graphite carbon coated negative electrode material is less than 100 ℃, for example less than 50 ℃;
and/or, in S2, after the carbonization treatment, screening and demagnetizing the obtained product.
8. The method according to claim 1, wherein the total mass of the coating layer formed by pitch and the coating layer formed by liquid resin accounts for 5 to 10% of the mass of the natural graphite carbon-coated negative electrode material, preferably 8.0 to 9.5%, such as 8.4% or 9.1%.
9. The natural graphite carbon-coated negative electrode material prepared by the preparation method of any one of claims 1 to 8.
10. A lithium ion battery, wherein the negative electrode material of the lithium ion battery is coated with the natural graphite carbon as described in any one of claims 1 to 8.
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