CN112928259B - Graphite material and preparation method and application thereof - Google Patents
Graphite material and preparation method and application thereof Download PDFInfo
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- CN112928259B CN112928259B CN202110181734.4A CN202110181734A CN112928259B CN 112928259 B CN112928259 B CN 112928259B CN 202110181734 A CN202110181734 A CN 202110181734A CN 112928259 B CN112928259 B CN 112928259B
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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
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/36—Selection of substances as active materials, active masses, active liquids
- H01M4/362—Composites
- H01M4/366—Composites as layered products
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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
- H01M10/00—Secondary cells; Manufacture thereof
- H01M10/05—Accumulators with non-aqueous electrolyte
- H01M10/052—Li-accumulators
- H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
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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
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- 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
- H01M4/583—Carbonaceous material, e.g. graphite-intercalation compounds or CFx
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
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Abstract
The invention relates to a graphite material and a preparation method and application thereof. The graphite material comprises core-shell structure particles, wherein the core-shell structure particles comprise a graphite core, a first coating layer and a second coating layer, the first coating layer is coated outside the graphite core, the second coating layer is coated outside the first coating layer, and the void ratio of the first coating layer is larger than that of the second coating layer. By utilizing the property that the graphite core precursor expands and contracts under the heating at different temperatures, the surface of the graphite core precursor is coated through multi-stage heat treatment, so that the graphite material with the first coating layer porosity being greater than the second coating layer porosity is formed. This cladding mode not only provides flexible shell for the graphite kernel, utilizes the great first cladding of porosity to alleviate graphite volume expansion, maintains the stable in structure of graphite kernel and second cladding simultaneously, promotes graphite material structural stability, promotes lithium ion battery cycle performance, has also guaranteed the processing manufacturability of material.
Description
Technical Field
The invention belongs to the field of battery materials, and particularly relates to a graphite material, and a preparation method and application thereof.
Background
Recycling of waste graphite electrode materials can realize recycling of resources, but after the battery is cycled for a long time, the interface of the graphite material is roughened and SEI film deposition occurs, and in addition, interlayer co-embedding of small molecular organic solvents can also cause graphite performance reduction, so that the graphite material can not be directly reused for battery manufacturing. The conventional graphite regeneration method is to use strong inorganic acid (H) first2SO4HCl) to leach impurities deposited on the surface of the graphite, and then to carry out coating treatment to improve the interface performance of the graphite. The coating material generally adopts rigid carbon materials as the main material, and the structure is generally direct coating and core-shell gap coating. The direct coating mode can not realize effective continuous coating on the graphite surface, so that the coating effect is poor, and in the circulating process, a rigid coating layer can be peeled off due to the volume expansion of graphite; core-shell void coating can alleviate material expansion, but core-shell void structures cannot be realizedThe current effective contact with the interface layer leads to poor electronic conductivity, and the core-shell void structure cannot relieve the uneven expansion of the graphite material, so that the shell structure can still crack in the circulating process.
Therefore, it is desired to develop a graphite material capable of alleviating the expansion of the material and maintaining the stable structure of the coating layer. And graphite material with good conductivity.
Disclosure of Invention
In order to solve the problems, the invention provides a graphite material and a preparation method and application thereof.
The invention provides a graphite material which comprises core-shell structure particles, wherein the core-shell structure particles comprise a graphite inner core, a first coating layer and a second coating layer, the first coating layer is coated outside the graphite inner core, the second coating layer is coated outside the first coating layer, and the void ratio of the first coating layer is larger than that of the second coating layer.
In another aspect, the present invention provides a method for preparing a graphite material, comprising: coating, namely coating a coating layer outside a graphite core precursor to obtain a coating material, wherein the graphite core precursor is a graphite material with a layer containing a solvent; and a heat treatment step, which comprises a first stage of heat treatment of the coating material to expand the volume of the graphite core precursor to obtain an expanded coating material, a second stage of heat treatment of the expanded coating material to solidify the coating shell structure to obtain a solidified coating material, and a third stage of heat treatment of the solidified coating material to shrink the volume of the graphite core precursor to obtain a first coating layer coated outside the graphite core and a second coating layer coated outside the first coating layer, wherein the void ratio of the first coating layer is greater than that of the second coating layer.
According to the invention, by utilizing the thermal expansion and contraction properties of the graphite core precursor at different temperatures, through multi-stage heat treatment, coating treatment is carried out on the surface of the graphite core precursor, so that the graphite material with the first coating layer porosity being greater than that of the second coating layer is formed. The coating mode not only provides a flexible shell for the graphite core, but also relieves the graphite volume expansion by utilizing the first coating layer with larger porosity, simultaneously maintains the structural stability of the graphite core and the second coating layer, promotes the structural stability of the graphite material, promotes the cycle performance of the lithium ion battery, and also ensures the processing and manufacturing performance of the material.
Drawings
The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings.
FIG. 1 is a schematic view of the process for preparing the graphite material of the present invention.
Wherein the reference numerals are as follows:
10-a graphite core precursor; 11-graphite; 12-graphite interlayer solvent; 20-coating a shell layer; 21-a first coating layer; 22-second coating layer
Detailed Description
The present invention will be described in detail with reference to the following embodiments.
The source of the graphite core precursor in the patent comprises waste batteries containing graphite electrodes, such as batteries containing graphite electrodes which have problems in post-injection formation, capacity grading and circulation processes and are out of service, and also comprises graphite materials containing solvents in layers of other sources.
"coating" in this patent includes complete coating and incomplete coating (i.e., coating a portion of the surface).
Hereinafter, the production method of the present invention is explained with reference to FIG. 1. As shown in fig. 1, the preparation method of the graphite material comprises a coating step and a heat treatment step. In the coating step, the graphite core precursor 10 is coated with the coating shell layer 20 to obtain the coating material. The graphite core precursor 10 includes graphite 11 and a solvent 12 embedded in the graphite 11 layer. Thereafter, the clad material is heat-treated. The heat treatment step comprises three heat treatment stages, wherein the first heat treatment stage expands the volume of the graphite core precursor to obtain an expanded coating material, the second heat treatment stage thermally treats the expanded coating material to solidify the structure of the coating shell layer 20 to obtain a solidified coating material, and the third heat treatment stage thermally treats the solidified coating material to shrink the volume of the graphite core precursor to obtain a first coating layer 21 coated outside the graphite core and a second coating layer 22 coated outside the first coating layer 21, wherein the void ratio of the first coating layer 21 is larger than that of the second coating layer 22.
The raw material graphite core precursor 10 adopted by the invention and the solvent 12 embedded between the layers of the graphite core precursor 10 can be decomposed in the heating process, so that the interlayer spacing of the graphite core precursor 10 is enlarged, and the volume of the graphite core material is expanded. According to the preparation method, the surface of the graphite core precursor 10 is coated with the coating shell layer 20, and then three-stage heat treatment processes are carried out. In the first stage of heat treatment, the graphite core precursor 10 coated inside is decomposed by the solvent 12 inserted between the layers thereof to cause the graphite core precursor 10 to expand in volume, and the coating shell 20 coated on the surface thereof expands as the graphite core precursor 10 expands. In the second stage of the heat treatment, the cladding shell 20 is cured to form a stable self-supporting cladding structure. In the third stage of heat treatment, the solvent 12 is completely released, the interlamellar regularity of the graphite core precursor 10 is improved at high temperature, the interlamellar spacing is reduced, the volume of the shaped coating shell 20 is not retracted any more, the material of the connecting point of the inner surface of the coating shell 20 and the graphite core precursor 10 can extend along with the volume shrinkage of the graphite core precursor 10, and the covering layer is pulled, so that the coating shell 20 forms a first coating layer 21 with relatively high porosity and a second coating layer 22 with relatively low porosity, which are connected with the graphite core, on the outer surface of the first coating layer. The connecting point that exists between first coating 21 and the graphite kernel plays the supporting role to the graphite kernel, and because first coating 21's void ratio is big, provides accommodation space for graphite kernel inflation at graphite lithium intercalation in-process, can effectively alleviate the coating structural damage that graphite inflation and lead to, has promoted the cycling stability of material. The graphite regeneration technology remodels the interface structure of the material, not only can realize effective cladding of a rough interface and relieve volume expansion brought in the circulation process, but also does not influence the processing and manufacturing performance of the material, and can realize good industrial application effect.
In an alternative embodiment, the encasing shell 20 comprises a flexible material. The flexible material is selected from one or more of one-dimensional carbon materials, one-dimensional transition metal materials and one-dimensional metal oxide materials. The one-dimensional carbon material may include one or more of carbon fiber, carbon nanotube, cellulose fiber, and lignin fiber. The one-dimensional transition metal material can comprise one or more of cobalt fiber, titanium fiber and tungsten fiber material. The one-dimensional metal oxide material may comprise one or more of alumina fibers, zirconia fibers, titania fibers. When the flexible material is an inorganic material, the coating shell layer 20 may further include a binder, and the binder and the flexible material form a slurry, so that the coating shell layer 20 is formed on the surface of the graphite core precursor 10, and the adhesion between the coating layer (i.e., the flexible material) and the core is increased. The adhesive is preferably an adhesive with good film forming property, so that a film can be formed on the surface of the flexible material better, and a better bonding point can be formed between the flexible materials, so that the structure of the cladding shell layer 20 is more stable. The adhesive can be one or more of but not limited to epoxy resin, polyvinyl alcohol and polyvinylpyrrolidone. When the flexible material is a polymer material, the polymer material may not contain an adhesive because of its certain viscosity, and the object of the present invention can be achieved by containing an adhesive. The coating means may be any conventional means such as, but not limited to, spray drying, solution dip coating, mechanical melt coating, solid phase coating, and the like.
In an alternative embodiment, the first stage heat treatment temperature is 200-450 ℃ and the treatment time is 2-6 h. This heat treatment stage decomposes the solvent 12 in the graphite 11 layer, thereby expanding the inter-layer spacing of the graphite core precursor 10, causing the volume of the graphite core precursor 10 to expand, which in turn causes the coated flexible material layer to expand. Preferably, the first stage heat treatment temperature is 300-400 ℃, and the treatment time is 4-5 h.
In an alternative embodiment, the temperature of the second-stage heat treatment is 600-850 ℃, and the treatment time is 2-6 h. In the stage, the bonding agent at the connecting point of the inner surface of the coating shell layer 20 and the inner core is carbonized and/or the high polymer fiber material is carbonized, so that the coating layer has certain structural strength, the structural flexibility of the coating shell layer 20 is reduced, the coating shell layer 20 is cured to form a stable self-supporting coating layer structure, and the connecting point of the inner surface of the coating shell layer 20 and the surface of the graphite inner core precursor 10 still exists stably. Preferably, the temperature of the second-stage heat treatment is 700-.
In an alternative embodiment, the temperature of the heat treatment in the third stage is 1000-1600 ℃ and the treatment time is 1-4 h. At this stage, the solvent 12 in the graphite 11 layer is completely released, the regularity between the graphite core precursor 10 layers is improved at a high temperature, so that the interlayer spacing of the expanded graphite core precursor 10 is reduced, the volume of the shaped coating shell layer 20 is not retracted any more, and the flexible material at the joint point of the inner surface of the coating shell layer 20 and the core is stretched along with the contraction of the volume of the graphite core precursor 10, so that the coating shell layer 20 forms a first coating layer 21 with relatively large porosity for connecting the graphite core and a second coating layer 22 with relatively small porosity for coating the outer surface of the first coating layer. Preferably, the third-stage heat treatment temperature is 1200-1500 ℃, and the treatment time is 2-3 h.
In an alternative embodiment, the coating shell layer 20 is included in an amount of 5 to 20% by mass, based on 100% by mass of the total mass of the coating material. When the mass content of the coating shell layer 20 is less than 5%, the coating effect on the graphite core precursor 10 is poor, a complete coating shell layer structure cannot be formed, and the graphite volume expansion cannot be effectively relieved. When the mass content of the coating shell 20 is higher than 20%, the material content of the coating shell 20 is higher, which causes the gram volume of the graphite material to be reduced. One skilled in the art may select any value within the above ranges as is practical, such as but not limited to 5%, 10%, 15%, 20%, etc.
In an alternative embodiment, the coating step further includes the steps of pickling, washing with water, and drying the graphite core precursor 10. Impurities in the graphite core precursor 10 are removed through acid leaching and water washing steps, and the electrochemical performance of the prepared graphite material is further improved. The acid leaching and water washing steps can be, but are not limited to, impurity removal by using inorganic acid and hydrogen peroxide. For example, the inorganic acid is sulfuric acid or hydrochloric acid, preferably sulfuric acid; the acid concentration is 0.5-2mol/L, preferably 0.8-1 mol/L; the amount of the hydrogen peroxide accounts for 2-5 percent of the mass of the graphite, and is preferably 3-4 percent; controlling the solid-to-liquid ratio to be 60-120g/L, preferably 80-100 g/L; the reaction temperature is 40-60 ℃, and preferably 50 ℃; the reaction time is 0.5-2.0h, and the preferable time is 1 h; and after the reaction is finished, washing the graphite core precursor with pure water for multiple times until the pH value is 6-7, and then drying the graphite core precursor for later use.
When the graphite material is the graphite material recovered from waste batteries, the recovery mode can be, but is not limited to, soaking the disassembled negative pole piece in pure water to wash and strip graphite. The washing and stripping conditions can be, but are not limited to, controlling the solid-liquid ratio to be 100-200g/L, and the preferable solid-liquid ratio to be 120-150 g/L; and after soaking for 1-3h, the copper foil is pulled away, and the preferred soaking time is 2 h. The graphite material may include natural modified graphite, artificial graphite, or mesocarbon microbeads (MCMB), or the like.
The present invention is further described below by way of specific examples. However, these examples are only illustrative and do not set any limit to the scope of the present invention.
In the following examples and comparative examples, reagents, materials and instruments used therefor were commercially available unless otherwise specified.
Example 1
And (3) soaking the disassembled negative pole piece in pure water, controlling the solid-to-liquid ratio to be 120g/L, soaking for 2h, washing, collecting and washing the graphite core precursor. And (3) carrying out acid washing on the washed graphite core precursor by using 1mol/L sulfuric acid and 5% (accounting for the mass of the graphite core precursor) hydrogen peroxide, reacting for 1h at 50 ℃ with a solid-to-liquid ratio of 80g/L, and collecting and drying after the acid washing is finished. And mixing 10 percent (accounting for the total amount of the graphite core precursor and the cellulose fibers) of cellulose fibers with the graphite core precursor after impurity removal, and performing spray drying at the drying temperature of 150 ℃.
And carrying out segmented high-temperature heat treatment on the graphite core precursor coated with the cellulose fibers. The temperature of the first stage of heat treatment is 200 ℃, the air atmosphere is maintained, the temperature rise rate is 2 ℃/min, and the heat preservation time is 6 h; the temperature of the second stage of heat treatment is 600 ℃, the argon atmosphere is maintained, the heating rate is 10 ℃/min, and the heat preservation time is 4 h; the temperature of the third stage of heat treatment is 1000 ℃, the heating rate is 10 ℃/min, and the heat preservation time is 4 h. Preparing the graphite material for later use.
The prepared graphite material was subjected to a powder resistivity test using a "semiconductor powder resistivity tester". The graphite material powder to be tested is pressed into a sample with a fixed shape and size, a pair of forced currents are used for passing through the sample, and the resistance of the sample is tested by measuring the voltage drop through a pair of leads.
The graphite material was subjected to a void fraction difference test to characterize the difference in void fractions of the first and second coating layers in the graphite material. Uniformly mixing 3 g of graphite material powder material to be measured with 2 g of resin, placing the mixture in a mold, heating the mixture at 60 ℃ for 4h, curing, taking out the cured section, and cutting the section by using an ion polisher to finish sample preparation; and (3) placing the prepared sample under a scanning electron microscope, and observing the appearance of the coating layer.
The prepared graphite material is assembled into a single-chip battery, and the assembled single-chip battery is tested to test the first discharge efficiency and the capacity retention rate after 500 cycles of the single-chip battery. The negative electrode of the monolithic cell included the graphite material prepared above: SP: the CMC is 94.5:1.5:1.5:2.5, a PE diaphragm is adopted, the positive active material is NCM811 nickel cobalt manganese ternary material, and the design capacity of the battery is 250 mAh. The voltage range of the battery test is 0.005-2.0V, the multiplying power of the normal temperature cycle test is 1C, the voltage range is 2.75-4.25V, and the test is 500 circles.
Through the test, the test result shows that the coating layer of the graphite material prepared in the embodiment is of a double-layer structure, the graphite core is supported by the first coating layer and is not contacted with the second coating layer, and the void ratio of the first coating layer is greater than that of the second coating layer; the powder resistivity was 0.58 m.OMEGA.cm; the reversible gram capacity is 350mAh/g, and the first discharge efficiency is 92.5 percent; the capacity retention rate after 500 cycles was 98.2%.
Example 2
The recovery, decontamination and coating processes were the same as in example 1.
High-temperature heat treatment conditions: the temperature of the first stage of heat treatment is 300 ℃, the air atmosphere is maintained, the temperature rise rate is 2 ℃/min, and the heat preservation time is 4 h; the temperature of the second stage of heat treatment is 700 ℃, the argon atmosphere is maintained, the heating rate is 10 ℃/min, and the heat preservation time is 3 h; the temperature of the third stage of heat treatment is 1200 ℃, the heating rate is 5 ℃/min, and the heat preservation time is 2 h. Preparing the graphite material for later use.
A single cell was assembled in the same manner as in example 1, and the graphite material obtained and the assembled single cell were tested in the same manner as in example 1. The test result shows that the coating layer of the graphite material prepared in the example has a double-layer structure, the graphite core is supported by the first coating layer and is not contacted with the second coating layer, and the void ratio of the first coating layer is larger than that of the second coating layer; the powder resistivity was 0.61 m.OMEGA.cm; the reversible gram capacity is 356mAh/g, and the first discharge efficiency is 93.7%; the capacity retention rate after 500 cycles was 99.8%.
Example 3
The recovery, decontamination and coating processes were the same as in example 1.
High-temperature heat treatment conditions: the temperature of the first stage of heat treatment is 400 ℃, the air atmosphere is maintained, the temperature rise rate is 2 ℃/min, and the heat preservation time is 5 h; the temperature of the second stage of heat treatment is 800 ℃, the argon atmosphere is maintained, the heating rate is 10 ℃/min, and the heat preservation time is 4 h; the temperature of the third stage of heat treatment is 1500 ℃, the heating rate is 5 ℃/min, and the heat preservation time is 3 h. Preparing the graphite material for later use.
A single cell was assembled in the same manner as in example 1, and the graphite material obtained and the assembled single cell were tested in the same manner as in example 1. The test result shows that the coating layer of the graphite material prepared in the example is of a double-layer structure, the graphite core is supported by the first coating layer and is not contacted with the second coating layer, and the void ratio of the first coating layer is larger than that of the second coating layer; the powder resistivity was 0.62 m.OMEGA.cm; the reversible gram capacity is 356mAh/g, and the first discharge efficiency is 93.8%; the capacity retention after 500 cycles was 99.7%.
Example 4
The recovery, decontamination and coating processes were the same as in example 1.
High-temperature heat treatment conditions: the temperature of the first stage of heat treatment is 450 ℃, the air atmosphere is maintained, the temperature rise rate is 2 ℃/min, and the heat preservation time is 2 h; the temperature of the second stage of heat treatment is 850 ℃, the argon atmosphere is maintained, the heating rate is 10 ℃/min, and the heat preservation time is 2 h; the temperature of the third stage of heat treatment is 1600 ℃, the heating rate is 5 ℃/min, and the heat preservation time is 1 h. Preparing the graphite material for later use.
A monolithic battery was assembled in the same manner as in example 1, and the graphite material obtained and the assembled monolithic battery were tested in the same manner as in example 1. The test result shows that the coating layer of the graphite material prepared in the example is of a double-layer structure, the graphite core is supported by the first coating layer and is not contacted with the second coating layer, and the porosity of the pores of the first coating layer is greater than that of the second coating layer; the powder resistivity was 0.61 m.OMEGA.cm; the reversible gram capacity is 352mAh/g, and the first discharge efficiency is 92.8%; the capacity retention rate after 500 cycles was 98.5%.
Example 5
The conditions were the same as in example 2 except that the content of the cellulose fibers as the coating layer was 5% (based on the total amount of the graphite core precursor and the cellulose fibers).
A single cell was assembled in the same manner as in example 1, and the graphite material obtained and the assembled single cell were tested in the same manner as in example 1. The test result shows that the coating layer of the graphite material prepared in the example is of a double-layer structure, the graphite core is supported by the first coating layer and is not contacted with the second coating layer, and the porosity of the pores of the first coating layer is greater than that of the second coating layer; the powder resistivity was 0.58 m.OMEGA.cm; the reversible gram capacity is 352mAh/g, and the first discharge efficiency is 92.1 percent; the capacity retention after 500 cycles was 97.2%.
Example 6
The conditions were the same as in example 2 except that the content of the cellulose fiber as the coating layer was 12.5% (based on the total amount of the graphite core precursor and the cellulose fiber).
A single cell was assembled in the same manner as in example 1, and the graphite material obtained and the assembled single cell were tested in the same manner as in example 1. The test result shows that the coating layer of the graphite material prepared in the example is of a double-layer structure, the graphite core is supported by the first coating layer and is not contacted with the second coating layer, and the porosity of the pores of the first coating layer is greater than that of the second coating layer; the powder resistivity was 0.60 m.OMEGA.cm; the reversible gram capacity is 357mAh/g, and the first discharge efficiency is 93.8%; the capacity retention rate after 500 cycles was 99.8%.
Example 7
The conditions were the same as in example 2 except that the content of the cellulose fibers as the coating layer was 20% (based on the total amount of the graphite core precursor and the cellulose fibers).
A single cell was assembled in the same manner as in example 1, and the graphite material obtained and the assembled single cell were tested in the same manner as in example 1. The test result shows that the coating layer of the graphite material prepared in the example is of a double-layer structure, the graphite core is supported by the first coating layer and is not contacted with the second coating layer, and the porosity of the pores of the first coating layer is greater than that of the second coating layer; the powder resistivity was 0.56 m.OMEGA.cm; the reversible gram capacity is 349mAh/g, and the first discharge efficiency is 92.0 percent; the capacity retention rate after 500 cycles was 98.1%.
Example 8
The recovery, impurity removal and heat treatment processes were the same as in example 2. Aluminum oxide fibers are adopted for coating, the aluminum oxide fibers are dispersed in a polyvinyl alcohol solution to form slurry (the aluminum oxide fibers account for 3% of the total amount of the aluminum oxide fibers, the polyvinyl alcohol and the graphite core precursor, and the polyvinyl alcohol accounts for 2% of the total amount of the aluminum oxide fibers, the polyvinyl alcohol and the graphite core precursor) in a liquid phase coating mode, the graphite core precursor is dispersed into the system, the temperature is kept at 100 ℃, and the prepared coating material is stirred, filtered, dried and thermally treated to prepare the graphite material for later use.
A single cell was assembled in the same manner as in example 1, and the graphite material obtained and the assembled single cell were tested in the same manner as in example 1. Test results show that the coating layer of the graphite material prepared in the embodiment is of a double-layer structure, the graphite core is supported by the first coating layer and is not contacted with the second coating layer, and the porosity of the pores of the first coating layer is larger than that of the second coating layer; the powder resistivity was 0.55 m.OMEGA.cm; the reversible gram capacity is 356mAh/g, and the first discharge efficiency is 93.8%; the capacity retention after 500 cycles was 99.7%.
Example 9
The titanium fiber was substituted for the alumina fiber, and the graphite material was prepared in the same manner as in example 8, except for the use thereof.
A single cell was assembled in the same manner as in example 1, and the graphite material obtained and the assembled single cell were tested in the same manner as in example 1. The test result shows that the coating layer of the graphite material prepared in the example is of a double-layer structure, the graphite core is supported by the first coating layer and is not contacted with the second coating layer, and the porosity of the pores of the first coating layer is greater than that of the second coating layer; the powder resistivity was 0.61 m.OMEGA.cm; the reversible gram capacity is 355mAh/g, and the first discharge efficiency is 93.4%; the capacity retention after 500 cycles was 99.6%.
Example 10
The carbon nanotubes were substituted for alumina fibers, and graphite materials were prepared for use under the same conditions as in example 8.
A single cell was assembled in the same manner as in example 1, and the graphite material obtained and the assembled single cell were tested in the same manner as in example 1. The test result shows that the coating layer of the graphite material prepared in the example is of a double-layer structure, the graphite core is supported by the first coating layer and is not contacted with the second coating layer, and the porosity of the pores of the first coating layer is greater than that of the second coating layer; the powder resistivity was 0.62 m.OMEGA.cm; the reversible gram capacity is 356mAh/g, and the first discharge efficiency is 93.6%; the capacity retention after 500 cycles was 99.6%.
Example 11
The epoxy resin was substituted for polyvinyl alcohol, and the graphite material was prepared for use in the same manner as in example 10 except that the conditions were changed.
A single cell was assembled in the same manner as in example 1, and the graphite material obtained and the cell assembled with the single cell were tested in the same manner as in example 1. The test result shows that the coating layer of the graphite material prepared in the example is of a double-layer structure, the graphite core is supported by the first coating layer and is not contacted with the second coating layer, and the porosity of the pores of the first coating layer is greater than that of the second coating layer; the powder resistivity was 0.61 m.OMEGA.cm; the reversible gram capacity is 355mAh/g, and the first discharge efficiency is 93.3%; the capacity retention rate after 500 cycles was 99.2%.
Example 12
The graphite material was prepared in the same manner as in example 10 except that polyvinyl pyrrolidone was used instead of polyvinyl alcohol, and was then used.
A single cell was assembled in the same manner as in example 1, and the graphite material obtained and the assembled single cell were tested in the same manner as in example 1. The test result shows that the coating layer of the graphite material prepared in the example is of a double-layer structure, the graphite core is supported by the first coating layer and is not contacted with the second coating layer, and the porosity of the pores of the first coating layer is greater than that of the second coating layer; the powder resistivity was 0.62 m.OMEGA.cm; the reversible gram capacity is 355mAh/g, and the first discharge efficiency is 93.2%; the capacity retention rate after 500 cycles was 99.1%.
Comparative example 1
The recovery, decontamination and coating processes were the same as in example 1. The graphite material coated with the cellulose fiber is directly subjected to high-temperature heat treatment, wherein the heat treatment temperature is 1500 ℃, the heating rate is 10 ℃/min, and the heat preservation time is 2 h.
A single cell was assembled in the same manner as in example 1, and the graphite material obtained and the assembled single cell were tested in the same manner as in example 1. Test results show that the prepared graphite material is of a core-shell structure coated with a coating layer, wherein the core and the shell are tightly combined; the powder resistivity was 0.57 m.OMEGA.cm; the reversible gram capacity of the material is 332mAh/g, and the first-turn efficiency is 86.5 percent; the capacity retention rate after 500 cycles of 1C circulation at normal temperature is 82.1%.
Comparative example 2
The recovery and impurity removal processes were the same as in example 1. In the coating step, first, use
Method for carrying out SiO on graphite core precursor material2Coating, then coating the obtained SiO in the same manner as in example 12And (3) coating the coated material, and directly carrying out high-temperature heat treatment on the coated graphite material at the temperature of 1500 ℃, at the heating rate of 10 ℃/min and for 2 h. The heat treated material was then etched with a 5% hydrofluoric acid solution to remove the SiO2The coating layer forms a core-shell structure with a gapA material. Preparing the graphite material for later use.
A single cell was assembled in the same manner as in example 1, and the graphite material obtained and the assembled single cell were tested in the same manner as in example 1. The test result shows that the prepared graphite material is a core-shell structure with a single-layer coating layer provided with a gap, a gap is formed between the graphite core and the single-layer coating layer, and the graphite core is locally contacted with the single-layer coating layer; the powder resistivity was 0.56 m.OMEGA.cm; the reversible gram capacity of the graphite material is 342mAh/g, and the first-turn efficiency is 88.5 percent; the capacity retention rate after 500 cycles of 1C circulation at normal temperature is 87.3 percent.
Comparative example 3
The recovery, decontamination and coating processes were the same as in example 1. The temperature of the first stage of heat treatment is 100 ℃, the air atmosphere is kept, the temperature rising rate is 2 ℃/min, and the heat preservation time is 6 h; the temperature of the second stage of heat treatment is 500 ℃, the argon atmosphere is maintained, the heating rate is 10 ℃/min, and the heat preservation time is 6 h; the temperature of the third stage of heat treatment is 900 ℃, the heating rate is 5 ℃/min, and the heat preservation time is 4 h. Preparing the graphite material for later use.
A single cell was assembled in the same manner as in example 1, and the graphite material obtained and the assembled single cell were tested in the same manner as in example 1. The test result shows that the coating layer of the prepared graphite material is discontinuous; the resistivity of the powder is 0.38m omega cm, the reversible gram capacity of the material is 346mAh/g, and the first-turn efficiency is 90.1 percent; the capacity retention rate after 500 cycles of 1C circulation at normal temperature is 92.4 percent.
Comparative example 4
The recovery, decontamination and coating processes were the same as in example 1. The temperature of the first stage of heat treatment is 550 ℃, the air atmosphere is maintained, the temperature rise rate is 2 ℃/min, and the heat preservation time is 2 h; the temperature of the second stage heat treatment is 950 ℃, the argon atmosphere is maintained, the heating rate is 10 ℃/min, and the heat preservation time is 2 h; the temperature of the third stage of heat treatment is 1700 ℃, the heating rate is 5 ℃/min, and the heat preservation time is 1 h.
A single cell was assembled in the same manner as in example 1, and the graphite material obtained and the assembled single cell were tested in the same manner as in example 1. The test result shows that the prepared material coating layer is discontinuous; the powder resistivity was 0.57 m.OMEGA.cm; the reversible gram capacity of the material is 333mAh/g, and the first-turn efficiency is 86.1%; the capacity retention rate after 500 cycles of 1C circulation at normal temperature is 89.2%.
Comparative example 5
The recovery, impurity removal and heat treatment processes were the same as in example 2. The amount of the cellulose fiber coating is adjusted to 1 percent (accounting for the total amount of the cellulose fiber and the graphite core precursor).
A single cell was assembled in the same manner as in example 1, and the graphite material obtained and the assembled single cell were tested in the same manner as in example 1. The test result shows that the coating layer of the prepared graphite material is incomplete; the resistivity of the powder is 0.42m omega cm, the reversible gram capacity of the material is 341mAh/g, and the first-turn efficiency is 85.5 percent; the capacity retention rate after 500 cycles of 1C circulation at normal temperature is 85.7%.
Comparative example 6
The recovery, impurity removal and heat treatment processes were the same as in example 2. The amount of the cellulose fiber coating is adjusted to 24 percent (accounting for the total amount of the cellulose fiber and the graphite core precursor).
A single cell was assembled in the same manner as in example 1, and the graphite material obtained and the assembled single cell were tested in the same manner as in example 1. Test results show that the prepared graphite material coating layer is of a double-layer structure, the porosity of the inner layer is higher than that of the outer layer, and the coating layer is thicker; the resistivity of the coated powder is 0.48m omega cm, the reversible gram capacity of the material is 338mAh/g, and the first-turn efficiency is 86.4 percent; the capacity retention rate after 500 cycles of 1C circulation at normal temperature is 88.6 percent.
The test results of the graphite materials prepared in examples 1 to 12 and comparative examples 1 to 6 after assembling into a monolithic cell are shown in table 1.
TABLE 1
And (4) analyzing results: the results of the examples 1 to 4 and the comparative example 1 show that the resistivity, the gram volume and the first effect of the graphite material powder prepared by the method are equivalent to those of the graphite material prepared by the conventional one-step high-temperature method, but the cycle performance of the graphite material prepared by the method is better because the graphite material coating shell prepared by the conventional one-step high-temperature calcination method has no void structure for absorbing graphite expansion, the structural strength of the coating shell is not enough to resist the volume expansion and contraction stress in the graphite cycle process, and the damage of the coating shell structure is easily caused in the cycle process, so that the cycle performance is attenuated.
The results of the analysis examples 1-4 show that when the first stage heat treatment temperature is 300-, the prepared material has excellent performances in all aspects, because the graphite material prepared under the condition has excellent structural characteristics of a coating shell layer, namely, the void ratio of the first coating layer outside the graphite core is larger than that of the second coating layer, and the first coating layer and the graphite core have more contact sites, the coating shell layer can effectively buffer the volume expansion of graphite, and abundant contact points can maintain the structural stability of the structural graphite material and improve the conductivity of the graphite material, so that the coating shell layer has better comprehensive performance.
The results of the examples 1 to 4 and the comparative example 2 are combined to show that the graphite material powder prepared by the method has lower resistivity, which indicates that the graphite material prepared by the method has higher electron conduction capability, more contact sites exist between the graphite core and the coating shell, and the core-shell structure material prepared by the comparative example 2 has a gap, and only partial contact exists between the core and the shell, so that the resistivity of the material powder is higher; the porosity of the graphite material prepared by the method is similar in all directions, the expansion stress borne by each part of the coating shell layer is uniform, and the structure of the coating shell layer can be better maintained, while the material prepared in the embodiment 2 does not have the characteristic, and the expansion stress borne by each direction of the coating shell layer is large in difference in the circulation process, so that the structure of the coating shell layer cannot be effectively maintained.
Combining the results of examples 1-4 and comparative examples 3-4, it can be seen that when the heat treatment temperature is above or below the temperature range of the present invention at each stage, the performance of the obtained material is poor because the volume of the graphite core precursor cannot be effectively expanded in the first stage of heat treatment but the covering shell layer is solidified in the second stage of heat treatment when the heat treatment temperature is low, and the volume of the graphite core precursor cannot be effectively contracted in the third stage of heat treatment due to the low temperature, so that the obtained material has no structural feature that the void ratio of the first covering layer is larger than that of the second covering layer, resulting in poor material performance. When the heat treatment temperature is higher, not only the volume expansion of the graphite core precursor can occur in the first stage of heat treatment process, but also the partial solidification of the coating shell layer structure can occur, and the coating shell layer has a cracking phenomenon in the process, so that the final coating shell layer structure has defects, thereby influencing the electrochemical performance of the material.
Comparative analysis of examples 8-10 and examples 10-12 shows that the graphite material prepared according to the experimental method of the present invention still has better electrochemical performance with different types of cladding materials and different types of binder materials.
Comparing example 2 with comparative examples 5-6, it can be seen that the first effect and cycle performance is poor when the coating material content is lower, although the resistivity of the graphite material is improved. The reason is that: when the content of the coating material is lower, a continuous and complete coating shell structure cannot be formed, and the coating shell cannot effectively prevent the occurrence of side reaction of the electrolyte on the surface of the graphite kernel and maintain the stability of the structure of the graphite kernel in the circulating process, so that the first effect, the gram capacity and the circulating performance of the graphite kernel are poorer; when the content of the coating material is higher, the powder resistivity, the first effect and the cycle performance of the coating material have certain advantages, but the gram capacity of the coating material is lower due to the higher proportion of the inactive material of the coating, so that the practical application range is limited.
In summary, it can be seen from the graphite materials prepared in examples 1 to 12 that the gram volume, the first discharge efficiency and the capacity retention rate of 500 cycles of the graphite material prepared by the method of the present invention are all significantly improved. The graphite material prepared by the method has a core-shell structure, wherein the void ratio of the first coating layer is larger than that of the second coating layer, the structure can effectively relieve the coating layer structure damage and the interface structure damage caused by graphite expansion in the battery charging and discharging processes, and the cycle performance and the capacity retention rate of the battery can be effectively improved. Meanwhile, the method can form the coating structure through three stages of heat treatment processes with specific parameters and corresponding coating proportion.
The preferred embodiments of the invention disclosed above are intended to be illustrative only. The preferred embodiments are not intended to be exhaustive or to limit the invention to the precise embodiments disclosed. Obviously, many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and the practical application, to thereby enable others skilled in the art to best utilize the invention. The invention is limited only by the claims and their full scope and equivalents.
Claims (14)
1. A method for preparing a graphite material, comprising:
coating, namely coating a coating layer outside a graphite core precursor to obtain a coating material, wherein the graphite core precursor is a graphite material with a layer containing a solvent;
a heat treatment step, which comprises a first stage of heat treatment of the coating material to expand the volume of the graphite core precursor to obtain an expanded coating material, a second stage of heat treatment of the expanded coating material to solidify the coating shell structure to obtain a solidified coating material, and a third stage of heat treatment of the solidified coating material to shrink the volume of the graphite core precursor to obtain a first coating layer coated outside the graphite core and a second coating layer coated outside the first coating layer, wherein the void ratio of the first coating layer is greater than that of the second coating layer;
the coating shell layer comprises an adhesive and a flexible material, the flexible material is selected from one or more of cellulose fibers, lignin fibers, one-dimensional carbon materials, one-dimensional transition metal materials and one-dimensional metal oxide materials, the one-dimensional transition metal materials comprise one or more of cobalt fibers, titanium fibers, vanadium fibers, chromium fibers, hafnium fibers, tantalum fibers and tungsten fiber materials, the one-dimensional metal oxide materials comprise one or more of alumina fibers, zirconia fibers and titanium oxide fibers, the total mass content of the coating material is 100%, and the total mass content of the coating shell layer is 5-20%.
2. The method as claimed in claim 1, wherein the first stage heat treatment temperature is 200-450 ℃ and the treatment time is 2-6 h.
3. The method as claimed in claim 2, wherein the first-stage heat treatment temperature is 300 ℃ and 400 ℃ and the treatment time is 4-5 h.
4. The method as claimed in claim 1, wherein the second-stage heat treatment temperature is 600-850 ℃ and the treatment time is 2-6 h.
5. The method as claimed in claim 4, wherein the second-stage heat treatment temperature is 700 ℃ and 800 ℃ and the treatment time is 3-4 h.
6. The method as claimed in claim 1, wherein the third heat treatment temperature is 1000 ℃ and 1600 ℃ and the treatment time is 1-4 h.
7. The method as claimed in claim 6, wherein the third heat treatment temperature is 1200 ℃ and 1500 ℃ and the treatment time is 2-3 h.
8. The preparation method according to claim 1, wherein the adhesive is one or more selected from epoxy resin, polyvinyl alcohol and polyvinylpyrrolidone.
9. The method according to claim 1, wherein the one-dimensional carbon material comprises one or more of carbon fiber and carbon nanotube.
10. The method of claim 1, further comprising the steps of pickling, washing with water, and drying the graphite core precursor before the coating step.
11. The graphite material obtained by the preparation method according to claim 1, which comprises core-shell structure particles, wherein the core-shell structure particles comprise a graphite inner core, a first coating layer coated outside the graphite inner core and a second coating layer coated outside the first coating layer, and the void ratio of the first coating layer is greater than that of the second coating layer.
12. The graphite material of claim 11, wherein the first coating layer and the second coating layer have the same compositional composition.
13. The graphite material of claim 11 or 12, wherein the first and second coating layers are comprised of a flexible material.
14. A lithium ion battery comprising the graphite material according to any one of claims 11 to 13.
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