CN108400294B - Preparation method of silicon cathode for lithium ion battery with multilevel structure - Google Patents

Preparation method of silicon cathode for lithium ion battery with multilevel structure Download PDF

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CN108400294B
CN108400294B CN201810098479.5A CN201810098479A CN108400294B CN 108400294 B CN108400294 B CN 108400294B CN 201810098479 A CN201810098479 A CN 201810098479A CN 108400294 B CN108400294 B CN 108400294B
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silicon
lithium ion
ion battery
graphene
multilevel structure
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CN108400294A (en
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杨全红
陈凡奇
韩俊伟
肖菁
张辰
陶莹
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Tianjin University
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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
    • 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
    • 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
    • H01M4/38—Selection of substances as active materials, active masses, active liquids of elements or alloys
    • H01M4/386—Silicon or alloys based on silicon
    • 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
    • H01M4/587—Carbonaceous material, e.g. graphite-intercalation compounds or CFx for inserting or intercalating light metals
    • 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
    • H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
    • H01M4/624—Electric conductive fillers
    • H01M4/625—Carbon or graphite
    • 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
    • Y02E60/10—Energy storage using batteries

Abstract

The invention belongs to the technical field of lithium ion batteries, and particularly relates to a preparation method of a silicon cathode for a lithium ion battery with a multilevel structure, which comprises the following steps: grinding and uniformly mixing micron silicon particles and sublimed sulfur, then placing the mixture in a tube furnace for heat treatment, and uniformly coating a layer of sulfur on the surface of the silicon particles; adding the dopamine, iron salt and the dopamine into a buffer solution, and polymerizing the dopamine on the surfaces of the composite powder particles under the condition of full stirring and carrying iron ions; preparing the solid obtained after separation and graphene oxide by a hydrothermal reduction method to obtain composite gel, and carrying out high-temperature heat treatment in a tubular furnace after washing and drying; soaking and washing the graphene/silicon composite material by using dilute hydrochloric acid, and drying the graphene/silicon composite material to obtain the graphene/silicon composite material. According to the invention, the first coulomb efficiency of the electrode material is improved by designing the yolk shell structure, the circulation stability of the electrode material is improved, and meanwhile, the densification of the yolk shell structure is realized by utilizing the graphene three-dimensional assembly, so that the silicon cathode for the lithium ion battery with the multilevel structure is finally obtained.

Description

Preparation method of silicon cathode for lithium ion battery with multilevel structure
Technical Field
The invention belongs to the technical field of lithium ion batteries, and particularly relates to a preparation method of a silicon cathode for a lithium ion battery with a multilevel structure.
Background
Silicon is used as the most potential next-generation high-energy-density lithium ion battery negative electrode material, has abundant reserves and has the mass specific capacity ten times that of graphite. The current research on the silicon negative electrode is about the specific mass capacity (>1000mAh g–1) And cycle performance (>500 circles) and the rate performance research. However, the above performance is often improved by reducing the size of the silicon active particles, that is, by using nano silicon as an active material to construct a negative electrode material. Nanotechnology has gained extensive attention in silicon cathodesNote that, excellent research results have been obtained, but the application of nanotechnology in silicon anodes leads to several problems: 1) the large specific surface area results in a large consumption of a limited lithium source. Meanwhile, silicon cannot generate a stable SEI film due to huge volume change in the charging and discharging processes, so that the SEI film is broken and grows repeatedly, and further consumption of a lithium source is formed. The irreversible consumption of the lithium source in the process is mainly reflected by lower coulombic efficiency, especially the first coulombic efficiency and the capacity retention rate of the first circles; 2) the increase of the interfaces among the particles leads to the increase of impedance, influences the electrical contact among materials and aggravates the defect of low electronic conductivity of silicon; 3) the material has low tap density, which is not beneficial to improving the volume performance of the electrode.
In view of the above, the present invention aims to provide a method for preparing a silicon negative electrode for a lithium ion battery, which uses micron silicon as a raw material for preparing the silicon negative electrode for the lithium ion battery, and not only can effectively avoid the above disadvantages of nano silicon, but also has a significant cost advantage. However, silicon active particles with the size of micron are generally cracked and pulverized during the charging and discharging processes, and the cycling stability of the silicon active particles is severely restricted. According to the invention, by designing the yolk shell structure, space required by volume expansion is reserved for silicon, and meanwhile, active particles are encapsulated in the graphitized carbon layer, so that the initial coulomb efficiency of the electrode material can be improved, and the cycle stability of the electrode material can be improved. More importantly, on the basis, the graphene three-dimensional assembly is used as a substrate compounded with active substance silicon by utilizing good electronic conductivity and ionic conductivity of the graphene three-dimensional assembly, and the active particles with the yolk shell structure are densified by regulating and controlling the interaction between the graphene sheet layer and water and utilizing a capillary evaporation drying mode of water to form a multilevel structure. Compared with the traditional silicon negative electrode material, the silicon-carbon composite material prepared by the invention has the remarkable advantages of high density and high silicon content, can realize accurate regulation and control of the porosity, the density and the silicon content of the composite material, realizes large increase of the volume specific capacity on the premise of ensuring full exertion of the mass specific capacity of the electrode material, and has very important application significance for electrochemical energy storage devices with space volume limitation in actual life.
In summary, it is necessary to provide a method for preparing a silicon negative electrode for a lithium ion battery with a multilevel structure, which improves the first coulomb efficiency of an electrode material by designing a "yolk shell" structure, improves the cycling stability of the electrode material, and simultaneously realizes densification of the "yolk shell" structure by using a graphene three-dimensional assembly, thereby finally obtaining a micron silicon negative electrode for a lithium ion battery with high volume performance.
Disclosure of Invention
The invention aims to: aiming at the defects of the prior art, the preparation method of the silicon negative electrode for the lithium ion battery with the multilevel structure is provided, the initial coulomb efficiency of the electrode material is improved by designing the yolk shell structure, the circulation stability of the electrode material is improved, the densification of the yolk shell structure is realized by utilizing the graphene three-dimensional assembly, and the micron silicon negative electrode for the lithium ion battery with high volume performance is finally obtained.
In order to achieve the purpose, the invention adopts the following technical scheme:
a preparation method of a silicon cathode for a lithium ion battery with a multilevel structure at least comprises the following steps:
firstly, grinding and uniformly mixing silicon micron particles and sublimed sulfur, then placing the mixture in a tubular furnace protected by inert gas for heat treatment, and uniformly coating a layer of sulfur on the surfaces of the silicon micron particles to obtain composite powder;
secondly, adding the powder obtained in the first step, dopamine and iron salt into a buffer solution together to maintain a constant pH value, and polymerizing the dopamine on the surfaces of the composite powder particles under the condition of full stirring and carrying a certain amount of iron ions to obtain a product to be treated;
thirdly, centrifugally separating the product to be treated obtained in the second step to obtain a solid, and preparing the solid and the graphene oxide dispersion liquid by a hydrothermal reduction method to obtain a composite gel;
fourthly, after the composite gel obtained in the third step is washed by deionized water to remove impurity ions and is dried by capillary evaporation, high-temperature heat treatment is carried out in a tubular furnace to remove elemental sulfur and carbonize polydopamine to form an amorphous carbon layer and a catalytic graphitization amorphous carbon layer;
and fifthly, carrying out acid soaking and washing on the material obtained in the fourth step to remove residual catalyst iron, and drying to obtain the graphene/silicon composite material.
According to the invention, sulfur is coated on the surface of micron silicon particles as a template, polydopamine is coated on the surface of the sulfur as a carbon layer precursor, the sulfur is compounded with graphene through hydrothermal reduction, and finally, the sulfur is removed through simple heat treatment, and the polydopamine is carbonized, so that the silicon-carbon composite material with an egg yolk shell structure can be obtained. The size of a reserved space required by volume expansion in the micron silicon circulation process can be adjusted accurately in a large range by accurately controlling the introduction amount of sulfur; by adjusting the addition amount of dopamine and the time of polymerization reaction, the thickness of the carbon layer can be accurately controlled, so that the structure of the graphene/silicon composite material can be regulated and controlled; the first coulomb efficiency of electrode material circulation can be improved by catalytic graphitization of the amorphous carbon layer formed by carbonizing the polydopamine, so that the electrode material can be well applied to lithium ion batteries.
As an improvement of the preparation method of the silicon cathode for the lithium ion battery with the multilevel structure, in the first step, the mass ratio of micron silicon to sublimed sulfur is 1 (2-4). The proportioning can accurately control the sulfur content in a certain range, thereby obtaining a proper reserved space after sulfur removal. The temperature of the heat treatment is 100-300 ℃, and the duration of the heat treatment is 3-24 h.
As an improvement of the preparation method of the silicon cathode for the lithium ion battery with the multilevel structure, in the second step, the concentration of dopamine in a buffer solution is 1-3 mg/mL, ferric salt is ferric chloride or ferric nitrate, the concentration of ferric ions is 0.3-0.5 mol/L, the mass ratio of composite powder to dopamine is (2-5): 1, and the buffer solution is Tris-HCl or PBS buffer solution. The dopamine in the concentration range can be well polymerized on the surface of the particles, and carries a proper amount of iron ions.
As an improvement of the preparation method of the silicon cathode for the lithium ion battery with the multilevel structure, in the second step, the polymerization reaction time of dopamine is 12-48 h. Within the reaction time range, a carbon layer with proper thickness can be obtained after the poly dopamine is carbonized, and a higher silicon-carbon ratio is maintained on the premise of ensuring the mechanical strength of the carbon layer.
As an improvement of the preparation method of the silicon negative electrode for the lithium ion battery with the multilevel structure, the concentration of the graphene oxide dispersion liquid is 1-3 mg/mL. Graphene oxide dispersions within this concentration range are most suitable for the formation of composite gels of three-dimensional graphene and silicon-containing active particles by graphene oxide lamellar overlap cross-linking in a hydrothermal process.
As an improvement of the preparation method of the silicon negative electrode for the lithium ion battery with the multilevel structure, in the third step, the mass ratio of solid powder obtained by centrifugation in the hydrothermal mixed solution to the graphene oxide is (1-10): 1. The proportion can obtain proper silicon content in the composite gel.
As an improvement of the preparation method of the silicon cathode for the lithium ion battery with the multilevel structure, in the third step, the temperature of the hydrothermal reaction is 150-250 ℃, and the duration time of the hydrothermal reaction is 4-8 h. The hydrothermal temperature of 150-250 ℃ can well drive polydopamine on the outermost layer of the active particles to be combined with graphene oxide, and meanwhile, graphene oxide lamella loaded with silicon-containing active particles can be fully lapped and crosslinked to form three-dimensional graphene/silicon composite gel.
As an improvement of the preparation method of the silicon cathode for the lithium ion battery with the multilevel structure, in the fourth step, the moisture removal method is drying, the drying temperature is 60-90 ℃, and the drying duration is 12-36 h. In the drying process, the densification of the silicon-containing active particles is realized by utilizing the capillary evaporation of water. At the drying temperature of 60-90 ℃, the block can realize better shrinkage, and simultaneously, the block crushing caused by rapid shrinkage at higher temperature is avoided; the drying time of 12 h-36 h can realize sufficient drying of the material.
The fourth step is that the desulfurization treatment is heat treatment desulfurization, and the heat treatment desulfurization method is that under the protection of inert atmosphere, the temperature is increased to 300-500 ℃ at the temperature increasing rate of 5-20 ℃/min, the temperature is kept for 2-10 h, then the temperature is increased to 700-900 ℃ at the temperature increasing rate of 3-10 ℃/min, the temperature is kept for 2-6 h, and finally the temperature is cooled to room temperature. The melting point and boiling point of the sulfur are low, and the heat treatment temperature of 300-500 ℃ can realize the complete removal of the sulfur. Meanwhile, under the condition of existence of an iron catalyst, the heat treatment temperature of 700-900 ℃ can realize complete carbonization of the polydopamine and catalytic graphitization of the amorphous carbon layer.
As an improvement of the preparation method of the silicon negative electrode for the lithium ion battery with the multilevel structure, the acid used in the fifth step is at least one of dilute hydrochloric acid and dilute sulfuric acid; the graphene/silicon composite material obtained in the fifth step has high density, and the bulk density is 1.0-1.6 g/cm3The silicon content is 40-70%.
Compared with the prior art, the invention has at least the following advantages:
firstly, the method has mild conditions, simple operation and green and environment-friendly preparation process, and the sulfur which is a flexible template can completely and uniformly coat the silicon component and is easy to remove.
Secondly, the method can realize the precise regulation and control of the size of the reserved space by controlling the introduction amount of the sulfur.
Thirdly, the yolk shell structure designed by the method reserves the space required by volume expansion for silicon, and simultaneously encapsulates active particles in the graphitized carbon layer, thereby not only improving the initial coulomb efficiency of the electrode material, but also improving the cycle stability of the electrode material.
Fourthly, the method can realize the densification and shrinkage of the three-dimensional graphene skeleton by utilizing the capillary evaporation effect of water, thereby realizing the densification of the active particles with the yolk shell structure. The silicon-carbon composite material prepared by the method has high density and high silicon content, and can remarkably improve the volume performance of the lithium ion battery on the premise of ensuring that the mass specific capacity is fully exerted.
Drawings
The invention and its advantageous effects are explained in detail below with reference to the accompanying drawings and the detailed description.
Fig. 1 is a TEM image of a graphene/silicon composite material prepared in example 1 of the present invention.
Fig. 2 is a constant current charge and discharge curve of the lithium ion battery made of the graphene/silicon composite material prepared in example 1 of the present invention.
Detailed Description
The technical solutions of the present invention are described below with specific examples, but the scope of the present invention is not limited thereto.
Example 1
The embodiment provides a multilevel-structured high-volume-performance lithium ion battery micron silicon negative electrode and a preparation method thereof, and the method at least comprises the following steps:
step one, 200mg of micron silicon powder and 800mg of sublimed sulfur are uniformly ground and mixed, and then are placed in a tube furnace to be subjected to heat treatment for 6 hours at 155 ℃ under inert atmosphere;
secondly, adding the powder obtained in the first step, 500mg of dopamine and 40g of ferric nitrate nonahydrate into a Tris-HCl buffer solution with the pH value of 250ml and 8.5, and reacting at room temperature for 24 hours under the condition of fully stirring;
and thirdly, centrifugally separating the product to be treated obtained in the second step to obtain a solid, adding the solid into 90mL of graphene oxide dispersion liquid with the concentration of 2mg/mL, and carrying out alternate stirring and ultrasonic treatment for 30min to fully mix the graphene oxide dispersion liquid and the graphene oxide dispersion liquid. Then adding the mixed dispersion liquid into a 100mL hydrothermal reaction kettle for hydrothermal reaction at 180 ℃ for 6 hours to obtain composite hydrogel;
and fourthly, washing the composite gel obtained in the third step by deionized water to remove impurity ions, carrying out capillary evaporation drying at 70 ℃ for 24 hours, and then carrying out high-temperature heat treatment in a tubular furnace. Under the protection of inert atmosphere, heating to 400 ℃ at the heating rate of 10 ℃/min, keeping the temperature for 3h, heating to 800 ℃ at the heating rate of 5 ℃/min, keeping the temperature for 3h, and finally cooling to room temperature.
And fifthly, soaking and washing the material obtained in the fourth step for 24 hours at the temperature of 60 ℃ by using 1mol/L diluted hydrochloric acid, removing residual catalyst iron, and drying to obtain the graphene/silicon composite material. The bulk density of the graphene/silicon composite material is 1.2g/cm3The silicon content was 57%.
A TEM image of the graphene/silicon composite material prepared in example 1 is shown in fig. 1, and it can be seen from fig. 1 that: the graphene/silicon composite material has a multi-stage structure, and active particles with a yolk shell structure are uniformly wrapped in graphene sheets.
The constant current charge and discharge curve of the lithium ion battery of the graphene/silicon composite material prepared in example 1 is shown in fig. 2, and it can be seen from fig. 2 that: the graphene/silicon composite material has high specific capacity and good cycling stability.
Example 2
The difference from example 1 is:
the amount of silicon micron was adjusted to 300mg and the amount of sublimed sulfur was adjusted to 700mg, which was the same as in example 1 and thus not described in detail. The bulk density of the graphene/silicon composite material is 1.4g/cm3The silicon content was 63%.
Example 3
The difference from example 1 is:
the amount of silicon micron was adjusted to 400mg and the amount of sublimed sulfur was adjusted to 600mg, which was the same as in example 1 and thus the details thereof were omitted. The bulk density of the graphene/silicon composite material is 1.5g/cm3The silicon content was 66%.
Example 4
The difference from example 1 is:
the amount of silicon micron was adjusted to 450mg and the amount of sublimed sulfur was adjusted to 550mg, which was the same as in example 1 and will not be described herein. The bulk density of the graphene/silicon composite material is 1.6g/cm3The silicon content was 68%.
Example 5
The difference from example 1 is:
the amount of dopamine was adjusted to 750mg and the polymerization time to 36h, which was the same as in example 1 and is not repeated here. The bulk density of the graphene/silicon composite material is 1.2g/cm3The silicon content was 53%.
Example 6
The difference from example 1 is:
the dosage of dopamine is adjusted to 850mg, the polymerization reaction time is adjusted to 40h, and the rest is mixed with the solidThe same as in example 1, and will not be described herein. The bulk density of the graphene/silicon composite material is 1.2g/cm3The silicon content was 52%.
Example 7
The difference from example 1 is:
the amount of the graphene oxide dispersion was adjusted to 180mL, the specification of the hydrothermal reaction kettle was adjusted to 200mL, and the rest was the same as in example 1, and thus, the description thereof is omitted. The bulk density of the graphene/silicon composite material is 1.3g/cm3The silicon content was 44%.
Example 8
The difference from example 1 is:
the amount of the graphene oxide dispersion was adjusted to 240mL, the specification of the hydrothermal reaction kettle was adjusted to 200mL, and the rest was the same as in example 1, and thus, the description thereof is omitted. The bulk density of the graphene/silicon composite material is 1.3g/cm3The silicon content was 40%.
Example 9
The embodiment provides a preparation method of a silicon negative electrode for a lithium ion battery with a multilevel structure, which at least comprises the following steps:
step one, 200mg of micron silicon powder and 800mg of sublimed sulfur are uniformly ground and mixed, and then are placed in a tube furnace to be subjected to heat treatment for 8 hours at 185 ℃ under inert atmosphere;
secondly, adding the composite powder obtained in the first step, 500mg of dopamine and 40g of ferric nitrate nonahydrate into 250mL of PBS (phosphate buffer solution) with pH value of 8.5, and reacting for 15h at room temperature under the condition of full stirring;
and thirdly, centrifugally separating the product to be treated obtained in the second step to obtain a solid, adding the solid into 90mL of graphene oxide dispersion liquid with the concentration of 2.5mg/mL, and carrying out alternate stirring and ultrasonic treatment for 60min to fully mix the graphene oxide dispersion liquid and the graphene oxide dispersion liquid. Then adding the mixed dispersion liquid into a 100mL hydrothermal reaction kettle for hydrothermal reaction at 200 ℃ for 5 hours to obtain composite hydrogel;
and fourthly, washing the composite gel obtained in the third step by deionized water to remove impurity ions, carrying out capillary evaporation drying for 15h at the temperature of 80 ℃, and then carrying out high-temperature heat treatment in a tubular furnace. Under the protection of inert atmosphere, the temperature is raised to 450 ℃ at the heating rate of 15 ℃/min, the temperature is kept for 5h, then the temperature is raised to 850 ℃ at the heating rate of 10 ℃/min, the temperature is kept for 4h, and finally the temperature is cooled to the room temperature.
And fifthly, soaking and washing the material obtained in the fourth step with 1.2mol/L dilute sulfuric acid at 50 ℃ for 36h, removing residual catalyst iron, and drying to obtain the graphene/silicon composite material. The bulk density of the graphene/silicon composite material is 1.3g/cm3The silicon content was 53%.
Example 10
The embodiment provides a preparation method of a silicon negative electrode for a lithium ion battery with a multilevel structure, which at least comprises the following steps:
step one, 200mg of micron silicon powder and 800mg of sublimed sulfur are ground and uniformly mixed, and then are placed in a tube furnace to be subjected to heat treatment for 20 hours at 135 ℃ under inert atmosphere;
secondly, adding the composite powder obtained in the first step, 500mg of dopamine and 40g of ferric nitrate into 250mL of PBS (phosphate buffer solution) with the pH value of 8.5, and reacting at room temperature for 18h under the condition of full stirring;
and thirdly, centrifugally separating the product to be treated obtained in the second step to obtain a solid, adding the solid into 90mL of graphene oxide dispersion liquid with the concentration of 1.5mg/mL, and carrying out alternate stirring and ultrasonic treatment for 40min to fully mix the graphene oxide dispersion liquid and the graphene oxide dispersion liquid. Then adding the mixed dispersion liquid into a 100mL hydrothermal reaction kettle for hydrothermal reaction at the reaction temperature of 140 ℃ for 7 hours to obtain composite hydrogel;
and fourthly, washing the composite gel obtained in the third step by deionized water to remove impurity ions, carrying out capillary evaporation drying at 75 ℃ for 18h, and then carrying out high-temperature heat treatment in a tubular furnace. Under the protection of inert atmosphere, the temperature is raised to 350 ℃ at the heating rate of 5 ℃/min, the temperature is kept for 3.5h, then the temperature is raised to 750 ℃ at the heating rate of 3 ℃/min, the temperature is kept for 5h, and finally the temperature is cooled to the room temperature.
And fifthly, soaking and washing the material obtained in the fourth step with 1mol/L dilute sulfuric acid at 55 ℃ for 16h, removing residual catalyst iron, and drying to obtain the graphene/silicon composite material. The bulk density of the graphene/silicon composite material is 1.3g/cm3The silicon content was 62%.
Comparative example 1
The amount of sublimed sulfur was 0mg, which is different from example 1, and the rest is the same as example 1, and thus the description thereof is omitted. The bulk density of the graphene/silicon composite material is 1.5g/cm3The silicon content was 57%.
Comparative example 2
Step one, 200mg of micron silicon powder and 800mg of sublimed sulfur are uniformly ground and mixed, and then are placed in a tube furnace to be subjected to heat treatment for 6 hours at 155 ℃ under inert atmosphere;
and step two, adding the powder obtained in the step one into 90mL of graphene oxide dispersion liquid with the concentration of 2mg/mL, and carrying out alternate stirring and ultrasonic treatment for 30min to fully mix the graphene oxide dispersion liquid and the graphene oxide dispersion liquid. Then adding the mixed dispersion liquid into a 100mL hydrothermal reaction kettle for hydrothermal reaction at 180 ℃ for 6 hours to obtain composite hydrogel;
and thirdly, washing the composite gel obtained in the second step by deionized water to remove impurity ions, carrying out capillary evaporation drying at 70 ℃ for 24 hours, and then carrying out high-temperature heat treatment in a tubular furnace. Under the protection of inert atmosphere, heating to 400 ℃ at the heating rate of 10 ℃/min, keeping the temperature for 3h, heating to 800 ℃ at the heating rate of 5 ℃/min, keeping the temperature for 3h, and finally cooling to room temperature. The bulk density of the graphene/silicon composite material is 1.0g/cm3The silicon content was 67%.
The graphene/silicon composite materials prepared in examples 1 to 10 and comparative examples 1 to 2 were mixed with a conductive additive (Super-P) and a binder (PVDF) in a mass ratio of 8:1:1, and a negative electrode sheet was prepared using a copper foil as a current collector. With LiPF6The electrolyte and the lithium sheet are used as the positive electrode to form a half cell, electrochemical performance test is carried out on the half cell, the mass specific capacity and the volume specific capacity of the composite electrode material are tested, and the obtained results are shown in table 1.
Table 1: test results of examples 1 to 10 and comparative examples 1 to 2
Figure GDA0002402010750000121
Figure GDA0002402010750000131
As can be seen from table 1: (1) as the amount of sulfur incorporation increases, the material headspace increases and the density decreases. (2) As the thickness of the carbon layer and the amount of graphene are increased, the silicon content of the material is reduced. (3) Without a headspace or carbon coating, the electrochemical performance of the material is less than optimal. Therefore, by accurately regulating and controlling the introduction amount of sulfur and the silicon-carbon ratio, a compact graphene/silicon composite material with high silicon content can be obtained, and high volume performance (1200 mAh/cm) is obtained while high specific capacity (1000 mAh/g) is ensured3)。
Variations and modifications to the above-described embodiments may occur to those skilled in the art, which fall within the scope and spirit of the above description. Therefore, the present invention is not limited to the specific embodiments disclosed and described above, and some modifications and variations of the present invention should fall within the scope of the claims of the present invention. Furthermore, although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

Claims (10)

1. A preparation method of a silicon cathode for a lithium ion battery with a multilevel structure is characterized by at least comprising the following steps:
firstly, grinding and uniformly mixing silicon micron particles and sublimed sulfur, then placing the mixture in a tubular furnace protected by inert gas for heat treatment, and uniformly coating a layer of sulfur on the surfaces of the silicon micron particles to obtain composite powder;
secondly, adding the composite powder obtained in the first step, dopamine and iron salt into a buffer solution together to maintain a constant pH value, and polymerizing the dopamine on the surfaces of composite powder particles under the condition of full stirring and carrying a certain amount of iron ions to obtain a product to be treated;
thirdly, centrifugally separating the product to be treated obtained in the second step to obtain a solid, and preparing the solid and the graphene oxide dispersion liquid by a hydrothermal reduction method to obtain a composite gel;
fourthly, after the composite gel obtained in the third step is washed by deionized water to remove impurity ions and is dried by capillary evaporation, high-temperature heat treatment is carried out in a tubular furnace to remove elemental sulfur and carbonize polydopamine to form an amorphous carbon layer and a catalytic graphitization amorphous carbon layer;
fifthly, acid leaching and washing the material obtained in the fourth step to remove residual catalyst iron, and drying to obtain the graphene/silicon composite material;
in the first step, the mass ratio of the micron silicon particles to the sublimed sulfur is 1 (2-4).
2. The method for producing a silicon negative electrode for a lithium ion battery having a multilevel structure according to claim 1, characterized in that: the temperature of the heat treatment is 100-300 ℃, and the duration of the heat treatment is 3-24 h.
3. The method for producing a silicon negative electrode for a lithium ion battery having a multilevel structure according to claim 1, characterized in that: in the second step, the concentration of dopamine in the buffer solution is 1-3 mg/mL, the ferric salt is ferric chloride or ferric nitrate, the concentration of ferric ions is 0.3-0.5 mol/L, the mass ratio of the composite powder to the dopamine is (2-5): 1, and the buffer solution is Tris-HCl or PBS buffer solution.
4. The method for producing a silicon negative electrode for a lithium ion battery having a multilevel structure according to claim 1, characterized in that: in the second step, the polymerization reaction time of dopamine is 12-48 h.
5. The method for producing a silicon negative electrode for a lithium ion battery having a multilevel structure according to claim 1, characterized in that: in the third step, the concentration of the graphene oxide dispersion liquid is 1-3 mg/mL.
6. The method for producing a silicon negative electrode for a lithium ion battery having a multilevel structure according to claim 1, characterized in that: in the third step, the mass ratio of the solid to the graphene oxide is (1-10): 1.
7. The method for producing a silicon negative electrode for a lithium ion battery having a multilevel structure according to claim 1, characterized in that: in the third step, the temperature of the hydrothermal reaction is 150-250 ℃, and the duration time of the hydrothermal reaction is 4-8 h.
8. The method for producing a silicon negative electrode for a lithium ion battery having a multilevel structure according to claim 1, characterized in that: in the fourth step, the capillary evaporation drying method is drying, the drying temperature is 60-90 ℃, and the drying duration is 12-36 h.
9. The method for producing a silicon negative electrode for a lithium ion battery having a multilevel structure according to claim 1, characterized in that: in the fourth step, the high-temperature heat treatment method comprises the steps of heating to 300-500 ℃ at the heating rate of 5-20 ℃/min under the protection of inert atmosphere, keeping the temperature for 2-10 h, heating to 700-900 ℃ at the heating rate of 3-10 ℃/min, keeping the temperature for 2-6 h, and finally cooling to room temperature.
10. The method for producing a silicon negative electrode for a lithium ion battery having a multilevel structure according to claim 1, characterized in that: the acid used in the fifth step is at least one of dilute hydrochloric acid and dilute sulfuric acid; the density of the graphene/silicon composite material block obtained in the fifth step is 1.0-1.6 g/cm3The silicon content is 40-70%.
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