CN112531164B - Silicon-carbon composite material, preparation method and application - Google Patents

Silicon-carbon composite material, preparation method and application Download PDF

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CN112531164B
CN112531164B CN202011215768.2A CN202011215768A CN112531164B CN 112531164 B CN112531164 B CN 112531164B CN 202011215768 A CN202011215768 A CN 202011215768A CN 112531164 B CN112531164 B CN 112531164B
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carbon composite
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carbon
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CN112531164A (en
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丁治英
马兴月
尹周澜
童汇
喻帅
李由
涂瑞萱
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Central South 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
    • H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
    • H01M4/628—Inhibitors, e.g. gassing inhibitors, corrosion inhibitors
    • 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
    • H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
    • H01M4/624—Electric conductive fillers
    • H01M4/625—Carbon or graphite
    • 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
    • H01M2004/021—Physical characteristics, e.g. porosity, surface area
    • 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
    • H01M2004/026—Electrodes composed of, or comprising, active material characterised by the polarity
    • H01M2004/027—Negative electrodes
    • 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 provides a silicon-carbon composite material, a preparation method and application. A preparation method of a silicon-carbon composite material comprises the following steps: s1, mixing strong base and silica micro powder, calcining, washing and drying a product to obtain a multi-component silicon-based material; s2, mixing the multi-component silicon-based material obtained in the step S1 with a carbon source and a template agent, sintering, washing and drying a product to obtain the silicon-carbon composite material. The silicon-carbon composite material prepared according to the steps has good conductivity, rate capability and cycle performance.

Description

Silicon-carbon composite material, preparation method and application
Technical Field
The invention belongs to the field of lithium ion battery electrode materials, and particularly relates to a silicon-carbon composite material, a preparation method and application thereof.
Background
The lithium ion battery has the advantages of high mass energy density, environmental friendliness and the like, and is widely applied to the fields of 3C, power and energy storage. In recent years, with the development of technology, further requirements on the endurance mileage of power batteries are also provided, which further advances the research on novel high-quality/volume energy density batteries. The energy density of the battery mainly depends on electrode materials, the theoretical gram specific capacity of the traditional graphite negative electrode is only 372mAh/g, and the requirement of the energy density of the battery cannot be met. The silicon and silicon-based oxide materials have high theoretical specific capacity (Si:3579mAh/g, SiO) 2 1965mAh/g, 2680mAh/g SiO, low lithium-removing potential (0.02-0.6V vs. Li) + Li), environment-friendly, abundant reserves and the like, and becomes a negative electrode material which is most likely to replace commercial graphite.
However, the commercial application of silicon and silicon-based oxide materials still needs to break through two technical barriers: (1) because the silicon-based material has huge volume change in the lithium ion extraction process, the phenomena of pulverization of the electrode material, falling off from a current collector and the like are easy to occur in the long circulation process, and the poor circulation stability is shown; (2) compared with a graphite cathode, the silicon-based material has very low conductivity, so the first coulombic efficiency of the material is low, and the rate capability of the material is poor.
In order to overcome the two technical barriers, researchers have explored various methods, mainly including compounding silicon-based materials with carbon, optimizing the structural design of the silicon-based materials, selecting novel binders, changing electrolyte components, and the like. But the defects of high volume expansion rate, poor conductivity and the like inherent in the silicon-based material are still not overcome. Therefore, its commercial application is still limited.
Disclosure of Invention
The present invention is directed to solving at least one of the above problems in the prior art. For this purpose, the first and second air-conditioning systems,
a first aspect of the invention provides a silicon carbon composite.
The second aspect of the invention provides a preparation method of the silicon-carbon composite material.
The third aspect of the invention provides an application of a silicon-carbon composite material in a lithium ion battery negative electrode material.
A silicon-carbon composite material comprises
A multi-component silicon-based material; and
porous carbon coated on the surface of the multi-component silicon-based material;
the multi-component silicon-based material is selected from at least two of silicon, silicon monoxide and silicon dioxide.
The multi-component silicon-based material serves as a main body for lithium ion deintercalation and contributes to capacity.
The silicon dioxide and the porous carbon have the function of relieving the volume change of the silicon-carbon composite material in the lithium ion deintercalation process.
The porous carbon has the further function of improving the electrical conductivity of the silicon-carbon composite material.
The third aspect of the porous carbon has the function of isolating the multi-component silicon-based material from being directly contacted with the electrolyte, so that the decomposition and consumption of electrolyte solvent/solute are reduced.
A preparation method of a silicon-carbon composite material comprises the following steps:
s1, mixing strong base and silica micro powder, calcining, washing and drying a product to obtain a multi-component silicon-based material;
s2, mixing the multi-component silicon-based material obtained in the step S1 with a carbon source and a template agent, sintering, washing a product, and drying to obtain the silicon-carbon composite material.
According to a preferred embodiment of the present invention, in step S1, the average particle size of the silica is 3 μm.
According to an embodiment of the invention, in step S1, the mass ratio of the strong base to the silica is 1:2 to 1: 10.
According to a preferred embodiment of the present invention, in step S1, the mass ratio of the strong base to the silica is 1: 4.
According to an embodiment of the present invention, in step S1, the mixing is any one of wet mixing and dry mixing.
According to a preferred embodiment of the present invention, in step S1, the mixing is wet mixing, and the operation method is: stirring anhydrous alcohol, silica and strong base uniformly, and drying.
According to an embodiment of the invention, in step S1, the calcination is performed at a constant temperature of 700 to 900 ℃ for 0.5 to 1.5 hours.
According to one embodiment of the present invention, in step S1, the temperature increase rate of the calcination before the constant temperature is 5 ℃/min.
According to one embodiment of the present invention, in step S1, the calcination is performed in a nitrogen or inert atmosphere.
According to an embodiment of the present invention, in step S1, the strong base includes at least one of lithium hydroxide, sodium hydroxide and potassium hydroxide.
According to a preferred embodiment of the present invention, in step S1, the strong base is sodium hydroxide.
According to a preferred embodiment of the present invention, in step S1, the reaction occurs as shown in formulas (1) to (4):
Figure BDA0002760326880000034
Figure BDA0002760326880000031
Figure BDA0002760326880000032
Figure BDA0002760326880000033
NaOH in formulas (1) to (4) can be replaced by LiOH or KOH.
On one hand, the strong base plays a role of a catalyst, and accelerates the disproportionation speed of the silicon monoxide at 700-900 ℃.
Under the protection of nitrogen or inert gas, when the temperature is raised to 600 ℃, the silicon monoxide starts to generate slow disproportionation reaction to generate silicon simple substance and silicon dioxide; however, if rapid disproportionation is to enable the silicon-based material to have better cycle performance and reversibility in the lithium ion battery cathode, the temperature needs to be heated to 1000-1200 ℃ to form a composite material with good crystallinity and a loose structure. According to the invention, a certain amount of strong base is added as a catalyst, so that the disproportionation speed of the silicon monoxide at 700-900 ℃ is increased; therefore, the composite material with good crystallinity and loose structure can be formed at the temperature of 700-900 ℃, and the energy consumption of the production process is further reduced.
The strong base plays a role of an etching agent on the other hand, and reacts with the multi-component silicon at high temperature to generate a porous and loose structure, so that the volume change of the silicon-carbon composite material in the charging and discharging process is relieved.
According to an embodiment of the present invention, the calcination in step S1 allows the composition of the multi-component silicon-based material to be controlled by adjusting the sintering time.
If the calcining time in the step S1 is 0.5-1 h, the multi-component silicon-based material is a compound of three components of silicon, silica and silicon dioxide; if the sintering time is 1-1.5 h, the multi-component silicon-based material is a compound of two components of silicon and silicon dioxide; if the reaction time is longer than 1.5h, the multi-component silicon-based material and the strong base can react as shown in formulas (2) to (4), so that a large amount of silicate is generated, and the properties of the multi-component silicon-based material are damaged.
According to one embodiment of the present invention, in step S2, the templating agent is a salt having a melting point > 800 ℃ and a solubility greater than 10g in 20 ℃ water.
According to a preferred embodiment of the present invention, in step S2, the template is at least one of sodium chloride, sodium sulfate, and potassium sulfate.
According to a preferred embodiment of the present invention, in step S2, the template agent is sodium chloride.
The template has the following functions: embedding the newly formed carbon layer in the form of unmelted crystalline particles during sintering at step S2; after the reaction is finished, the carbon layer obtains porosity after the template agent is removed by washing with deionized water. Thus, the templating agent should have a melting point of > 800 ℃ and be readily soluble in water, i.e., greater than 10g in water at 20 ℃.
According to an embodiment of the present invention, in step S2, the carbon source is one or more of glucose, sucrose, starch, citric acid, or ascorbic acid.
According to an embodiment of the invention, in the step S2, the mass ratio of the multi-component silicon-based material to the carbon source is 1:2 to 2: 1.
According to an embodiment of the present invention, in step S2, the mass ratio of the multi-component silicon-based material to the template is: 1: 8-1: 4.
According to an embodiment of the present invention, in step S2, the sintering temperature is 650 to 750 ℃.
According to an embodiment of the present invention, in step S2, the sintering time is not less than 2 hours at a constant temperature to ensure that the carbon source is completely carbonized to form the porous carbon coating layer.
According to an embodiment of the present invention, in step S2, the temperature rise rate before the constant temperature is 5 ℃/min for the sintering.
According to an embodiment of the present invention, in step S2, the mixing is wet mixing, and the operation method is: fully mixing the multi-component silicon-based material, glucose, sodium chloride, deionized water and absolute ethyl alcohol, stirring, heating and evaporating to paste, and drying.
According to one embodiment of the invention, the washing is water washing; and drying at the temperature of 50-140 ℃.
An application of a silicon-carbon composite material in a lithium ion battery cathode material.
An application of a lithium ion battery prepared from a silicon-carbon composite material in the field of power energy.
Compared with the prior art, the invention has at least the following beneficial effects:
(1) the silicon-carbon composite material provided by the invention has the advantages that all components have synergistic effect, and the silicon-carbon composite material has good rate performance and cycle performance.
When the current density is 0.1A/g, the gram specific capacity is 698.2 mAh/g; when the current density is 5A/g, the gram specific capacity is 432.7 mAh/g; when the current density is recovered to 0.1A/g, the corresponding specific charge capacity is recovered to 660.3 mAh/g.
Under the high current density of 0.2A/g, after 100 cycles, the charging specific capacity is 640.2mAh/g, and the capacity retention rate is 91.5%.
(2) The method adopts strong base as the catalyst for the disproportionation of the silicon monoxide, accelerates the disproportionation of the silicon monoxide at 700-900 ℃, and reduces the energy consumption of the production process.
(3) According to the silicon-carbon composite material provided by the invention, the silicon dioxide and the porous carbon are used as volume expansion buffer media, so that the problem of volume expansion of a silicon-based negative electrode material is solved, under the high current density of 0.2A/g, after 100 cycles, particles of the silicon-carbon composite material are not cracked, and the surface of a pole piece made of the material is not cracked.
(4) The silicon-carbon composite material provided by the invention keeps the advantage of high capacity of a silicon-based material, and the gram specific capacity of the silicon-based material reaches 699.5mAh/g under the current density of 0.2A/g.
(5) According to the silicon-carbon composite material provided by the invention, the porous carbon coating layer isolates the contact between the multi-component silicon-based material and the electrolyte, the decomposition and failure of the components of the electrolyte are relieved, and the cycle performance of the material can be effectively improved.
(6) The preparation method of the silicon-carbon composite material provided by the invention has the advantages of cheap and easily-obtained raw materials and low process cost.
(7) The preparation method of the silicon-carbon composite material only needs two steps, and is simple in process and high in controllability; the obtained material has good stability and strong oxidation resistance, and is suitable for commercial application.
Drawings
FIG. 1 is a scanning electron micrograph of a silicon carbon composite obtained in example 2.
FIG. 2 is an X-ray diffraction pattern of the multi-component silicon-based material obtained in example 1.
FIG. 3 is an X-ray diffraction pattern of the multi-component silicon-based material obtained in example 2.
FIG. 4 is an X-ray diffraction pattern of the multi-component silicon-based material obtained in example 3 and comparative example 1.
FIG. 5 is an X-ray diffraction pattern of the multi-component silicon-based material obtained in example 2 and comparative example 2.
Fig. 6 is an X-ray diffraction pattern of the silicon carbon composite material obtained in comparative example 3.
FIG. 7 is a graph of the AC impedance of the multi-component silicon-based material and the silicon-carbon composite material obtained in example 2.
FIG. 8 is a graph showing the rate capability results of the silicon carbon composite obtained in example 2.
FIG. 9 is a graph showing the cycle performance results of the silicon carbon composite obtained in example 2.
Fig. 10 is a graph showing the cycle performance results of the silicon carbon composite obtained in comparative example 2.
Fig. 11 is a graph showing the cycle performance results of the silicon carbon composite obtained in comparative example 3.
Figure 12 is a graph of cycling performance results for pure silicon materials.
FIG. 13 is a graph of the cycling performance results for silica.
FIG. 14 is scanning electron micrographs of the silicon-carbon composite material obtained in example 2 before and after cycling through a pole piece.
Detailed Description
Next, the present invention will be further described with reference to examples, comparative examples and drawings, but the present invention is not limited to these examples, comparative examples and drawings.
In the following examples and comparative examples, all the starting materials used were derived from commercially available products unless otherwise specified.
Example 1
The embodiment prepares the silicon-carbon composite material, and specifically comprises the following steps:
(1) weighing 4g of silicon monoxide and 1g of sodium hydroxide, adding 30mL of absolute ethyl alcohol, uniformly stirring, and drying at 60 ℃;
(2) placing the mixture obtained in the step (1) in a tubular furnace to calcine under the argon atmosphere, wherein the constant temperature is 700 ℃, the constant temperature duration is 1h, and the heating rate is 5 ℃/min;
(3) washing the product obtained in the step (2) with deionized water, and drying at 60 ℃ to obtain a multi-component silicon-based material;
(4) weighing 2g of multicomponent silicon-based material, 2g of glucose and 8g of sodium chloride, adding 30mL of water and 10mL of absolute ethyl alcohol, uniformly stirring, and evaporating to dryness;
(5) and (4) sintering the mixture obtained in the step (4) in an argon atmosphere, wherein the constant temperature is 700 ℃, the constant temperature duration is 2 hours, and the heating rate is 5 ℃/min.
(6) And (5) washing the sintered product obtained in the step (5) with deionized water, and drying at 60 ℃ to obtain the silicon-carbon composite material.
Example 2
The embodiment of the present invention provides a silicon-carbon composite material, which includes the following specific steps:
in the step (2), the constant temperature is 900 ℃;
in the step (4), 2g of the multicomponent silicon-based material, 2g of glucose and 10g of sodium chloride were mixed in proportion, and 20mL of water and 10mL of anhydrous ethanol were added.
Example 3
The embodiment prepares the multi-component silicon-based material, and specifically comprises the following steps:
(1) weighing 4g of silicon monoxide and 1g of sodium hydroxide, adding 30mL of absolute ethyl alcohol, uniformly stirring, and drying at 60 ℃;
(2) placing the mixture obtained in the step (1) in a tubular furnace to calcine under the argon atmosphere, wherein the constant temperature is 800 ℃, the constant temperature duration is 30min, and the heating rate is 5 ℃/min;
(3) washing the product obtained in the step (2) with deionized water, and drying at 60 ℃ to obtain a multi-component silicon-based material;
comparative example 1
This comparative example prepared a multi-component silicon-based material, the specific steps differing from example 3:
in the step (1), the amount of sodium hydroxide added was 0.2 g.
Comparative example 2
The comparative example prepares a silicon-carbon composite material, and the specific steps are as follows compared with the example 2:
in the step (1), sodium hydroxide is not added.
Comparative example 3
In the comparative example, a silicon-carbon composite material is prepared by taking the sintered product obtained in the step (5) in the example 2 as a raw material and adopting a magnesiothermic reduction method, and the method specifically comprises the following steps:
(1) mixing the sintered product obtained in the step (5) of the example 2 with magnesium powder, wherein the mass ratio of the sintered product to the magnesium powder is 4: 0.2.
(2) and (2) sintering the mixture obtained in the step (1) in a tubular furnace, wherein the constant temperature is 600 ℃, the constant temperature duration is 3h, and the heating rate is 5 ℃/min.
(3) And (3) cooling the material obtained in the step (2) to room temperature, sequentially washing with 0.1mol/L hydrochloric acid and deionized water, and drying at 60 ℃ to obtain the silicon-carbon composite material.
Test example
In the test example, the multi-component silicon-based material and the silicon-carbon composite material obtained in the above examples and comparative examples were subjected to physical and chemical property tests, and the specific test methods and results are as follows.
And (3) morphology characterization:
the morphology of the silicon-carbon composite material obtained in example 2 was characterized by a Scanning Electron Microscope (SEM), and the characterization results are shown in fig. 1. The information in the figure shows: the silicon-carbon composite material prepared in example 2 has an irregular shape and a loose structure, and a porous and loose coating layer is arranged on the surface of the silicon-carbon composite material.
The characterization of the electrode plate made of the silicon-carbon composite material obtained in example 2 as the active material is shown in fig. 14 (fig. 14a is the morphology before cycle, and fig. 14b is the morphology after cycle). The information in the figure shows: (1) the shapes of the pole pieces are almost not different before and after circulation; (2) after circulation, the active material particles are complete, the surface of the pole piece is smooth, and the pole piece is not cracked. The information shows that the silicon-carbon composite material prepared by the preparation method can effectively relieve the volume expansion problem of the silicon-based material, so that the electrochemical performance of the material is improved.
And (3) component characterization:
the components of the surface coating layer of the silicon-carbon composite material obtained in example 2 were characterized by EDS spectra, and the characterization results are shown in table 1.
Table 1 surface coating layer composition of the silicon carbon composite material obtained in example 2.
Figure BDA0002760326880000081
The results in table 1 show that the atomic number percentage of carbon in the coating layer on the surface of the silicon-carbon composite material 2 is 59.19%, which indicates that the main component of the coating layer is carbon; in conjunction with the morphology shown in fig. 1, the coating is a porous carbon coating.
And (3) characterization of a crystallization state:
and (3) representing the multi-component silicon-based material obtained in the examples 1-3 and the comparative examples 1-2 and the crystallization performance of the silicon-carbon composite material obtained in the comparative example 3 by adopting an X-ray diffraction pattern.
The XRD patterns of the multicomponent silicon-based materials obtained in example 1 (fig. 2) and example 2 (fig. 3) showed characteristic peaks of silica and elemental silicon. Information in the figure illustrates: (1) in the temperature parameter range provided by the invention, after a certain content of sodium hydroxide is added, the reaction of the formula (1) can be rapidly carried out on the silicon monoxide, and the silicon monoxide is disproportionated into a silicon simple substance and silicon dioxide; (2) the generated silicon simple substance and silicon dioxide have good crystallization performance.
The comparative XRD patterns of the multi-component silicon-based materials obtained in example 3 and comparative example 1 are shown in fig. 4. Wherein the multicomponent silicon-based material obtained in example 3 shows characteristic peaks of silicon dioxide and a simple substance of silicon, while the multicomponent silicon-based material obtained in ratio 1 shows no distinct characteristic peak; it is shown that the silica in comparative example 1 was not efficiently disproportionated, or the disproportionated product was amorphous. The results in FIG. 4 illustrate that if the amount of strong base is not within the parameters provided by the present invention, the temperature of rapid disproportionation of silica cannot be effectively reduced.
The comparative XRD patterns of the multi-component silicon-based materials obtained in example 2 and comparative example 2 are shown in fig. 5. Wherein the multi-component silicon-based material obtained in comparative example 2 does not show a characteristic peak of silica or elemental silicon, it is demonstrated that under the sintering condition of 900 ℃/1h as provided in example 2, if a strong base is not added, the silica cannot disproportionate to produce silica or elemental silicon having good crystallinity. Thus, the addition of a strong base can lower the rapid disproportionation temperature of amorphous silica.
The XRD pattern of the silicon-carbon composite material obtained in comparative example 3 is shown in fig. 6, which shows only the characteristic peak of the simple substance silicon, and illustrates that the sintered product obtained in step (5) of example 2 is reduced to the simple substance silicon coated with porous carbon after the magnesiothermic reduction reaction.
Application example
In the application example, the button type lithium ion battery is prepared by respectively taking the embodiment 2, the comparative example 2 and the comparative example 3, and elemental silicon and silicon dioxide as negative active materials, and electrochemical performance test is carried out, wherein the specific implementation method comprises the following steps:
(1) 70mg of active material, 20mg of conductive carbon black and 10mg of sodium alginate are taken, mixed and homogenized by taking water as a solvent, and smeared.
(2) And (2) drying the pole piece obtained in the step (1) for 4h at 80 ℃ in an air atmosphere, and then drying for 12h at 120 ℃ in a negative pressure less than or equal to 100 Pa.
(3) And (3) cutting the dried pole pieces obtained in the step (2) into round pieces with the diameter of 16mm, wherein the loading capacity of the active substances on each round piece is about 0.5 mg.
(4) And (4) assembling the 2025 type button cell by taking the wafer obtained in the step (3) as a positive electrode and the lithium metal sheet as a negative electrode.
The electrochemical performance of the battery prepared by the application example was tested, and the test results and analysis were as follows:
conductivity:
the ac impedance profile of the material obtained in example 2 is shown in fig. 7, from which it can be seen that: the resistance value of the silicon-carbon composite material obtained in the example 2 is obviously lower than that of the multi-component silicon-based material, which shows that the coating of the porous carbon obviously improves the conductivity of the material, and further reduces the resistance value.
Rate capability:
the rate capability of the silicon-carbon composite material obtained in example 2 was tested within the voltage range of 0.01-2.5V, and the performance results are shown in fig. 8. The information in the graph shows that when the initial current density is 0.1A/g, the gram specific capacity of the material is 698.2 mAh/g; when the current density is 5A/g, the gram specific capacity of the material is 432.7 mAh/g; when the current density is restored to 0.1A/g, the gram specific capacity of the material is also restored to 660.3 mAh/g. The silicon-carbon composite material provided by the invention has excellent rate performance.
Cycle performance:
the cycle performance of the silicon-carbon composite material obtained in example 2 was tested at a current density of 0.2A/g in a voltage range of 0.01-2.5V, and the test results are shown in FIG. 9. The information in the figure shows that the first cycle specific capacity of the material is 699.5mAh/g, after 100 cycles, the gram specific capacity is 640.2mAh/g, and the cycle retention rate reaches 91.5%. The silicon-carbon composite material provided by the invention has excellent cycle performance on the premise of keeping higher gram specific capacity.
The cycle performance of the silicon-carbon composite material obtained in comparative example 2 was tested at a current density of 0.2A/g in a voltage range of 0.01 to 2.5V, and the results are shown in FIG. 10. The information in the figure shows that although the first cycle specific capacity of the material is as high as 1566.6mAh/g, after 100 cycles, the gram specific capacity is only 479mAh/g, and the cycle retention rate is as low as 30.6%. The silicon-carbon composite material can exert higher first cycle gram specific capacity when the single-component silicon oxide is taken as a matrix, but the problem of volume expansion cannot be overcome, so that the cycle performance is poor.
The cycle performance of the silicon-carbon composite material obtained in comparative example 3 was tested at a current density of 0.2A/g within a voltage range of 0.01-2.5V, and the test results are shown in FIG. 11. The information in the figure shows that the first cycle specific capacity of the material is 1319.5mAh/g, after 100 cycles, the gram specific capacity is 841.4mAh/g, and the cycle retention rate is 63.8%. The above information shows that the silicon-carbon composite material prepared in comparative example 3 has poor cycle performance, but is superior to the silicon-carbon composite material prepared in comparative example 2. The reason is that the raw material of comparative example 3 is the sintered product obtained in step (5) of example 2, and therefore, after the magnesium thermal reduction and the cleaning, a part of the raw material is loose and porous, and the cycle capacity attenuation caused by volume expansion is relieved.
The cycle performance of pure silicon was tested at a current density of 0.2A/g in a voltage range of 0.01-2.5V, and the test results are shown in FIG. 12. The data show that the capacity retention of pure silicon is only 8.7% after 100 weeks of cycling. Due to low conductivity of pure silicon (10) -3 S/cm) and high volume expansion (up to 300% if fully intercalated at room temperature), and therefore has extremely poor cycle performance.
The cycle performance of the pure silicon dioxide is tested in a voltage range of 0.01-2.5V and under a current density of 0.2A/g. Test results figure 13 shows that the silica is almost electrochemically inactive, showing only a gram specific capacity of about 65 mAh/g.
The results of the cycle performance of each active material show that the cycle performance of the silicon-carbon composite material prepared by the method of the invention is better than that of the silicon-carbon composite materials of comparative example 2 (carbon-coated silicon monoxide) and comparative example 3 (carbon-coated silicon simple substance), and is better than that of the silicon simple substance and silicon dioxide. Therefore, the silicon-carbon composite material prepared by the preparation method provided by the invention can show excellent cycle performance on the basis of ensuring the performance of higher gram specific capacity.
The results and analysis show that the silicon-carbon composite material provided by the invention has obviously improved performance in cycle performance and rate capability compared with simple silicon, silicon dioxide, carbon-coated silicon and carbon-coated silicon monoxide. In the preparation method of the silicon-carbon composite material, strong alkali can effectively reduce the rapid disproportionation temperature of the silicon monoxide, so that silicon and silicon dioxide with excellent electrochemical performance and good crystallinity are generated; the strong base can also play a role of an etching agent, so that the multi-component silicon-based material has a porous property, and the problem of volume expansion of the multi-component silicon-based material is further solved.
The present invention has been described in detail with reference to the embodiments, comparative examples and drawings, but the present invention is not limited to the above description and various changes can be made within the knowledge of those skilled in the art without departing from the gist of the present invention.

Claims (6)

1. The silicon-carbon composite material is characterized by comprising the following components in parts by weight:
a multi-component silicon-based material; and
porous carbon coated on the surface of the multi-component silicon-based material;
the multi-component silicon-based material is selected from at least two of silicon, silicon monoxide and silicon dioxide;
the preparation method of the silicon-carbon composite material comprises the following steps:
s1, mixing strong base and silica micropowder, calcining, washing a product, and drying to obtain a multi-component silicon-based material;
s2, mixing the multi-component silicon-based material obtained in the step S1 with a carbon source and a template agent, sintering, washing and drying a product to obtain the silicon-carbon composite material; the sintering temperature is 650-750 ℃;
in step S2, the template agent is salt with melting point more than 800 ℃ and solubility more than 10g in water at 20 ℃;
in step S2, the carbon source is one or more of glucose, sucrose, starch, citric acid, and ascorbic acid.
2. The silicon-carbon composite material according to claim 1, wherein in step S1, the calcination temperature is 700-900 ℃ and the calcination time is 0.5-1.5 h.
3. The silicon-carbon composite material according to claim 1, wherein in step S1, the calcination is performed in nitrogen or an inert atmosphere.
4. The silicon-carbon composite material according to claim 1, wherein in the step S1, the mass ratio of the strong base to the silicon monoxide is 1: 2-1: 10.
5. The silicon-carbon composite material according to claim 1, wherein in step S2, the mass ratio of the multi-component silicon-based material, the carbon source and the template is: 1:2: 8-2: 1: 8.
6. The application of the silicon-carbon composite material of claim 1 in a lithium ion battery negative electrode material.
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