CN106848277B

CN106848277B - Magnesium-iron-oxygen/carbon composite material and preparation method thereof

Info

Publication number: CN106848277B
Application number: CN201710045078.9A
Authority: CN
Inventors: 于洋; 耿孝岭; 王巍
Original assignee: Qufu Normal University
Current assignee: Qufu Normal University
Priority date: 2017-01-22
Filing date: 2017-01-22
Publication date: 2020-07-07
Anticipated expiration: 2037-01-22
Also published as: CN106848277A

Abstract

The invention relates to a magnesium-iron-oxygen/carbon composite material and a preparation method thereof, wherein an iron source, magnesium powder and a liquid carbon source are mixed, transferred into a closed reaction container, sealed and placed in a crucible furnace, and reacted for 8-12h at the temperature of 550-650 ℃ to obtain a composite product. In the invention, the magnesium powder, the liquid carbon source and the ferric salt react at the same time, the produced magnesium oxide and ferric oxide form homogeneous-phase magnesium ferrite, and a carbon layer formed by the liquid carbon source is just coated on the surface of the particle. In the process, magnesium oxide and ferric oxide are synchronously synthesized, when binary oxide particles are generated, a liquid carbon source is synchronously pyrolyzed to obtain a carbon coating layer, the two substances are mutually attached under the atmosphere of chemical reaction, the carbon coating layer and the binary oxide particles form an ideal core-shell structure, and the carbon composite material is constructed in one step.

Description

Magnesium-iron-oxygen/carbon composite material and preparation method thereof

Technical Field

The application relates to a preparation method of a carbon composite material capable of being used as a lithium ion battery cathode material and lithium battery property detection thereof, in particular to a preparation method of a carbon composite magnesium ferrite material and application thereof, wherein the carbon composite magnesium ferrite material has the characteristics of high multiplying power and stable circulation when being used as a lithium battery cathode material.

Background

At present, the metal oxide as a lithium ion negative electrode material is a hot research topic (the theoretical specific capacity of the metal oxide is as high as 1000mAh/g, while the theoretical specific capacity of the traditional graphite negative electrode material is only 372mAh/g), but the metal oxide has certain defects so as to limit the application of the metal oxide.

The oxide as a lithium battery negative electrode material, such as pure iron oxide, corresponds to the charge-discharge behavior between the iron oxide and the metallic iron particles during the cycle, i.e. the phase of the battery before and after one cycle changes, thereby causing the collapse of the material structure and the gradual deterioration of the cyclicity. The most effective method at present is to coat a layer of conductive carbon on the surface of oxide particles, wherein the carbon layer can prevent the aggregation of oxides and can also be used as a support body of the oxides to keep the microscopic appearance of the oxides unchanged; thirdly, the carbon coating layer has excellent conductivity, and can improve the high-rate charge and discharge performance of the material, and the improvement way is generally accepted by researchers.

It is believed that the structural characteristics of the surface of the material are primarily altered by the coated carbon material, and in order to achieve similar improvements in the interior of the particles of the material, researchers have discovered that the activity and structural stability of the material can be improved by utilizing the synergistic effect of two or more metals. Such a multi-component oxide produces a corresponding oxide with intercalation/deintercalation of lithium ions during the cycle. For example, a copper-iron-oxygen material, the initial material is CuFe2O4 pure phase, the battery discharges firstly, and the following reaction (1-2) occurs; the first charge, the (3-4) reaction takes place, which is the end of the first cycle. The cell continues to cycle, and only the reaction of (3-4), i.e., the subsequent reaction, is the interconversion of the oxide with its corresponding metal.

CuFe₂O₄+nLi^--ne^-→Li_nCuFe₂O₄(1)

Li_nCuFe₂O₄-(8-n)Li^-+(8-n)e^-→xCu⁰+(2-x)Fe⁰+Fe_xCu_1-x+4Li₂O(0≤n.x≤1) (2)

At present, in order to obtain a multi-component oxide and a carbon composite/carbon coated structure, researchers generally adopt the following experimental schemes:

A. solid phase synthesis, e.g. CuFe₂O₄Firstly, raw materials are mixed by ball milling, and then inert gas is required to be introduced: such as nitrogen. (reference: One-step solid state reaction to selective catalytic composite CuFe₂O₄anode material for high power lithium ion batteries)。

B. The sol-gel method comprises preparing a homogeneous solution, treating to form a gel, and calcining at high temperature in nitrogen (Combustion Synthesis of MgFe)₂O₄graphene nanocomposite as a high-performancenegative for lithium ion batteries)。

C. And (3) carrying out a solvothermal method, mixing magnesium salt and iron salt in EG, adding a small amount of additive additionally, and carrying out a solvothermal reaction at 200 ℃. However, the number of cycles reported is only 70 (Hollow spheres of MgFe)₂O₄as anode material for lithium-ion batteries)。

D. Sol-gel method: after the ferric nitrate, the magnesium nitrate and the citric acid form gel, the product (MgFe) can be obtained through three-step heat treatment₂O₄nanoparticles as anode materials for lithium-ion batteries)。

E. The coprecipitation method comprises the steps of firstly preparing a binary precursor and then calcining at high temperature to obtain the product. Coating the carbon material by mixing the product with a conventional solid carbon source and carbonizing the mixture at a high temperature (Preparation of carbon-coated MgFe)₂O₄withexcellent cycling and rate performance)。

F. Microwave-assisted methods use graphene oxide as the carbon material. Graphene oxide is expensive, and the reaction of the method is complex (MgFe)₂O₄reduced graphene oxide as high-performance and material for the solution of batteries; microwave catalyst MgFe₂O₄-Fe₂O₃Microwave catalytic oxidation degradation of crystal violet wastewater _ \ "Gaoshangfei \").

Through the experimental procedures, it can be seen that many research works can improve the performances of the material, such as conductivity, specific discharge capacity and the like, to a certain extent, but most experimental processes are complex, and special gas is required to be introduced sometimes, which can increase the synthesis cost. The complex with carbon material requires even more steps, which results in complex synthetic route, low yield and high cost.

Disclosure of Invention

In order to overcome the problems of complex synthesis process, particularly carbon composite process, even need of using inert gas, low yield and high cost in the prior art, the invention provides a preparation method of a magnesium-iron-oxygen/carbon composite material, which comprises the steps of mixing an iron source, magnesium powder and a liquid carbon source, transferring the mixture into a closed reaction container, sealing and placing the container into a crucible furnace, and reacting for 8-12 hours at the temperature of 550-650 ℃ to obtain a composite product.

Different from the prior art that the method of mixing the salts of two metals and then processing the mixture is adopted, the invention utilizes the reducibility of metal magnesium powder, Mg reacts with a liquid carbon source such as ethanol under a closed condition, Mg is converted into magnesium carbonate or magnesium oxide, the ethanol releases gas, part of the ethanol is converted into a carbon material, and ferric nitrate (ferric salt) in the raw material is decomposed into ferric oxide. Namely, the magnesium powder, the ethanol and the ferric salt react at the same time, the produced magnesium oxide and the ferric oxide form uniform-phase magnesium ferrite, and a carbon layer formed by the ethanol is just coated on the surface of the particles. In the process, magnesium oxide and ferric oxide are synchronously synthesized, when binary oxide particles are generated, a carbon source (ethanol) is synchronously pyrolyzed to obtain a carbon coating layer, the two substances are attached to each other in a chemical reaction atmosphere, the carbon coating layer and the binary oxide particles form an ideal core-shell structure, and the carbon composite material is constructed in one step.

The equation for the reaction is:

Mg+CH₃CH₂OH→2C+MgO+3H₂↑

4Fe(NO₃)₃＝2Fe₂O₃+l2NO₂↑+3O₂↑

MgO+Fe₂O₃＝MgFe₂O₄

the total reaction is as follows:

2Mg+2CH₃CH₂OH+4Fe(NO₃)₃→2MgFe₂O₄/C+2C+6H₂↑+l2NO₂↑+3O₂↑

the carbon composite process is an important process in the synthesis of carbon composite materials. Currently, most of the reported carbon recombination processes require 2 or more steps to be achieved, such as (E and F in the above references). The preparation is much more complicated than our one-step preparation. At present, many reports show that the carbon material is obtained under a special inert atmosphere, the method has low energy consumption, gas consumption and yield, and the conductivity of the obtained carbon material is much lower than that of the carbon material obtained in a closed container.

Meanwhile, the better the crystallinity of the oxide is, which shows that the micro crystal defects are few, and the electrochemical performance is also positively influenced. In terms of improving the crystallinity of the material, there are reports that: first, a precursor of the oxide or an oxide having poor crystallinity is synthesized, and annealed at a high temperature, thereby achieving the purpose of improving crystallinity, such as (B and D in the above references). The work of the inventor is to put the raw materials into a reaction vessel and heat the raw materials to a fixed temperature at one time, so that oxide particles with good crystallinity can be obtained.

The material of the invention can be synthesized by one-step reaction. In the conventional carbon compounding process, the obtained oxide particles are mixed with the carbon material, which is a pure mixture, and the oxide and the carbon material are only in surface contact. The carbon composite material is a product obtained by mixing raw materials and performing intermolecular contact reaction in a closed reaction container. Therefore, the contact between the oxide particles and the carbon layer is more uniform, and the carbon layer is uniformly covered on the surface layers of all the magnesium ferrite particles through a transmission electron microscope, and the thickness of the magnesium ferrite particles is uniform. In general, oxides obtained at low temperatures (about 200 ℃) require a high-temperature (generally greater than 500 ℃) annealing process, which removes a small amount of moisture from the interior of the oxide and improves the crystallinity of the oxide. In the present invention, the reaction temperature is controlled to be directly raised from room temperature to 600 ℃, and oxide particles having excellent crystallinity are directly obtained. The preparation method is simple, only the tightness of the reaction container needs to be paid attention to, no special atmosphere is needed for assistance, the experimental process is simple and convenient, the operability is strong, and the product can be obtained by only one step.

It is preferable that: the iron source is at least one selected from ferric nitrate, ferric chloride and other ferric iron sources.

It is preferable that: the liquid carbon source is at least one of absolute ethyl alcohol, diethyl ether, acetone, ethylene glycol and the like.

It is preferable that: the mol ratio of the iron to the magnesium is 2-2.5: 1-1.5.

It is preferable that: the ratio of the volume of the liquid carbon source to the number of moles of magnesium is 0.1-0.5 mol/L.

It is preferable that: the mixing condition of the iron source, the magnesium powder and the liquid carbon source is ultrasonic mixing for 30min at room temperature.

It is preferable that: the heating rate in the crucible furnace is 6-8 ℃/min.

It is preferable that: and washing the composite product by a mixed solution of water and alcohol, and drying for 4-10h at the temperature of 60-80 ℃.

The invention also provides the magnesium-iron-oxide/carbon composite material prepared by the method, wherein the carbon exists in a form comprising a carbon coating layer and carbon spheres, the carbon coating layers are connected with each other to form a three-dimensional configuration and are uniformly coated on the surfaces of magnesium-iron oxide particles, so that the oxide particles do not grow any longer, are maintained to be small in particle size, are round and are relatively stable. The oxide is easy to aggregate and grow, particularly the iron-containing oxide, the product can grow to the micron level due to the magnetism, and if the iron oxide is not coated by the carbon layer, the iron oxide is easy to aggregate to form larger octahedron or irregular aggregates.

The diameter of the magnesium iron oxide particles is 30-80nm, the diameter of the carbon spheres is micron-sized, the thickness of the carbon coating layer is 7-9nm, and the mass ratio of the carbon coating layer to the carbon spheres is 1: 8-10.

Different from the prior art, the magnesium-iron-oxide material provided by the invention generates MgO and iron oxide in a circulation process, wherein the magnesium oxide is mainly used as a support body of iron oxide particles, maintains the micro-morphology of the iron oxide and does not participate in charge and discharge behaviors (the magnesium oxide is not used as a lithium battery negative electrode material and therefore does not participate in charge and discharge). Thus, the presence of magnesium oxide serves to stabilize the structure from the interior of the composite material, resulting in extremely high cycle stability of the product.

The carbon material in the invention is a carbon layer and a carbon sphere, both of which play a role of stabilizing the material, but the emphasis points are different:

the carbon coating is thin, and the size of the magnesium-iron-oxygen particles can be controlled in the experimental product generation process. If the particles of the magnesium ferrite are large, the collapse of the structure is serious in the charging and discharging processes of the battery, and the transportation of lithium ions is also not facilitated. Therefore, the function of the carbon coating layer is important, the nanoscale carbon coating layer can limit the size of magnesium, iron and oxygen particles, and can be used as a conductive material to rapidly transmit electrons, so that the discharge specific capacity of the material is improved, and the cycle life of the material is prolonged.

The carbon sphere particles are large, can prevent the active particles from aggregating, can also serve as a material to provide capacity, and can serve as an attachment of the active particles to prevent the active particles from collapsing and play a role in stabilizing the structure. Therefore, the carbon spheres provide discharge capacity on one hand and also play a role in prolonging the cycle life of the material.

The existence of the carbon-coated magnesium ferrite and the carbon spheres greatly improves the discharge capacity and the cycle life of the composite material when the composite material is used as a lithium ion battery cathode material.

Drawings

FIG. 1 is the EDX diagram of example 1

FIG. 2 is MgFe of example 1₂O₄Scanning electron microscope picture of @ C product

FIG. 3 is a scanning electron micrograph of a carbon coating layer according to example 1

FIG. 4 is MgFe of example 1₂O₄Transmission electron microscope picture of @ C product

FIG. 5 is a transmission electron micrograph of a carbon clad layer according to example 1

FIGS. 6-7 are all MgFe of example 1₂O₄High resolution transmission diagram of @ C product

FIGS. 8-9 are thermogravimetric analysis plots (10ml ethanol in FIG. 8 for the thermogravimetric plot of example 1 and 5ml ethanol in FIG. 9 for the thermogravimetric plot of example 6) for examples 1 and 6, FIG. 10 is an XRD diffractogram for examples 1-3

FIG. 11 is the XRD diffractograms of examples 4-5

FIG. 12 is the XRD diffractogram of examples 3 and 6

FIGS. 13-16 are XRD diffractograms for examples 7-10, respectively

FIGS. 17 to 18 are constant current charge/discharge cycle diagrams of example 1 (FIG. 17 shows a specific charge/discharge capacity at a current magnification of 100mA/g, FIG. 18 shows a variation magnification in the order of 0.1C-0.2C-0.5C-1C-1.5C-2C-2.5C-2C-1.5C-1C-0.5C-0.C (1℃ 1000mA/g))

FIGS. 19 to 20 are constant current charge/discharge cycle diagrams of example 1 (FIG. 19 shows a specific charge/discharge capacity at a current magnification of 1000mA/g, FIG. 20 shows a specific charge/discharge capacity of 3000mA/g, and show that the theoretical capacity of conventional graphite is 372mAh/g and that of magnesium iron oxide is 1000 mAh/g.)

Detailed Description

Specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings.

The Mg powder dosage is 0.02-0.056g, and pure phase products can be obtained, which can be matched just: PCPDF standard card number: 88-1936.

Example 1: weighing 2.0130g Fe (NO)₃)₃·9H₂0.0560g of O and metal magnesium powder, and 10ml of absolute ethyl alcohol are poured. Ultrasonic treating at room temperature for 30min, transferring and sealing into a high-temperature reaction kettle with a volume of 20 ml. Placing the reaction kettle in a crucible furnace, heating to 600 deg.C at a heating rate of 6 deg.C/min, maintaining for 10 hr, naturally cooling, collecting the product, washing with mixed solution of deionized water and anhydrous ethanol for 3 times, and drying in a drying oven at 70 deg.C for 6 hr to obtain black powder product (MgFe)₂O₄@ C, XRD, fig. 10).

Example 2: weighing 2.0154g Fe (NO)₃)₃·9H₂O and 0.0325g of metal magnesium powder, and 10ml of absolute ethyl alcohol is poured into the mixture. The subsequent procedure was the same as in example 1, giving a pure phase product, XRD.

Example 3: weighing 2.0155g Fe (NO)₃)₃·9H₂O and 0.0287g of metal magnesium powder, and 10ml of absolute ethyl alcohol is poured into the mixture. The subsequent procedure was the same as in example 1, giving a pure phase product, XRD.

The Mg powder amount is more than 0.056g, and the obtained product contains magnesium carbonate (MgCO)₃) Impurities, corresponding to the 401 crystal plane in magnesium carbonate standard cards 08-0479.

Example 4: weighing 2.0558gFe(NO₃)₃·9H₂0.1194g of O and metal magnesium powder, and 10ml of absolute ethyl alcohol are poured. The subsequent procedure was the same as in example 1, and a product containing magnesium carbonate impurities was obtained, as confirmed by XRD, as shown in FIG. 11.

Example 5: weighing 2.0023g Fe (NO)₃)₃·9H₂O and 0.0726g of metal magnesium powder are poured into 10ml of absolute ethyl alcohol. The subsequent procedure was the same as in example 1, and a product containing magnesium carbonate impurities was obtained, as confirmed by XRD, as shown in FIG. 11.

The dosage range of ethanol is as follows: 5-10ml, less influence on the product purity, the magnesium iron oxide particle size and the carbon layer thickness, and less influence on the carbon content in the product (see fig. 8-9, thermogravimetric analysis of carbon content), compared to example 2.

Example 6: weighing 2.0141g Fe (NO)₃)₃·9H₂0.0338g of O and metal magnesium powder, and 5ml of absolute ethyl alcohol are poured. The subsequent procedure was the same as in example 1, and a pure phase product was obtained, and XRD was shown in FIG. 12.

Example 7: the iron source being FeCl₃·6H₂O (1.330g, 4.92mmol), magnesium powder (0.0308g) and 6ml of ethanol (the effect of the ethanol amount is not large, and the result of 10ml is basically consistent with that of 6 ml). The subsequent process was the same as example 1, and the product was pure phase magnesium ferrite. XRD is shown in figure 13.

Example 8: the iron sources were ferric nitrate (2.058g), magnesium powder (0.032g), diethyl ether 5ml (10ml resulted in substantially the same result as 5 ml). The subsequent process is the same as example 1, the product contains a small amount of magnesium carbonate impurities, and the magnesium carbonate impurities are removed by adjusting the using amount of magnesium powder. XRD is shown in FIG. 14.

Example 9: the iron sources were ferric nitrate (2.007g), magnesium powder (0.031g), and acetone 5ml (10ml results are essentially the same as 5 ml). The subsequent process was the same as example 1, and the product was pure phase magnesium ferrite. XRD is shown in figure 15.

Example 10: the iron sources were ferric nitrate (2.038g), magnesium powder (0.032g), and ethylene glycol 5ml (10ml resulted in substantially the same result as 5 ml). The subsequent process was the same as example 1, and the product was pure phase magnesium ferrite. XRD is shown in figure 16.

The composite material of the present application combines the advantages of either phase alone while avoiding the disadvantages of the respective phases. For example, the carbon material has the advantages of extremely high conductivity and stable cycle when being used as a lithium battery negative electrode material, but the specific discharge capacity of the carbon material is lower (the theoretical specific capacity of the carbon material is limited and is 372 mAh/g); the magnesium-iron-oxygen material has higher specific capacity (the theoretical specific capacity is about 1000mAh/g), but the cycle performance is poorer. Tests show that the composite material constructed by the method not only has higher specific discharge capacity, but also has higher capacity retention rate after being cycled for thousands of times, and is a lithium battery cathode substitute material with high potential, high multiplying power and long service life. The method has the advantages of few types of materials, low cost and environmental friendliness; the product is easy to prepare, has strong operability and is easy for large-scale production.

Mixing the product with acetylene black and PVDF powder to prepare a negative pole piece; and assembling a button lithium battery in the glove box. Constant current charge and discharge behavior tests were performed as shown in fig. 17-20.

Fig. 17 shows that the charge-discharge cycle performance of the material at a current multiplying factor of 100mA/g tends to gradually increase the capacity (after 60 cycles, the discharge specific capacity is 941mAh/g, which is 94% of the theoretical specific capacity), and the capacity increases smoothly, which shows that the structure of the material is gradually stable.

Fig. 18 shows that the structure of the material is stable, and even after a large-rate discharge (2.5C: 2500mA/g) and then a return to a small-rate (0.1C), the material still has a similar initial discharge capacity (920mAh/g), and the coulomb efficiency is almost 99.9%, indicating that the structure is stable.

Fig. 19-20 are the cycle performance at higher current rates of the cells. FIG. 19 shows the specific charge/discharge capacity at a current magnification of 1000mA/g, and FIG. 20 shows the specific charge/discharge capacity of 3000 mA/g. Under 2 kinds of higher current multiplying power, the discharge specific capacity of the battery is smooth and stable, the cycle is respectively carried out for 650 times and 1000 times, the capacity is stable at 665mAh/g and 440mAh/g, and the capacity is generally higher than that of the current report; and the cycle life is not reduced for thousands of times.

The presence of the four elements magnesium, iron, oxygen, carbon in the product can be illustrated by the EDX diagram, as shown in figure 1.

Analyzing the surface structure and the core-shell structure of the composite material through a scanning electron microscope and a transmission electron microscope:

FIG. 2 shows that there are larger carbon spheres (at black circles) in the product, as well as smaller carbon-coated MgFeO particles; FIG. 3 is a diagram for easy observation of the core-shell structure, in which the Mg-Fe-O particles in the product have been washed away by hydrochloric acid.

As in fig. 2, it can be seen that there are larger carbon spheres (black circles) in the product, carbon spheres: 5-7 μm; and smaller carbon-coated magnesium iron oxide particles. In order to conveniently observe the core-shell structure, magnesium, iron and oxygen particles, the rest carbon coating layer and carbon spheres in the product are removed by hydrochloric acid, and the product is detected and scanned by ultrasound. As can be seen from fig. 3, the carbon coating layer has a significant void phenomenon, and the hollow part is the original mg-fe-o particles, because after the carbon composite oxide is soaked in acid, mg-fe-o reacts with acid to produce mg ions and fe ions that can be dissolved in water, and only the remaining carbon materials, such as carbon layers and carbon spheres, are precipitated.

Ethanol produces carbon materials in two forms, one is large carbon spheres and the other is a carbon layer which can be clearly observed by transmission pictures. Since oxygen is limited in the closed container, ethanol is not completely decomposed into gas, but partially carbonized. Meanwhile, the furnace temperature rises quickly, the generation of oxides and the decomposition and carbonization of ethanol are almost synchronously carried out, so that magnesium-iron-oxygen particles are just formed, and the carbon layer covers the surface of the magnesium ferrite, inhibits the growth of the magnesium-iron-oxygen particles and is controlled to be about 50 nm.

Carbon materials have two morphologies: carbon coating layer: 7-9nm thick, the average size of the carbon-coated MgFeOx particles is 50nm (see transmission diagram, FIGS. 4-5, FIG. 4 can clearly see the uniform carbon layer; FIG. 5 is for easy observation, the MgFeOx particles have been washed away by hydrochloric acid, so the carbon layer in the diagram is a hollow structure). FIG. 4 is MgFe₂O₄The @ C product, FIG. 5 is a carbon coating layer, the material microscopically presents a three-dimensional carbon coating structure, wherein MgFe₂O₄The particles are uniformly embedded inside the carbon layer and connected to each other through the carbon layer (see fig. 4). FIGS. 6 to 7 are all MgFe₂O₄The high resolution transmission picture of the @ C product, the thickness of the carbon layer is 7-9 nm.

The thermogravimetric analysis method is adopted to test the carbon content, the ethanol content is 10mL in figure 8, the ethanol content is 5mL in figure 9, and the carbon percentage content is 56.94% and 53.27% respectively, which shows that the influence of the ethanol dosage on the carbon content in the experiment is not great.

Claims

1. A method of preparing a magnesium ferrite/carbon composite material, comprising:

magnesium ferrite/carbon composite material: the carbon exists in a form of carbon coating layers and carbon spheres, the carbon coating layers are connected with each other to present a three-dimensional configuration and are uniformly coated on the surfaces of the magnesium-iron oxide particles, the diameter of the carbon spheres is micron-sized, the thickness of the carbon coating layers is 7-9nm, the diameter of the magnesium-iron oxide particles is 30-80nm, and the mass ratio of the carbon coating layers to the carbon spheres is 1: 8-10;

the method comprises the following steps: mixing an iron source, magnesium powder and a liquid carbon source, transferring the mixture into a closed reaction container, sealing the closed reaction container and placing the closed reaction container into a crucible furnace, and reacting the mixture for 8 to 12 hours at the temperature of 550-650 ℃ to obtain a composite product;

the iron source is a trivalent iron source, and the liquid carbon source is at least one of alcohols and ethers.

2. The method of claim 1, wherein: the iron source is at least one of ferric nitrate or ferric chloride.

3. The method of claim 1, wherein: the liquid carbon source is at least one of absolute ethyl alcohol, ethylene glycol, acetone and diethyl ether.

4. The method of claim 1, wherein: the molar ratio of iron to magnesium is 2-2.5: 1-1.5.

5. The method of claim 1, wherein: the ratio of the volume of the liquid carbon source to the number of moles of magnesium is 0.1-0.5 mol/L.

6. The method of claim 1, wherein: the mixing condition of the iron source, the magnesium powder and the liquid carbon source is ultrasonic mixing for 30min at room temperature.

7. The method of claim 1, wherein: the heating rate in the crucible furnace is 6-8 ℃/min.

8. The method of claim 1, wherein: and washing the composite product by a mixed solution of water and alcohol, and drying for 4-10h at the temperature of 60-80 ℃.