CN111020749A - Cobalt-loaded hollow carbon nanofiber composite catalyst and preparation method and application thereof - Google Patents
Cobalt-loaded hollow carbon nanofiber composite catalyst and preparation method and application thereof Download PDFInfo
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Abstract
The invention provides a cobalt-loaded hollow carbon nanofiber composite catalyst and a preparation method and application thereof, belonging to the technical field of lithium-oxygen batteries. The invention utilizes the electrostatic spinning technology to prepare the self-supporting, binderless, porous and light hollow carbon nanofiber composite catalyst loaded with cobalt, which is in a three-dimensional network shape with large specific surface areaThe composite fiber has the advantages of small contact resistance, small electrode polarization, light weight and high efficiency, the structure of the porous channel can increase the specific surface area, increase the active sites and be beneficial to the transmission of ions, oxygen and the like, thereby optimizing the air electrode structure of the battery, greatly improving the performance of the lithium-oxygen battery, and obtaining 100 mA.g after the battery is formed‑1The highest discharge capacity can reach 4427 mA.h.g during charging and discharging‑1(ii) a When the cut-off capacity is 500mA · h · g‑1At 200mA · g‑1Under the charging and discharging conditions, the battery can be cycled for 60 weeks, and the cycling stability is good.
Description
Technical Field
The invention relates to the technical field of lithium-oxygen batteries, in particular to a cobalt-loaded hollow carbon nanofiber composite catalyst and a preparation method and application thereof.
Background
In recent years, lithium-oxygen batteries have attracted more and more attention. The lithium-oxygen battery is a novel efficient, relatively environment-friendly green battery with extremely high theoretical specific capacity. The anode material is derived from oxygen in the air, and the cathode material is metallic lithium. Theoretically, O at the positive electrode of a lithium-oxygen battery2The amount is not limited, the capacity thereof mainly depends on the negative electrode metal Li flake, and therefore the capacity thereof should be very large, but the actual capacity thereof is far from the theoretical value, and there are still many problems. First, the greatest problem is that the reduction reaction of oxygen on the air electrode side is very slow, and in addition, the main discharge product Li generated during discharge is2O2The conductivity is poor and the decomposition is extremely difficult. Thus, to reduce the electrochemical polarization during the positive reaction and overpotential during charging, an effective oxygen catalyst can be added to promote O2Reduction and precipitation at the cathode, and reduction of charge and discharge voltage. However, the prior art catalysts limit the performance of lithium oxygen batteries.
Disclosure of Invention
In view of the above, the present invention aims to provide a cobalt-supported hollow carbon nanofiber composite catalyst, and a preparation method and an application thereof. The self-supporting and binderless porous light cobalt-loaded hollow carbon nanofiber composite catalyst prepared by the invention has the advantages of small contact resistance, small electrode polarization, light weight and high efficiency, and the porous channel structure can increase the specific surface area, increase the active sites and is beneficial to the transmission of ions, oxygen and the like, so that the air electrode structure of the battery is optimized, and the performance of the lithium-oxygen battery is greatly improved.
In order to achieve the above object, the present invention provides the following technical solutions:
the invention provides a preparation method of a cobalt-loaded hollow carbon nanofiber composite catalyst, which comprises the following steps:
mixing polyacrylonitrile, cobalt acetate tetrahydrate and N, N-dimethylformamide to obtain a shell precursor;
mixing polymethyl methacrylate with N, N-dimethylformamide to obtain a core body precursor;
injecting the shell precursor into an outer-layer injector, injecting the core precursor into an inner-layer injector, and performing electrostatic spinning to obtain fibers;
and carrying out heat treatment on the fibers to obtain the cobalt-loaded hollow carbon nanofiber composite catalyst.
Preferably, the weight content of polyacrylonitrile in the shell precursor is 10-12%, and the weight content of cobalt acetate tetrahydrate is 5-7%.
Preferably, the weight content of the polymethyl methacrylate in the core body precursor is 20-25%.
Preferably, the voltage of the electrostatic spinning is 15-20 KV, and the distance between the needle head and the receiving plate is 15-20 cm.
Preferably, the advancing speed of the outer-layer injector is 0.11-0.13 mm/min, and the advancing speed of the inner-layer injector is 0.08-0.10 mm/min.
Preferably, the heat treatment comprises a first heat treatment and a second heat treatment which are sequentially carried out, wherein the first heat treatment is carried out in an air atmosphere, the temperature of the first heat treatment is 225-255 ℃, the heat preservation time is 4-5 h, and the temperature rising speed of the temperature rising to the first heat treatment temperature is 1 ℃/min; the second heat treatment is carried out in high-purity nitrogen, the temperature of the second heat treatment is 700-900 ℃, the heat preservation time is 1-2 h, and the temperature rising speed of rising to the second heat treatment temperature is 2 ℃/min.
The invention also provides the cobalt-loaded hollow carbon nanofiber composite catalyst prepared by the preparation method in the technical scheme, wherein the cobalt-loaded hollow carbon nanofiber composite catalyst takes polyacrylonitrile doped with cobalt nanoparticles as a shell and polymethyl methacrylate as a core, and pores are formed on the wall of the hollow structure.
Preferably, the wall thickness of the cobalt-loaded hollow carbon nanofiber composite catalyst is 50-100 nm.
Preferably, the Co content in the cobalt-loaded hollow carbon nanofiber composite catalyst is 20-30 wt%.
The invention also provides application of the cobalt-loaded hollow carbon nanofiber composite catalyst in the technical scheme as a cathode material of a lithium-oxygen battery.
The invention provides a preparation method of a cobalt-loaded hollow carbon nanofiber composite catalyst, which comprises the following steps: mixing polyacrylonitrile, cobalt acetate tetrahydrate and N, N-dimethylformamide to obtain a shell precursor; mixing polymethyl methacrylate with N, N-dimethylformamide to obtain a core body precursor; injecting the shell precursor into an outer-layer injector, injecting the core precursor into an inner-layer injector, and performing electrostatic spinning to obtain fibers; and carrying out heat treatment on the fibers to obtain the cobalt-loaded hollow carbon nanofiber composite catalyst.
The invention utilizes the electrostatic spinning technology to prepare the self-supporting, binderless, porous and light hollow carbon nanofiber composite catalyst loaded with cobalt, the three-dimensional network-shaped composite fiber with large specific surface area has the advantages of small contact resistance, small electrode polarization, light weight and high efficiency, the structure of the porous channel can increase the specific surface area, increase the active sites and be beneficial to the transmission of ions, oxygen and the like, thereby optimizing the air electrode structure of the battery, greatly improving the performance of the lithium-oxygen battery, and after the battery is formed, 100 mA.g-1The highest discharge capacity can reach 4427 mA.h.g during charging and discharging-1(ii) a When the cut-off capacity is 500mA · h · g-1At 200mA · g-1Under the charging and discharging conditions, the battery can be cycled for 60 weeks, and the cycling stability is good.
Drawings
FIG. 1 is a first charge-discharge curve diagram of a lithium-oxygen battery assembled by Co-HCNF and 20 wt% Pt/C air electrode;
FIG. 2 is a constant capacity charge-discharge cycle diagram of a lithium-oxygen battery assembled by Co-HCNF and 20 wt% Pt/C air electrode;
FIG. 3 is a graph showing the discharge capacity of a lithium-oxygen battery assembled by Co-HCNF and 20 wt% Pt/C as a function of the number of cycles;
FIG. 4 is a scanning electron microscope micrograph of cobalt-loaded hollow carbon nanocomposite fibers;
FIG. 5 is a transmission electron micrograph of Co-HCNF;
FIG. 6 is a spectrum of the distribution energy of C, O, Co and N elements in Co-HCNF;
FIG. 7 is an XRD pattern of Co-HCNF.
Detailed Description
The invention provides a preparation method of a cobalt-loaded hollow carbon nanofiber composite catalyst, which comprises the following steps:
mixing polyacrylonitrile, cobalt acetate tetrahydrate and N, N-dimethylformamide to obtain a shell precursor;
mixing polymethyl methacrylate with N, N-dimethylformamide to obtain a core body precursor;
injecting the shell precursor into an outer-layer injector, injecting the core precursor into an inner-layer injector, and performing electrostatic spinning to obtain fibers;
and carrying out heat treatment on the fibers to obtain the cobalt-loaded hollow carbon nanofiber composite catalyst.
The invention mixes Polyacrylonitrile (PAN), cobalt acetate tetrahydrate and N, N-dimethyl formamide (DMF) to obtain the precursor of the shell.
In the invention, the weight content of polyacrylonitrile in the shell precursor is preferably 10-12%, and cobalt acetate tetrahydrate (Co (CH)3COO)2·4H2O) is preferably 5 to 7% by weight. In the invention, the Mw of the polyacrylonitrile is preferably 130000-160000, and more preferably 150000.
In the present invention, the mixing is preferably performed by mixing polyacrylonitrile and N, N-dimethylformamide first and then adding cobalt acetate tetrahydrate. The specific mixing method is not particularly limited, and the mixing method known to those skilled in the art can be adopted, specifically, for example, stirring at 40-50 ℃ for 6-12 hours.
According to the invention, polymethyl methacrylate (PMMA) and N, N-dimethylformamide are mixed to obtain a core body precursor.
In the invention, the weight content of the polymethyl methacrylate in the core body precursor is preferably 20-25%. The specific mixing method is not particularly limited, and the mixing method known to those skilled in the art can be adopted, specifically, for example, stirring at 40-50 ℃ for 6-12 hours.
After a shell precursor and a core precursor are obtained, the shell precursor is injected into an outer-layer injector, the core precursor is injected into an inner-layer injector, and electrostatic spinning is carried out to obtain the fiber.
In the invention, the voltage of the electrostatic spinning is preferably 15-20 KV, and the distance between the needle head and the receiving plate is preferably 15-20 cm.
In the invention, the advancing speed of the outer-layer injector is preferably 0.11-0.13 mm/min, more preferably 0.12mm/min, and the advancing speed of the inner-layer injector is preferably 0.08-0.10 mm/min, more preferably 0.09 mm/min. In the present invention, the needle of the outer syringe is preferably a 21-gauge stainless steel needle, and the needle of the inner syringe is preferably a 26-gauge stainless steel needle.
After the fibers are obtained, the fibers are subjected to heat treatment to obtain the cobalt-loaded hollow carbon nanofiber composite catalyst.
In the present invention, the heat treatment preferably includes a first heat treatment and a second heat treatment performed in sequence, the first heat treatment is preferably performed in an air atmosphere, the temperature of the first heat treatment is preferably 225 to 255 ℃, more preferably 250 ℃, the holding time is preferably 4 to 5 hours, and the temperature rise rate when the temperature rises to the first heat treatment temperature is preferably 1 ℃/min; the second heat treatment is preferably carried out in high-purity nitrogen, the temperature of the second heat treatment is preferably 700-900 ℃, more preferably 800 ℃, the heat preservation time is preferably 1-2 h, and the heating speed of heating to the second heat treatment temperature is preferably 2 ℃/min.
The invention also provides the cobalt-loaded hollow carbon nanofiber composite catalyst prepared by the preparation method in the technical scheme, wherein the cobalt-loaded hollow carbon nanofiber composite catalyst takes polyacrylonitrile doped with cobalt nanoparticles as a shell and polymethyl methacrylate as a core, and pores are formed on the wall of the hollow structure.
In the invention, the wall thickness of the cobalt-loaded hollow carbon nanofiber composite catalyst is preferably 50-100 nm.
In the invention, the content of Co in the cobalt-loaded hollow carbon nanofiber composite catalyst is preferably 20-30 wt%.
The invention also provides application of the cobalt-loaded hollow carbon nanofiber composite catalyst in the technical scheme as a cathode material of a lithium-oxygen battery.
In the present invention, the application is preferably: cutting the cobalt-loaded hollow carbon nanofiber composite catalyst into round pieces with the diameter of 16mm to form a CR2032 lithium oxygen battery, wherein the battery assembly process is carried out in a glove box (O)2<0.01ppm，H2O<0.01ppm) was completed.
In the present invention, the cells were assembled in the order of positive cell casing-the cobalt-supported hollow carbon nanofiber composite catalyst disk-glass fiber membrane-electrolyte (1M LiTFSI in DME) -lithium sheet-nickel foam-negative cell casing.
In order to further illustrate the present invention, the cobalt-supported hollow carbon nanofiber composite catalyst provided by the present invention, the preparation method and the application thereof are described in detail below with reference to examples, but they should not be construed as limiting the scope of the present invention.
Example 1
Preparing an electrostatic spinning precursor:
polyacrylonitrile (PAN, Mw 150000), PMMA, N-dimethylformamide solution (DMF, 99.8%), cobalt acetate tetrahydrate from alfaaaesar.
First, two different polymer solutions were prepared as the inner and outer precursor solutions of the coaxial injector. PAN and PMMA were dissolved in DMF at concentrations of 10 wt% and 20 wt%, respectively, and the mixture was heated at 40 ℃ to a temperature ofStirring vigorously for 12 h. Subsequently, 5 wt% of Co (CH)3COO)2·4H2O was uniformly mixed in DMF solution containing 10 wt% PAN as a shell precursor. Meanwhile, a DMF solution containing 20 wt% of PMMA was used as a core precursor.
The electrostatic spinning process comprises the following steps:
the two precursors are respectively injected into an inner layer injector and an outer layer injector, and the needle heads are respectively 26 stainless steel needles and 21 stainless steel needles with different types. The electrostatic spinning voltage is 15KV, the distance between the needle head and the receiving plate is 15cm, and the advancing speeds of the outer layer and the inner layer are 0.12mm/min and 0.10mm/min respectively.
And (3) heat treatment process:
in the air atmosphere, at the temperature of 250 ℃, the heating rate is 1 ℃/min, and the heat preservation time is 4 h; and then carrying out heat treatment at 800 ℃ in a high-purity nitrogen atmosphere at the heating rate of 2 ℃/min for 2h to obtain the final product, namely the cobalt-loaded hollow carbon nanofiber composite catalyst (Co-HCNF). The wall thickness of the cobalt-loaded hollow carbon nanofiber composite catalyst is 50-100 nm, and the content of Co is 22.1 wt%.
Step 2: battery system
Assembling the battery:
cutting the cobalt-loaded hollow carbon nanofiber composite catalyst into round pieces with the diameter of 16mm to form a CR2032 lithium oxygen battery, and assembling the battery in a glove box (O)2<0.01ppm，H2O<0.01ppm) was completed.
The cells were assembled in the order of positive cell casing-disc of cobalt supported hollow carbon nanofiber composite catalyst-glass fiber membrane-electrolyte (1M LiTFSI in DME) -lithium disc-nickel foam-negative cell casing.
The lithium-oxygen battery was assembled using a 20% wt% Pt/C air electrode using conventional methods, with the addition of a binder and a conductive carbon material.
Electrochemical testing:
(1) and (3) cutting off the voltage within the voltage range of 2.2-4.5V under the discharge current of 100mA/g, and carrying out a first charge-discharge test.
(2) The constant-capacity charge and discharge test was carried out with a discharge current of 200mA/g and a cut-off of 1000 mAh/g.
And step 3: battery testing
FIG. 1 is a graph showing the first charge-discharge curve of a lithium-oxygen battery assembled by Co-HCNF and 20 wt% Pt/C air electrode (current density: 100 mA. g)-1) The charging and discharging interval is 2.2-4.5V, and the testing environment is 1atm high-purity oxygen (relative pressure, if no special description exists, all the tests below are under the conditions). As can be seen, at 100mA g-1Under the current density of (1), the first charging overvoltage and the discharging overvoltage of the lithium-oxygen battery assembled by the sample Co-HCNF are respectively 1.28V and 0.43V, and the charging and discharging overvoltage (1.38V and 0.44V) of the lithium-oxygen battery assembled by the Pt/C electrode with the concentration of less than 20wt percent; and its specific discharge capacity (4427mAh g)-1) Also greater than 20 wt% Pt/C electrode capacity 3058mAh · g-1。
FIG. 2 is a constant capacity charge-discharge cycle chart of a lithium-oxygen battery assembled by Co-HCNF and 20 wt% Pt/C air electrode (current density: 200mA g)-1Cutoff capacity: 500 mAh.g-1) As shown in FIG. 2a, the assembly of Co-HCNF into a Li-oxygen cell can be cycled 60 times, while the assembly of 20 wt% Pt/C into a Li-oxygen cell can only be cycled 40 times. This is mainly due to the by-products and incompletely oxidized Li produced as the cycle progresses during the charge and discharge2O2Etc. gradually accumulate on the surface of the Pt/C electrode, resulting in severe passivation of the electrode surface, stopping the reactant (Li)+And O2) Into the interior of the electrode, the reactants cannot come into contact with the internal active sites and rapid decay of performance results. Co-HCNF does not need to add a binder and a conductive carbon material, avoids secondary reaction of the additive and a reaction product, avoids generation of a byproduct, and most importantly, the hollow carbon nanofiber structure provides a quick channel for the transportation of a reactant (Li)+And O2) Can quickly enter the inside of the electrode, improves the utilization rate of active sites and is beneficial to the long circulation of the battery. On the other hand, Co has been demonstrated to be an ORR and OER catalyst with excellent performance, playing a key role in optimizing the performance of the battery.
FIG. 3 is a graph showing the discharge capacity of a lithium-oxygen battery assembled by Co-HCNF and 20 wt% Pt/C as a function of the number of cycles (current density: 200 mA. cndot.)g-1Cutoff capacity: 500 mAh.g-1). It can be seen from the figure that the discharge capacity of the lithium-oxygen battery assembled by Co-HCNF is always maintained at 500mAh g during the battery cycle-1No fluctuation, up to 60 cycles. And the assembly of 20 wt% Pt/C into a Li-oxygen battery only maintains 40 cycles. The excellent cycling performance of the sample Co-HCNF assembled into a lithium oxygen cell can be mainly attributed to the following three aspects: 1, since any adhesive and conductive additive are not mixed, the blocking of the transfer passage by the non-decomposed product can be avoided; 2, the three-dimensional network penetrated hollow nano-fiber structure can promote O2And Li+The reaction efficiency is improved by the rapid transfer; 3, a large number of nano Co particles can provide a plurality of high-efficiency ORR/OER catalytic active sites, and a long-time stable battery circulation process is ensured.
Step 4 topography characterization
Scanning Electron microscope testing (SEM)
Fig. 4 is a scanning electron microscope microscopic image of the cobalt-loaded hollow carbon nanocomposite fiber. The hollow nanofibers with continuous and uniform appearance form a three-dimensional network, the diameter of the fibers is about 200-500 nm, the structure of the fibers can be seen from the fiber ports to be a hollow structure, and the appearance structure can be continuously verified by a transmission electron microscope image.
Transmission electron microscope Test (TEM)
FIG. 5 is a transmission electron micrograph of Co-HCNF. It was further confirmed that the sample was a hollow carbon fiber, and the tube wall was about several tens of nanometers thick. Meanwhile, the Co nanoparticles are uniformly distributed on the carbon nanofibers, the growth of the Co nanoparticles is inhibited by the carbon nanofibers, overgrowth or accumulation cannot occur, the Co nanoparticles are small, and the diameter of the Co nanoparticles is 20-80 nm.
Fig. 6 is a distribution energy spectrum of C, O, Co, N elements in Co-HCNF, and the distribution of C, O, Co, N elements can be obtained by energy spectrum analysis, and it can be seen that the four elements are distributed uniformly, which can prove that the cobalt-loaded hollow carbon nanocomposite fiber is successfully synthesized.
4)XRD
FIG. 7 is an XRD pattern of Co-HCNF, and it is understood that the apparent diffraction peaks at 44 ° (111), 51 ° (200) and 77 ° (220) reflect the cubic phase of Co (PDF # 15-0806). In addition, a distinct graphitization diffraction (002) peak was observed at about 26 ° 2 θ, since cobalt acts as a catalyst during PAN carbonization, which would be about 3000 ℃ if cobalt were absent. Graphitization of the carbon substrate also contributes to improvement of electrode conductivity, and thus, the performance of the battery can be greatly improved.
Example 2
Preparing an electrostatic spinning precursor:
polyacrylonitrile (PAN, Mw 130000), PMMA, N-dimethylformamide solution (DMF, 99.8%), cobalt acetate tetrahydrate from alfaaaesar.
First, two different polymer solutions were prepared as the inner and outer precursor solutions of the coaxial injector. PAN and PMMA were dissolved in DMF at concentrations of 10 wt% and 20 wt%, respectively, and stirred vigorously at 40 ℃ for 12 h. Subsequently, 4 wt% of Co (CH)3COO)2·4H2O was uniformly mixed in DMF solution containing 10 wt% PAN as a shell precursor. Meanwhile, a DMF solution containing 20 wt% of PMMA was used as a core precursor.
The electrostatic spinning process comprises the following steps:
the two precursors are respectively injected into an inner layer injector and an outer layer injector, and the needle heads are respectively 26 stainless steel needles and 21 stainless steel needles with different types. The electrostatic spinning voltage is 20KV, the distance between the needle head and the receiving plate is 20cm, and the advancing speeds of the outer layer and the inner layer are 0.11 mm/min and 0.08mm/min respectively.
And (3) heat treatment process:
in the air atmosphere, at 225 ℃, the heating rate is 1 ℃/min, and the heat preservation time is 5 h; and then carrying out heat treatment at 700 ℃ in a high-purity nitrogen atmosphere at the heating rate of 2 ℃/min for 2h to obtain the final product, namely the cobalt-loaded hollow carbon nanofiber composite catalyst (Co-HCNF). The wall thickness of the cobalt-loaded hollow carbon nanofiber composite catalyst is 50-100 nm, and the content of Co is 15.7 wt%.
Example 3
Preparing an electrostatic spinning precursor:
polyacrylonitrile (PAN, Mw ═ 160000), PMMA, N-dimethylformamide solution (DMF, 99.8%), cobalt acetate tetrahydrate available from AlfaAesar.
First, two different polymer solutions were prepared as the inner and outer precursor solutions of the coaxial injector. PAN and PMMA were dissolved in DMF at concentrations of 10 wt% and 20 wt%, respectively, and stirred vigorously at 40 ℃ for 12 h. Subsequently, 6 wt% of Co (CH)3COO)2·4H2O was uniformly mixed in DMF solution containing 10 wt% PAN as a shell precursor. Meanwhile, a DMF solution containing 20 wt% of PMMA was used as a core precursor.
The electrostatic spinning process comprises the following steps:
the two precursors are respectively injected into an inner layer injector and an outer layer injector, and the needle heads are respectively 26 stainless steel needles and 21 stainless steel needles with different types. The electrostatic spinning voltage is 20KV, the distance between the needle head and the receiving plate is 20cm, and the advancing speeds of the outer layer and the inner layer are 0.11 mm/min and 0.08mm/min respectively.
And (3) heat treatment process:
in the air atmosphere, at 255 ℃, the heating rate is 1 ℃/min, and the heat preservation time is 4 h; and then carrying out heat treatment at 900 ℃ in a high-purity nitrogen atmosphere at the heating rate of 2 ℃/min for 1h to obtain the final product, namely the cobalt-loaded hollow carbon nanofiber composite catalyst (Co-HCNF). The wall thickness of the cobalt-loaded hollow carbon nanofiber composite catalyst is 50-100 nm, and the content of Co is 25.7 wt%.
The foregoing is merely a preferred embodiment of the invention and is not intended to limit the invention in any manner. It should be noted that, for those skilled in the art, without departing from the principle of the present invention, several improvements and modifications can be made, and these improvements and modifications should also be construed as the protection scope of the present invention.
Claims (10)
1. A preparation method of a cobalt-loaded hollow carbon nanofiber composite catalyst is characterized by comprising the following steps:
mixing polyacrylonitrile, cobalt acetate tetrahydrate and N, N-dimethylformamide to obtain a shell precursor;
mixing polymethyl methacrylate with N, N-dimethylformamide to obtain a core body precursor;
injecting the shell precursor into an outer-layer injector, injecting the core precursor into an inner-layer injector, and performing electrostatic spinning to obtain fibers;
and carrying out heat treatment on the fibers to obtain the cobalt-loaded hollow carbon nanofiber composite catalyst.
2. The preparation method according to claim 1, wherein the polyacrylonitrile in the shell precursor accounts for 10-12 wt%, and the cobalt acetate tetrahydrate accounts for 5-7 wt%.
3. The method according to claim 1, wherein the core body precursor contains polymethyl methacrylate in an amount of 20 to 25 wt%.
4. The preparation method according to claim 1, wherein the voltage of the electrostatic spinning is 15-20 KV, and the distance between the needle and the receiving plate is 15-20 cm.
5. The method for preparing the syringe of claim 1, wherein the advancing speed of the outer layer syringe is 0.11 to 0.13mm/min, and the advancing speed of the inner layer syringe is 0.08 to 0.10 mm/min.
6. The method according to claim 1, wherein the heat treatment comprises a first heat treatment and a second heat treatment which are sequentially performed, wherein the first heat treatment is performed in an air atmosphere, the temperature of the first heat treatment is 225 to 255 ℃, the holding time is 4 to 5 hours, and the temperature rising rate of the temperature rising to the first heat treatment temperature is 1 ℃/min; the second heat treatment is carried out in high-purity nitrogen, the temperature of the second heat treatment is 700-900 ℃, the heat preservation time is 1-2 h, and the temperature rising speed of rising to the second heat treatment temperature is 2 ℃/min.
7. The cobalt-loaded hollow carbon nanofiber composite catalyst prepared by the preparation method of any one of claims 1 to 6, wherein polyacrylonitrile doped with cobalt nanoparticles is used as a shell, polymethyl methacrylate is used as a core, and pores are formed on the wall of the hollow structure.
8. The cobalt-supported hollow carbon nanofiber composite catalyst according to claim 7, wherein the wall thickness of the cobalt-supported hollow carbon nanofiber composite catalyst is 50 to 100 nm.
9. The cobalt-supported hollow carbon nanofiber composite catalyst according to claim 7, wherein the content of Co in the cobalt-supported hollow carbon nanofiber composite catalyst is 20 to 30 wt%.
10. Use of the cobalt-supported hollow carbon nanofiber composite catalyst as claimed in any one of claims 7 to 9 as a cathode material for a lithium-oxygen battery.
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