An iron cluster-loaded porous carbon fiber composite material and its preparation method and application

Abstract

The present invention discloses an iron cluster-loaded porous carbon fiber composite material and its preparation method and application, relating to the technical field of lithium-sulfur battery materials; the iron cluster-loaded porous carbon fiber composite material is porous carbon fiber, iron atoms aggregate into iron clusters in the form of nanoclusters, and the iron clusters are dispersed on the porous carbon fiber; its preparation method specifically utilizes the complexation of organic iron ions and β-cyclodextrin, uses polyacrylonitrile as a carbon source, disperses small molecule groups in polyacrylonitrile fibers, and obtains the iron cluster-loaded porous carbon fiber composite material through electrospinning and pyrolysis technology. The composite material improves the electron / ion transport capacity in the battery with the rich pore structure and excellent conductivity of carbon fiber. The iron clusters efficiently adsorb polysulfides and catalyze their conversion ability, greatly alleviating polysulfide shuttling and improving the electrode conversion reaction kinetics. These characteristics work together to achieve high-performance lithium-sulfur batteries.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-sulfur battery materials, and in particular relates to an iron cluster-loaded porous carbon fiber composite material, a preparation method thereof, and applications thereof. Background Art

[0002] Lithium-sulfur batteries have high energy density (2600 Wh kg -1 Lithium-sulfur batteries are considered to be the most promising next-generation high-energy-density battery system due to their advantages, such as high energy density, environmental friendliness, and abundant sulfur reserves. However, the polysulfide shuttle effect and slow reaction kinetics still limit the commercial application of lithium-sulfur batteries. Although researchers have alleviated these problems to some extent by introducing catalytic materials to accelerate the conversion of lithium polysulfides, the active sites of the catalytic materials gradually become ineffective as the insulating products Li2S / Li2S2 are formed. Therefore, the development of catalysts with abundant active sites is particularly important.

[0003] Clusters are groups of atoms bound together with well-defined atomicity. Due to their small size (<2 nanometers), they possess abundant active sites. Cluster catalysis allows the inactive portions of nanoparticles to be fully utilized, effectively improving catalytic efficiency.

[0004] The present invention provides an iron cluster-loaded porous carbon fiber composite material and a preparation method thereof, which synergizes the excellent electron / ion transmission ability of the porous carbon fibers with the excellent polysulfide adsorption and conversion catalytic ability of the iron clusters, greatly alleviating the polysulfide shuttling, improving the electrode conversion reaction kinetics, and realizing a high-performance lithium-sulfur battery. Summary of the Invention

[0005] In view of the shortcomings of the prior art, the first object of the present invention is to provide a method for preparing an iron cluster-loaded porous carbon fiber composite material. The preparation method of the present invention is scientific, reasonable, easy to implement, low-cost, and can achieve large-scale production.

[0006] The second object of the present invention is to provide an iron cluster-loaded porous carbon fiber composite material prepared by the above preparation method.

[0007] The third object of the present invention is to provide an application of an iron cluster-loaded porous carbon fiber composite material prepared by the above-mentioned preparation method. The iron cluster-loaded porous carbon fiber composite material provided by the present invention, through the excellent electron / ion transmission ability of porous carbon fibers, synergistically with the excellent polysulfide adsorption and polysulfide conversion ability of iron clusters, greatly alleviates polysulfide shuttling and improves the electrode conversion reaction kinetics.

[0008] In order to achieve the above object, the present invention adopts the following technical solutions:

[0009] The present invention provides a method for preparing an iron cluster-loaded porous carbon fiber composite material, comprising adding beta-cyclodextrin and an organic iron salt to an organic solvent to obtain a mixed solution A, adding polyacrylonitrile to the mixed solution A to obtain a mixed solution B, electrospinning the mixed solution B to obtain an organic fiber film, and sequentially pre-oxidizing and calcining the organic fiber film to obtain the composite material;

[0010] In the mixed solution A, the concentration of the organic iron salt is 0.01-0.04 mol / L.

[0011] The preparation method of the present invention utilizes the special hollow cylindrical three-dimensional ring structure of β-cyclodextrin and the different hydrophilic and hydrophobic properties of the inner and outer surfaces to confine the hydrophobic organic iron salt in its cavity to form an inclusion complex containing iron ions; and utilizes the excellent fiber-forming property and structural stability of PAN to prepare an organic fiber membrane through electrostatic spinning technology. Since β-cyclodextrin and the organic iron salt preferentially form an inclusion complex, the sublimation of the organic iron salt during the pre-oxidation process is hindered. In the subsequent carbonization process, the organic iron is released from the β-cyclodextrin, decomposes, and combines with the carbonized fiber to form clusters of 6-20 iron atoms that are anchored on the carbon fiber surface.

[0012] Experiments have found that in order to obtain clusters of iron atoms anchored on the surface of carbon fibers, first, the mixing order of the raw materials needs to be effectively controlled. Only by mixing β-cyclodextrin with organic iron salts first can the iron ions be effectively anchored. Direct mixing will lead to incomplete anchoring. Second, β-cyclodextrin and organic iron salts must be used as raw materials. β-cyclodextrin has both airspace and hydrophilic and hydrophobic properties, so that the organic iron salt can be confined in its cavity. If β-cyclodextrin is replaced with a substance with only airspace, it has no hydrophilic and hydrophobic properties and cannot anchor the organic iron salt in the airspace. Therefore, if β-cyclodextrin is not added or β-cyclodextrin is replaced with others, only single atoms can be loaded in the porous carbon fibers. If inorganic iron salts are used, it is not conducive to the dispersion of metal atoms and can only form metal particles.

[0013] In addition, the molar concentration of the organic iron salt in the mixed solution A also needs to be effectively controlled. If there is too much organic iron salt, it will exceed the dispersion limit of the carbon fiber, resulting in the iron element being unable to aggregate in the form of nanoclusters and can only be loaded into the porous carbon fiber composite material in the form of iron metal nanoparticles.

[0014] Preferably, the organic iron salt is selected from at least one of iron acetylacetonate, iron phthalocyanine, ferrocene, and iron porphyrin complex.

[0015] Preferably, the organic solvent is selected from N,N-dimethylformamide.

[0016] Preferably, polyacrylonitrile is added to mixed solution A and stirred in an oil bath to obtain mixed solution B. The stirring temperature is 40-60°C, the stirring rate is 50-100 r / min, and the stirring time is 12-24 hours. Through the above process, PAN is uniformly dispersed in the N,N-dimethylformamide solution.

[0017] Preferably, the β-cyclodextrin concentration in the mixed solution A is 0.02-0.06 mol / L. Optimal performance is achieved when the molar amount of β-cyclodextrin is controlled within this range. Excessive β-cyclodextrin can cause turbidity in the spinning solution, increase viscosity, and make it difficult to form a uniform spinning solution for subsequent electrospinning. Excessive β-cyclodextrin can make it difficult to complex with the organic iron salt, resulting in a large amount of organic iron salt subliming during the pre-oxidation stage, making it difficult to form iron clusters during the subsequent carbonization process.

[0018] Preferably, in the mixed solution A, the concentration of the organic iron salt is 0.02-0.04 mol / L, preferably 0.03-0.04 mol / L.

[0019] Preferably, the mass ratio of polyacrylonitrile to β-cyclodextrin is 2-5:1, preferably 2-3:1.

[0020] Preferably, the electrospinning process has a propulsion speed of 0.5-2 mL / h and a voltage of 16-24 KV.

[0021] Preferably, the pre-oxidation temperature is controlled at 260-300°C, the heating rate is controlled at 1-5°C / min, and the pre-oxidation time is 2-3 hours. Experiments have found that the pre-oxidation temperature needs to be controlled within the range of the present invention. Excessively high pre-oxidation temperatures will cause the inclusion complex of β-cyclodextrin and organic iron to decompose and sublime, making it difficult to form iron clusters during the subsequent carbonization process.

[0022] Preferably, the calcination is carried out under an argon protective atmosphere, the calcination temperature is 900-1200° C., and the calcination time is 1-4 h.

[0023] After calcination, a membrane-like iron cluster-loaded porous carbon fiber composite material is obtained. In actual application, the calcined product is cut to the required size for application.

[0024] The present invention also provides an iron cluster-loaded porous carbon fiber composite material, which consists of porous carbon fibers and iron clusters loaded on the porous carbon fibers. The iron clusters are formed by aggregation of 6-20 iron atoms in the form of nanoclusters.

[0025] Preferably, the iron cluster-loaded porous carbon fiber composite material has the following composition, calculated in atomic percentage: N 5%-10%, Fe 0.1%-2%, and the balance being C and unavoidable impurities.

[0026] Preferably, the iron cluster-loaded porous carbon fiber composite material is in a membrane shape.

[0027] Preferably, the fiber diameter of the porous carbon fiber is 50-300 nm, and the specific surface area is 500-1500 m 2 g -1 , pore volume is 0.1~1 cm 3 g -1 .

[0028] The present invention also provides an application of an iron cluster-loaded porous carbon fiber composite material, wherein the iron cluster-loaded porous carbon fiber composite material is used as a sulfur positive electrode carrier in a lithium-sulfur battery.

[0029] Preferably, the application to lithium-sulfur batteries specifically includes the following steps:

[0030] S1. Dissolve sublimed sulfur in a carbon disulfide solution with a sulfur concentration of 1-10 g / mL. Immerse the iron cluster-loaded porous carbon fiber composite disc in the carbon disulfide solution for 5 minutes. After removal, place it in a forced air oven and dry it at 40°C for 12 hours. Place the dried sample in an ampoule and seal it. Heat it at 155°C in an argon atmosphere for 12 hours to obtain an iron cluster-loaded porous carbon fiber / sulfur positive electrode sheet.

[0031] S2. A lithium-sulfur battery was assembled by assembling an iron cluster-loaded porous carbon fiber / sulfur cathode, a polypropylene separator, and a metal lithium anode. The electrolyte (DME / DOL (v / v = 1 / 1) containing 1.0 M LiTFSI and 1 wt% LiNO3) was added in 20 μL drops on the positive electrode side and 20 μL drops on the negative electrode side on both sides of the separator. The assembled battery was allowed to stand for 6 hours before electrochemical testing.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] 1) The iron cluster-loaded porous carbon fiber composite material in the present invention, synergistically combining the high sulfur loading brought by the rich pore structure of the carbon fibers and the excellent polysulfide adsorption and polysulfide conversion catalytic capabilities of the iron clusters, greatly alleviates polysulfide shuttling, improves the electrode conversion reaction kinetics, and realizes high-performance lithium-sulfur batteries, which has application prospects.

[0034] 2) The preparation method of the iron cluster-loaded porous carbon fiber composite material in the present invention utilizes the special hollow cylindrical three-dimensional ring structure of β-cyclodextrin and the different hydrophilic and hydrophobic properties of the inner and outer surfaces to confine the hydrophobic organic iron salt in its cavity. Combined with electrospinning technology, the iron cluster-loaded porous carbon fiber composite material is prepared. The preparation method is scientific, reasonable, easy to implement, low-cost, and can be produced on a large scale.

[0035] 3) The iron cluster-loaded porous carbon fiber composite material prepared in the present invention has excellent electrocatalytic activity as a positive electrode carrier material for lithium-sulfur batteries. The battery prepared using this material has a discharge capacity of up to 1097 mAh / g at a discharge rate of 0.2C, and the capacity can still be maintained at 995 mAh / g after 100 cycles. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 This is a transmission electron microscope (TEM) image of the iron cluster-loaded porous carbon fiber composite material prepared in Example 1 of the present invention.

[0037] Figure 2 TEM images and high-resolution TEM images of the iron-loaded porous carbon fiber composite material prepared in Comparative Example 1 of the present invention, wherein Figure 2 (a) is the TEM image, Figure 2 (b) is a high-resolution TEM image.

[0038] Figure 3 TEM images and high-resolution TEM images of the iron-loaded porous carbon fiber composite material prepared in Comparative Example 2 of the present invention, wherein Figure 3 (a) is the TEM image, Figure 3 (b) is a high-resolution TEM image.

[0039] Figure 4 This is a spherical aberration corrected scanning transmission electron microscope (AC-STEM) image of the iron cluster-loaded porous carbon fiber composite material prepared in Example 1 of the present invention.

[0040] Figure 5 This is the AC-STEM image of the sample prepared in Comparative Example 3 of the present invention.

[0041] Figure 6 The X-ray diffraction (XRD) patterns of the samples prepared in Example 1 and Comparative Example 1 of the present invention are shown.

[0042] Figure 7 This is a cycle performance diagram of the battery prepared in Example 1 of the present invention under a discharge rate of 0.2C.

[0043] Figure 8This is a cycle performance diagram of the battery prepared in Comparative Example 1 of the present invention under a 0.2C discharge rate condition.

[0044] Figure 9 This is a cycle performance diagram of the battery prepared in Comparative Example 2 of the present invention under a 0.2C discharge rate condition.

[0045] Figure 10 This is a cycle performance diagram of the battery prepared in Comparative Example 3 of the present invention under a 0.2C discharge rate condition. DETAILED DESCRIPTION

[0046] The following will provide a clear and complete description of the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0047] Example 1

[0048] The preparation method of the iron cluster-loaded porous carbon fiber composite material is specifically carried out according to the following steps:

[0049] 1) Add 0.454 g β-cyclodextrin, 0.074 g ferrocene, and 10 mL N,N-dimethylformamide to a 25 mL beaker. Place the beaker on a stirrer and stir at 50-100 rpm for 1 h to obtain solution A.

[0050] 2) Add 1.2 g of polyacrylonitrile to mixed solution A and stir in an oil bath at 40°C, 100 rpm, and 20 h to obtain mixed solution B.

[0051] 3) spinning the mixed solution B prepared in step 3 using electrospinning technology at a propulsion speed of 0.5 mL / h and a voltage of 16 KV;

[0052] 4) Pre-oxidizing the spun organic fiber film at a temperature of 280°C, a heating rate of 5°C / min, and a pre-oxidation time of 2 h;

[0053] 5) The pre-oxidized film prepared in step 4 was calcined at 1000°C for 2 h under an argon atmosphere to obtain an iron cluster-loaded porous carbon fiber composite material. The morphology and structure of the composite material were analyzed and characterized. Figure 1 、 4 , as shown in 6.

[0054] Figure 1This is the TEM image of the iron cluster loaded porous carbon fiber composite material. It can be seen from the figure that the iron cluster loaded porous carbon fiber composite material presents a fibrous structure with a fiber diameter of 50 to 200 nm.

[0055] Figure 4 This is the AC-STEM image of the iron cluster-loaded porous carbon fiber composite material. Using a higher magnification, we can clearly see that 6-20 iron atoms are aggregated in the form of nanoclusters and loaded on the porous carbon fiber. At the same time, no lattice fringes representing the crystal structure are found, indicating that the main body of the sample is amorphous carbon loaded with Fe clusters.

[0056] Figure 6 The upper curve is the XRD pattern of the iron cluster-loaded porous carbon fiber composite material in Example 1. A hump can be observed at around 21°, corresponding to the (002) crystal plane of graphite. The diffraction peak is broad, indicating a high degree of disorder. No diffraction peaks of elemental iron and its compounds are observed in the figure, indicating that the iron clusters in the present invention are clusters formed by iron atoms, have no metallic properties, and are not iron metal clusters, further demonstrating the successful preparation of the iron cluster-loaded porous carbon fiber composite material.

[0057] The iron cluster-loaded porous carbon fiber composite material prepared above is applied in a lithium-sulfur battery, specifically by the following steps:

[0058] 1) Cut the iron cluster-loaded porous carbon fiber membrane into 10 mm diameter discs for later use. Dissolve sublimated sulfur in a carbon disulfide solution at a sulfur concentration of 10 g / mL. Immerse the iron cluster-loaded porous carbon fiber composite disc in the carbon disulfide solution for 5 minutes. Remove the disc and dry it in an air-dried oven at 40°C for 12 hours under an argon atmosphere. The dried sample is sealed in an ampoule. Heat the ampoule at 155°C for 12 hours under an argon atmosphere to obtain the iron cluster-loaded porous carbon fiber / sulfur cathode.

[0059] 2) In an anhydrous and oxygen-free glove box, the prepared iron cluster-loaded porous carbon fiber / sulfur cathode sheet was placed in the center of the cathode shell, 20 μL of electrolyte (DME / DOL (v / v = 1 / 1) containing 1.0 M LiTFSI and 1 wt% LiNO3) was added, and a 19 mm diameter polypropylene separator was placed. 10 μL of the above electrolyte was added, followed by a 15 mm diameter lithium metal sheet. After adding a gasket and shrapnel, the negative electrode shell was covered and hydraulically assembled into a lithium-sulfur battery. The assembled battery was left to stand for 6 h before electrochemical testing. The test results are shown in the figure. Figure 7 shown.

[0060] Figure 7The figure shows the cycling performance of the prepared battery at a 0.2C discharge rate. As can be seen from the figure, the battery has an initial discharge capacity of 1097 mAh / g at a 0.2C discharge rate. After 100 cycles, it still maintains a discharge capacity of 995 mAh / g, with a capacity retention rate of 90.7%. This excellent cycling stability is inseparable from the strong adsorption and efficient conversion of polysulfides by the Fe clusters and the excellent electronic and ionic conductivity of the porous carbon fibers.

[0061] Example 2:

[0062] The preparation method of the iron cluster-loaded porous carbon fiber composite material is specifically carried out according to the following steps:

[0063] 1) Add 0.454 g β-cyclodextrin, 0.06 g iron phthalocyanine, and 10 mL N,N-dimethylformamide to a 25 mL beaker. Place the beaker on a stirrer and stir at 50–100 rpm for 1 h to obtain solution A.

[0064] 2) Add 1.0 g of polyacrylonitrile to mixed solution A and stir in an oil bath at 40°C, 100 rpm, and 24 h to obtain mixed solution B.

[0065] 3) Spinning the mixed solution B prepared in step 3 using electrospinning technology at a propulsion speed of 0.5 mL / h and a voltage of 20 KV;

[0066] 4) Pre-oxidizing the spun organic fiber film at a temperature of 280°C, a heating rate of 5°C / min, and a pre-oxidation time of 2 h;

[0067] 5) The pre-oxidized film prepared in step 4 was calcined at 1000° C. for 2 h in an argon atmosphere to obtain an iron cluster-loaded porous carbon fiber composite material.

[0068] The iron cluster-loaded porous carbon fiber composite material prepared above was used in a lithium-sulfur battery using the same steps as in Example 1. A battery prepared using the iron cluster-loaded porous carbon fiber composite material prepared in Example 2 as a sulfur carrier had an initial discharge capacity of 1146 mAh / g at a 0.2C discharge rate. After 100 cycles, it had a discharge capacity of 930 mAh / g, with a capacity retention rate of 81.2%.

[0069] Example 3:

[0070] The preparation method of the iron cluster-loaded porous carbon fiber composite material is specifically carried out according to the following steps:

[0071] 1) Add 0.454 g β-cyclodextrin, 0.037 g ferrocene, and 10 mL N,N-dimethylformamide to a 25 mL beaker. Place the beaker on a stirrer and stir at 50-100 rpm for 1 h to obtain solution A.

[0072] 2) Add 1.0 g of polyacrylonitrile to mixed solution A and stir in an oil bath at 40°C, 100 rpm, and 24 h to obtain mixed solution B.

[0073] 3) Spinning the mixed solution B prepared in step 3 using electrospinning technology at a propulsion speed of 0.5 mL / h and a voltage of 20 KV;

[0074] 4) Pre-oxidizing the spun organic fiber film at a temperature of 280°C, a heating rate of 5°C / min, and a pre-oxidation time of 2 h;

[0075] 5) The pre-oxidized film prepared in step 4 was calcined at 1000° C. for 2 h in an argon atmosphere to obtain an iron cluster-loaded porous carbon fiber composite material.

[0076] The iron cluster-loaded porous carbon fiber composite material prepared above was used in a lithium-sulfur battery using the same steps as in Example 1. A battery prepared using the iron cluster-loaded porous carbon fiber composite material prepared in Example 3 as a sulfur carrier had an initial discharge capacity of 1139 mAh / g at a 0.2C discharge rate. After 100 cycles, it had a discharge capacity of 943 mAh / g, with a capacity retention rate of 82.8%.

[0077] Comparative Example 1:

[0078] Other conditions are the same as those in Example 1, except that the organic iron salt is replaced with ferric nitrate. The steps are as follows:

[0079] 1) Add 0.454 g β-cyclodextrin, 0.097 g ferric nitrate, and 10 mL N,N-dimethylformamide to a 25 mL beaker. Place the beaker on a stirrer and stir at 50-100 rpm for 1 h to obtain Solution A.

[0080] 2) Add 1.2 g of polyacrylonitrile to mixed solution A and stir in an oil bath at 40°C, 100 rpm, and 20 h to obtain mixed solution B.

[0081] 3) spinning the mixed solution B prepared in step 3 using electrospinning technology at a propulsion speed of 0.5 mL / h and a voltage of 16 KV;

[0082] 4) Pre-oxidizing the spun organic fiber film at a temperature of 280°C, a heating rate of 5°C / min, and a pre-oxidation time of 2 h;

[0083] 5) The pre-oxidized film prepared in step 4 was calcined at 1000°C for 2 h under an argon atmosphere to obtain an iron metal nanoparticle-loaded porous carbon fiber composite material. The morphology and structure of the composite material were analyzed and characterized, as shown in the following figure: Figure 2 、 6 shown.

[0084] Figure 2 (a) is a TEM image of the iron metal nanoparticle-loaded porous carbon fiber composite material prepared in Comparative Example 1, and Figure 1 The magnification is the same as that of the figure. It can be seen from the figure that the iron metal nanoparticle-loaded porous carbon fiber composite material presents a fibrous structure with a fiber diameter of 50 to 200 nm. In addition, it can be seen that metal nanoparticles of different sizes are scattered on the carbon fibers. The size of the metal nanoparticle agglomerates is much larger than the size of the iron clusters provided by the embodiment of the present invention. High-resolution analysis of these nanoparticles, such as Figure 2 As shown in (b), the interplanar spacing of the nanoparticles is 0.349 nm, corresponding to the (200) crystal plane of elemental iron.

[0085] Figure 6 The curve below is the XRD pattern of the iron metal nanoparticle-loaded porous carbon fiber composite material prepared in Comparative Example 1. A hump can be observed at around 26°, corresponding to the (111) crystal plane of carbon. The diffraction peak is relatively broad, indicating a high degree of disorder. In addition, a sharp diffraction peak is observed at around 44.7°, corresponding to the (110) crystal plane of Fe element. XRD and TEM analysis show that during the carbonization process, inorganic metal salts such as ferric nitrate are not conducive to the dispersion of metal atoms and are prone to induce metal agglomeration to form iron metal nanoparticles.

[0086] The iron metal nanoparticle-loaded porous carbon fiber composite material prepared above was used in a lithium-sulfur battery, following the same steps as in Example 1.

[0087] Figure 8 The figure shows the cycling performance of the prepared battery at a 0.2C discharge rate. As can be seen from the figure, the battery prepared in Comparative Example 1 using the iron metal nanoparticle-loaded porous carbon fiber composite as a sulfur carrier exhibits an initial discharge capacity of 1005 mAh / g at a 0.2C discharge rate. After 100 cycles, it maintains a discharge capacity of 792 mAh / g, with a capacity retention rate of 78.8%. Both the initial discharge capacity and cycling stability of this battery are significantly lower than those of Example 1, indicating that efficient catalyst utilization using iron metal nanoparticles is difficult.

[0088] Comparative Example 2:

[0089] Other conditions are the same as those in Example 1, except for the amount of ferrocene added. The steps are as follows:

[0090] 1) Add 0.454 g β-cyclodextrin, 0.12 g ferrocene, and 10 mL N,N-dimethylformamide to a 25 mL beaker. Place the beaker on a stirrer and stir at 50-100 rpm for 1 h to obtain solution A.

[0091] 2) Add 1.2 g of polyacrylonitrile to mixed solution A and stir in an oil bath at 40°C, 100 rpm, and 20 h to obtain mixed solution B.

[0092] 3) spinning the mixed solution B prepared in step 3 using electrospinning technology at a propulsion speed of 0.5 mL / h and a voltage of 16 KV;

[0093] 4) Pre-oxidizing the spun organic fiber film at a temperature of 280°C, a heating rate of 5°C / min, and a pre-oxidation time of 2 h;

[0094] 5) The pre-oxidized film prepared in step 4 was calcined at 1000°C for 2 h under an argon atmosphere to obtain an iron metal nanoparticle-loaded porous carbon fiber composite material. The morphology and structure of the composite material were analyzed and characterized, as shown in the following figure: Figure 3 .

[0095] Figure 3 (a) is a TEM image of the iron metal nanoparticle-loaded porous carbon fiber composite material prepared in Comparative Example 2. It can be seen from the figure that the iron metal nanoparticle-loaded porous carbon fiber composite material presents a fibrous structure with a fiber diameter of about 100 nm. In addition, it can be seen that the metal nanoparticles are uniform and evenly distributed on the carbon fibers. High-resolution analysis of these nanoparticles, such as Figure 3 As shown in (b), the interplanar spacing of the nanoparticles is 0.349 nm, corresponding to the (200) plane of elemental iron. This analysis indicates that excessive amounts of organometallic salts make it difficult to evenly disperse during the carbonization process, leaving the iron atoms in metallic form. Only with the amount of organometallic salts prepared by the present invention can iron clusters form.

[0096] The iron metal nanoparticle-loaded porous carbon fiber composite material prepared above was used in a lithium-sulfur battery, following the same steps as in Example 1.

[0097] Figure 9The figure shows the cycling performance of the prepared battery at a 0.2C discharge rate. As can be seen from the figure, the battery prepared with iron nanoparticle-loaded porous carbon fiber composite as the sulfur carrier has an initial discharge capacity of 1063 mAh / g at a 0.2C discharge rate. After 100 cycles, it has a discharge capacity of 862 mAh / g, with a capacity retention rate of 81.1%. The initial discharge capacity of this battery is the same as that of Example 1, but the cycling stability is much lower than that of Example 1, indicating that it is difficult to achieve efficient catalyst utilization using iron nanoparticles.

[0098] Comparative Example 3:

[0099] Other conditions are the same as those in Example 1, except that β-cyclodextrin is not added. The steps are as follows:

[0100] 1) Add 0.12 g of ferrocene and 10 mL of N,N-dimethylformamide to a 25 mL beaker, place on a stirrer, and stir at 50-100 rpm for 1 h to obtain solution A.

[0101] 2) Add 1.2 g of polyacrylonitrile to mixed solution A and stir in an oil bath at 40°C, 100 rpm, and 20 h to obtain mixed solution B.

[0102] 3) spinning the mixed solution B prepared in step 3 using electrospinning technology at a propulsion speed of 0.5 mL / h and a voltage of 16 KV;

[0103] 4) Pre-oxidizing the spun organic fiber film at a temperature of 280°C, a heating rate of 5°C / min, and a pre-oxidation time of 2 h;

[0104] 5) The pre-oxidized film prepared in step 4 was calcined at 1000°C for 2 h under an argon atmosphere to obtain an iron single atom-loaded porous carbon fiber composite material. The morphology and structure of the composite material were analyzed and characterized. Figure 5 .

[0105] Figure 5 This AC-TEM image of a porous carbon fiber composite material loaded with single iron atoms shows that the iron is loaded in the porous carbon fibers as single atoms. This analysis demonstrates the necessity of β-cyclodextrin for the formation of iron clusters. The inclusion complexation of the organic iron salt causes several iron ions to aggregate within the β-cyclodextrin cavity, where they are then carbonized to form the iron clusters. Without β-cyclodextrin, the iron exists only in single atomic form.

[0106] The material prepared above was used in a lithium-sulfur battery, following the same steps as in Example 1.

[0107] Figure 10 The figure shows the cycling performance of the prepared battery at a 0.2C discharge rate. As can be seen from the figure, the battery prepared using the single-atom iron-loaded porous carbon fiber composite as the sulfur carrier has an initial discharge capacity of 1060 mAh / g at a 0.2C discharge rate. After 100 cycles, it has a discharge capacity of 840 mAh / g, with a capacity retention rate of 79.2%. The initial discharge capacity and cycling stability of this battery are much lower than those of Example 1, indicating that single-atom iron makes it difficult to achieve efficient catalyst utilization and its cycling performance is poor.

[0108] In summary, in combination with Examples 1-3 and Comparative Examples 1-3, the following analysis can be obtained:

[0109] In Examples 1-3, the organic iron salt is ferrocene or iron phthalocyanine, and the content of the organic iron salt is adjusted. It can be seen that the organic iron salts selected in the present invention can form inclusion compounds with β-cyclodextrin by utilizing their hydrophobic properties, and form iron clusters through carbonization, thereby preparing iron cluster-loaded porous carbon fiber composite materials;

[0110] Adjusting the organic iron salt in Comparative Example 1 reveals that the choice of iron salt affects the preparation of the iron cluster-loaded porous carbon fiber composite. Beta-cyclodextrin is unable to incorporate hydrophilic ferric nitrate, causing the iron to aggregate during carbonization, forming iron metal nanoparticles. When applied to lithium-sulfur batteries, the iron metal nanoparticle-loaded porous carbon fiber composite exhibited significantly lower initial discharge capacity and cycling stability than lithium-sulfur batteries using the iron cluster-loaded porous carbon fiber composite.

[0111] In Comparative Example 2, the content of organic iron salt was adjusted. It can be seen that the content of organic iron salt affects the preparation of iron cluster-loaded porous carbon fiber composite materials. The ability of carbon fiber to load iron clusters is limited. Too much organic salt exceeds the dispersion limit of carbon fiber, resulting in the inability of iron elements to aggregate in the form of nanoclusters. Only iron metal nanoparticles-loaded porous carbon fiber composite materials can be obtained.

[0112] By adjusting the application of β-cyclodextrin in Comparative Example 3, it can be seen that β-cyclodextrin is necessary for cluster formation. The present invention utilizes the inclusion complexation of β-cyclodextrin on organic iron salts to enrich organic iron ions in its cavity, and forms clusters after carbonization. Without adding β-cyclodextrin, only single-atom loaded porous carbon fiber composite materials can be obtained. When single-atom iron-loaded porous carbon fiber materials are applied to lithium-sulfur batteries, the initial discharge capacity of the battery is high, but its cycle stability is poor, which is much lower than that of lithium-sulfur batteries using iron cluster loaded porous carbon fiber composite materials.

[0113] In summary, the preparation method of the present invention utilizes the special hollow cylindrical three-dimensional ring structure of β-cyclodextrin and the different hydrophilic and hydrophobic properties inside and outside to confine hydrophobic organic iron ions in its cavity, and combines electrospinning technology to prepare iron clusters loaded on porous carbon fiber composites. The iron clusters loaded on the porous carbon fiber composite have iron clusters in which iron atoms are aggregated in the form of nanoclusters, and the high sulfur load brought by the rich pore structure of carbon fiber, and the excellent polysulfide adsorption and catalytic polysulfide conversion ability of the iron clusters, which greatly alleviate the polysulfide shuttling, improve the electrode conversion reaction kinetics, and realize high-performance lithium-sulfur batteries.

[0114] The above description is only used to help understand the method and core essence of the present invention, but the scope of protection of the present invention is not limited thereto. For those skilled in the art, equivalent replacements or modifications based on the technical solutions and inventive concepts of the present invention within the technical scope disclosed by the present invention should be included in the scope of protection of the present invention. In summary, the contents of this specification should not be understood as limiting the present invention.

Claims

1. A method for preparing an iron cluster-loaded porous carbon fiber composite material, characterized in that: β-cyclodextrin and an organic iron salt are added to an organic solvent to obtain a mixed solution A, polyacrylonitrile is added to the mixed solution A, and the mixture is stirred in an oil bath to obtain a mixed solution B, the mixed solution B is electrospun to obtain an organic fiber film, and the organic fiber film is sequentially pre-oxidized and calcined to obtain the obtained product; The organic solvent is selected from N,N-dimethylformamide; In the mixed solution A, the concentration of the organic iron salt is 0.01-0.04 mol / L; In the mixed solution A, the concentration of β-cyclodextrin is 0.02-0.06 mol / L; The mass ratio of polyacrylonitrile to β-cyclodextrin is 2~5:1; The stirring temperature is 40-60°C, the stirring rate is 50-100 r / min, and the stirring time is 12-24 h; The pre-oxidation temperature is controlled at 260-300°C, the heating rate is controlled at 1-5°C / min, and the pre-oxidation time is 2-3h; The iron cluster-loaded porous carbon fiber composite material consists of porous carbon fibers and iron clusters loaded on the porous carbon fibers, wherein the iron clusters are aggregated in the form of nanoclusters of 6-20 iron atoms; The porous carbon fibers have a fiber diameter of 50 to 300 nm and a specific surface area of ​​500 to 1500 m 2 g -1 , pore volume is 0.1~1cm 3 g -1 .

2. The method for preparing an iron cluster-loaded porous carbon fiber composite material according to claim 1, wherein: The organic iron salt is selected from at least one of ferric acetylacetonate, ferrophthalocyanine, ferrocene and iron porphyrin complex.

3. The method for preparing an iron cluster-loaded porous carbon fiber composite material according to claim 1, wherein: The electrospinning process is carried out at a propulsion speed of 0.5-2 mL / h and a voltage of 16-24 KV.

4. The method for preparing an iron cluster-loaded porous carbon fiber composite material according to claim 1, wherein: The calcination is carried out under an argon protective atmosphere, the calcination temperature is 900-1200° C., and the calcination time is 1-4 h.

5. The method for preparing an iron cluster-loaded porous carbon fiber composite material according to claim 1, wherein: The iron cluster-loaded porous carbon fiber composite material has the following composition, calculated by atomic percentage: N 5% to 10%, Fe 0.1% to 2%, and the balance being C and unavoidable impurities; The iron cluster-loaded porous carbon fiber composite material is in a membrane shape.

6. Use of an iron cluster-loaded porous carbon fiber composite material prepared by the preparation method according to any one of claims 1 to 5, characterized in that: The iron cluster-loaded porous carbon fiber composite material is used as a sulfur positive electrode carrier in lithium-sulfur batteries.

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