Three-dimensional Bi (at) C (at) CNFs composite material as well as preparation method and application thereof

Abstract

The invention discloses a three-dimensional Bi (at) C (at) CNFs composite material as well as a preparation method and application thereof. The three-dimensional nitrogen-doped nanofiber bismuth carbon material (Bi (at) C (at) CNFs) is prepared through electrostatic spinning and high-temperature calcination technologies. Bismuth is packaged into polyacrylonitrile-derived nitrogen-doped carbon nanofibers through in-situ substitution of metal (copper, zinc, cobalt and the like) atoms, the specific capacity of Bi (at) C (at) CNFs (Cu) is still kept at 388 mA h g <-1 > after 200 cycles at 1.0 A g <-1 >, the specific capacity is still as high as 301.2 mA h g <-1 > when the specific capacity is 2.0 A g <-1 >, and excellent sodium storage performance is attributed to the fact that bismuth particles are uniformly embedded in a conductive carbon fiber network, so that the specific capacity of Bi (at) C (at) CNFs (Cu) is greatly improved. The electronic conductivity and the buffer space of Bi are effectively improved, and the three-dimensional structure can solve the problem of structural pulverization of the bismuth carbon material in the circulation process and promote further permeation of electrolyte. Therefore, the Bi (at) C (at) CNFs as the negative electrode material of the sodium ion battery shows excellent cycle performance and rate capability.

Description

Technical Field

[0001] The present invention relates to a three-dimensional Bi@C@CNFs composite material, a preparation method and an application thereof, belonging to the technical field of new materials. Background Art

[0002] In order to meet the growing demands of people for convenient electronic devices and electric vehicles, the development of energy storage materials with high energy density has become the focus of common concern in society. Sodium-ion batteries (SIBs) have a "rocking chair" working mechanism similar to that of lithium-ion batteries, and sodium is also one of the most abundant elements in the earth's crust. Sodium is inexpensive and abundant globally, especially in the ocean. Therefore, SIBs have greater competitiveness in the future large-scale energy storage market. However, compared with the radius of lithium ions (Li + ) in LIBs, the radius of sodium ions (Na + ) in SIBs is larger. Therefore, the anode materials suitable for LIBs are not entirely suitable for SIBs anode materials. At the same time, SIBs also have problems such as low energy density and poor initial Coulomb efficiency of the anode. Therefore, finding and developing anode materials for sodium-ion batteries with excellent performance is a problem that needs to be solved in the current market for large-area applications.

[0003] Bismuth metal, as a "green metal" that can be safely used, has the characteristic of low melting point. Therefore, it can be directly reduced from natural ores with carbon, etc. Compared with transition metal materials such as cobalt-based and zinc-based materials, bismuth metal materials have the advantages of stable chemical properties, relatively small volume, easy availability, and non-toxicity. Moreover, the theoretical capacity of bismuth metal is relatively ideal (≈385 mA h g -1 ). Therefore, bismuth metal is considered to be one of the most promising anode materials. However, bismuth-based metals have problems such as volume expansion, structural collapse, fragmentation, and even detachment during the electrochemical testing process when used as SIBs anode materials. It is necessary to alleviate or eliminate the volume expansion problem of germanium-based materials during charge and discharge. Summary of the Invention

[0004] Aiming at the deficiencies of the prior art, the present invention provides a three-dimensional Bi@C@CNFs composite material, a preparation method and an application thereof. M-H3BTC is selected as a precursor, and M-H3BTC@PAN is prepared by electrospinning technology. Then, it is subjected to pre-oxidation and carbonization treatment, and then Bi is carried out by a solvothermal method 3+Through in-situ substitution of M ions and subsequent high-temperature calcination, a Bi@C@CNFs composite material is obtained. In the Bi@C@CNFs composite material, bismuth nanoparticles are uniformly dispersed in the nanofiber conductive network, providing conditions for rapid electron transport. During the cycling process, the porous carbon fibers provide additional space for the volume expansion of Bi. As the anode material for SIBs, the Bi@C@CNFs composite material forms a stable SEI layer on its surface, further ensuring the structural stability of the material and exhibiting excellent cycling stability.

[0005] To solve the problems of the existing technology, the technical solution adopted in the present invention is as follows:

[0006] A three-dimensional Bi@C@CNFs composite material is prepared by first dissolving nitrate, polyvinylpyrrolidone, and 1,3,5-benzenetricarboxylic acid in methanol and then mixing them to obtain an M-H3BTC precursor; then uniformly dispersing the M-H3BTC precursor in polyacrylonitrile and using electrospinning technology to obtain M-H3BTC@PAN; after carbonizing M-H3BTC@PAN, it is further infiltrated into a mixed solution of bismuth trichloride in glycerol and methanol for bismuth substitution, and finally, a Bi@C@CNFs composite material is obtained by high-temperature calcination.

[0007] As an improvement, the nitrate is copper nitrate, zinc nitrate, or cobalt nitrate.

[0008] The preparation method of the above three-dimensional Bi@C@CNFs composite material includes the following steps:

[0009] (1) Preparation of the M-H3BTC precursor: Add nitrate and polyvinylpyrrolidone to a methanol solvent and stir until completely dissolved, marked as solution A; weigh 1,3,5-benzenetricarboxylic acid and place it in methanol and stir until completely dissolved, marked as solution B; slowly add solution B to solution A, stir at room temperature for 30 min, let it stand for 24 h, then centrifuge and dry for 6 h to obtain the M-H3BTC precursor for standby;

[0010] (2) Preparation of the M-H3BTC@PAN precursor: Take the M-H3BTC precursor and disperse it in N,N-dimethylformamide, ultrasonically treat it for 20 min until the M-H3BTC precursor is uniformly dispersed, then add PAN, stir at 40 °C for 12 h to obtain an electrospinning dispersion liquid containing M-H3BTC, and then collect the M-H3BTC@PAN precursor on a receiver. The mass ratio of the M-H3BTC precursor to PAN is 1-2:1;

[0011] (3) Preparation of Bi@C@CNFs composite material: The M-H3BTC@PAN precursor was pre-oxidized and then carbonized in a mixed hydrogen and argon atmosphere for 2 h. 1.89 g of BiCl3 was dissolved in 50 mL of a mixed solvent of glycerol and methanol. 100 mg of the carbonized spinning material was immersed therein, and the mixture was heated to 100 °C and reacted for 12 h. After cooling, washing and drying, it was placed in an inert atmosphere again and carbonized at 400-600 °C and held for 2 h to obtain a black Bi@C@CNFs composite material, where the volume ratio of glycerol to methanol was 6:1.

[0012] As an improvement, the mass ratio of the M-H3BTC precursor to PAN in step (2) is 1.25:1, and the volume of N,N-dimethylformamide is 5 mL.

[0013] As an improvement, the pushing speed of the syringe in step (2) is 0.5 mL / h -1 , the receiving distance is 11 cm, and the rotation speed of the rotating shaft receiver is 400 r / min -1 , after setting, a continuous voltage of 16 kV is applied between the receiver and the needle.

[0014] As an improvement, the heating rate of pre-oxidation in step (3) is 2 °C / min -1 , and it is heated to 250 °C and held for 2 h.

[0015] As an improvement, the heating rate of high-temperature carbonization in step (3) is 5 °C / min -1 , and it is heated to 500 °C.

[0016] As an improvement, the inert gas in step (3) is argon.

[0017] The above-mentioned three-dimensional Bi@C@CNFs composite material is used as a negative electrode material for sodium ion batteries.

[0018] Design principle:

[0019] Bismuth nanoparticles are uniformly dispersed in the nanofiber conductive network, providing conditions for the rapid transmission of electrons, and the conductive fibers provide additional volume expansion space for bismuth during the cycling process. A stable SEI layer is formed on the surface of Bi@C@CNFs as the negative electrode material for SIBs, further ensuring the structural stability of the material.

[0020] Beneficial effects

[0021] Compared with the prior art, a three-dimensional Bi@C@CNFs composite material, its preparation method and application of the present invention synthesize a self-supporting three-dimensional nitrogen-doped nanofiber bismuth-carbon material (Bi@C@CNFs) by electrospinning, ion exchange and high-temperature carbonization technologies. This method can evenly distribute bismuth-carbon nanoparticles in carbon fibers derived from polyacrylonitrile polymers, reduce the agglomeration of bismuth-carbon materials, thereby improving the capacity of the bismuth-carbon negative electrode, enhancing its stability during the cycling process, and the carbon nanocage skeleton can provide sufficient expansion space for the bismuth-carbon materials. The 3D nanofilamentous structure can effectively shorten the ion / electron transport distance, and still retain the initial structural morphology after the cycling test, and exhibit excellent cycling performance and rate performance. Bi@C@CNFs(Cu) still shows a specific capacity of 388 mA h g -1 after cycling 200 cycles at a current density of 1 A g -1 , and has a specific capacity of 301.2 mAh g -1 at a high rate of 2 A g -1 , showing relatively excellent cycling stability. Description of the Drawings

[0022] Figure 1 SEM image of the product Bi@C@CNFs(Cu) prepared in Example 1;

[0023] Figure 2 SEM image of the product Bi@C@CNFs(Co) prepared in Example 2;

[0024] Figure 3 SEM image of the product Bi@C@CNFs(Zn) prepared in Example 3;

[0025] Figure 4 XRD spectra of the products Bi@C@CNFs(Cu), Bi@C@CNFs(Zn) and Bi@C@CNFs(Co) prepared in Examples 1, 2 and 3;

[0026] Figure 5 TG curve of the product Bi@C@CNFs(Cu) prepared in Example 1;

[0027] Figure 6 High-resolution C1s spectrum of the product Bi@C@CNFs(Cu) prepared in Example 1;

[0028] Figure 7 High-resolution Bi 4f spectrum of the product Bi@C@CNFs(Cu) prepared in Example 1;

[0029] Figure 8 High-resolution N1s spectrum of the product Bi@C@CNFs(Cu) prepared in Example 1;

[0030] Figure 9 Cyclic voltammograms of the first four cycles of the product Bi@C@CNFs(Cu) prepared in Example 1;

[0031] Figure 10 Charge-discharge curves of the product Bi@C@CNFs(Cu) prepared in Example 1 measured at a current density of 0.1 A g-1;

[0032] Figure 11 Comparison chart of the rate performance of the negative electrode materials Bi@C@CNFs(Cu), Bi@C@CNFs(Zn), and Bi@C@CNFs(Co) of the products prepared in Examples 1, 2, and 3;

[0033] Figure 12 For 1.0 A g -1 Comparison chart of the performance of the products Bi@C@CNFs(Cu), Bi@C@CNFs(Zn), and Bi@C@CNFs(Co) prepared in Examples 1, 2, and 3 after 200 cycles;

[0034] Figure 13 Cyclic voltammograms of the product Bi@C@CNFs(Cu) prepared in Example 1 at different scan rates;

[0035] Figure 14 Fitting curve of the logarithm of the peak current and the logarithm of the scan rate of the product Bi@C@CNFs(Cu) prepared in Example 1;

[0036] Figure 15 Electrochemical impedance before cycling of the products Bi@C@CNFs(Cu), Bi@C@CNFs(Zn), and Bi@C@CNFs(Co) prepared in Examples 1, 2, and 3;

[0037] Figure 16 Electrochemical impedance of the products Bi@C@CNFs(Cu), Bi@C@CNFs(Zn), and Bi@C@CNFs(Co) prepared in Examples 1, 2, and 3 after 200 cycles;

[0038] Figure 17 Cross-sectional view of the product Bi@C@CNFs(Cu) prepared in Example 1 before cycling;

[0039] Figure 18 For the product Bi@C@CNFs(Cu) prepared in Example 1 at 1 A g -1 Cross-sectional thickness image after cycling 200 times under the current density condition;

[0040] Figure 19SEM image of the product Bi@C@CNFs(Cu) prepared in Example 1 before cycling;

[0041] Figure 20 SEM image of the product Bi@C@CNFs(Cu) prepared in Example 1 after 200 cycles; Detailed implementation manners

[0042] The following examples can enable those skilled in the art to understand the present invention more comprehensively, but do not limit the present invention to the scope of the described examples.

[0043] Example 1

[0044] (1) Preparation of Cu-H3BTC precursor

[0045] Take 0.9 g of copper nitrate trihydrate Cu(NO3)2·3H2O and 0.4 g of polyvinylpyrrolidone (PVP, MW = 4000) and add them to 50 mL of methanol solvent, stir until completely dissolved, and label it as solution A; weigh 0.43 g of 1,3,5-benzenetricarboxylic acid (H3BTC) and place it in 50 mL of methanol, stir until completely dissolved, and label it as solution B; slowly add solution B to solution A, stir at room temperature for 30 min, centrifuge after standing for 24 h, and dry for 6 h for standby.

[0046] (2) Preparation of Cu-H3BTC@PAN precursor:

[0047] Take 0.25 g of the above Cu-H3BTC precursor and disperse it in 5 mL of N,N-dimethylformamide (DMF), ultrasonically treat for 20 min until Cu-H3BTC is uniformly dispersed, then take 0.2 g of polyacrylonitrile (PAN) and add it to the above dispersion, stir at 40 °C for 12 h to obtain an electrospinning dispersion containing Cu-H3BTC. Load the electrospinning dispersion obtained above into a 10 mL syringe, fix the above device on a precision syringe pump using a pressure rod, connect the syringe and a 21-gauge blunt injection needle in sequence through a plastic heat hose and a copper wire, and evenly wind the copper wire around the other end of the needle, and then fix it on a specific insulating bracket to adjust the injection direction of the needle and the distance to the top rotating shaft receiver. At the same time, wrap a layer of aluminum foil of the same width on the rotating shaft receiver to facilitate the collection of the precursor's spinning. The advancing speed of the syringe is 0.5 mL / h -1 , the receiving distance is 11 cm, and the rotation speed of the rotating shaft receiver is 400 r / min -1 . After setting, set a constant voltage between the receiver and the needle, apply a high voltage of 16 kV, and as the syringe pusher automatically advances, collect the Cu-H3BTC@PAN precursor on the receiver.

[0048] (3) Preparation of Bi@C@CNFs(Cu) composites

[0049] The blue-green Cu-H3BTC@PAN electrospinning precursor was peeled off from the aluminum foil and sandwiched between two alumina plates. It was placed in a muffle furnace at 2 °C min -1 The temperature was raised to 250℃ and kept for 2h for pre-oxidation. -1 The heating rate was raised to 500℃ for carbonization for 2h. Then 1.89g BiCl3 was taken and dissolved in a 6:1 ratio of propylene glycol and methanol mixed solvent, stirred at room temperature until completely dissolved, and 100mg of the carbonized spinning material was soaked in it, and then transferred to a stainless steel reactor, heated to 100℃ for reaction for 12h. After natural cooling, it was washed and dried and placed in a tube furnace again, and heated at 5℃min in an argon atmosphere. -1 The heating rate was increased to 500℃ for carbonization and kept warm for 2h to obtain a black Bi@C@CNFs(Cu) composite material.

[0050] Figure 1 This is a SEM image of the product prepared in Example 1. It can be seen from the figure that the carbon nanofibers are not sintered together, and some wrinkles appear on the surface, but they are still evenly distributed and not agglomerated and bonded.

[0051] Figure 4 The XRD spectrum of Bi@C@CNFs(Cu) is given. The results show that there are three obvious diffraction peaks at 2θ=27.2°, 38° and 39.7°, corresponding to the (012), (104) and (110) crystal planes of metallic bismuth (Bi) (JCPDS: 44-1246). The characteristic peaks of Cu are not present, indicating that Bi 3+ Complete exchange of copper ions can be achieved through in situ substitution to obtain Bi@C@CNFs(Cu) composite materials.

[0052] Figure 5 TG curve of Bi@C@CNFs(Cu) prepared in Example 1. The figure shows that a slight weight loss of 4.90wt% was observed at 248°C, which was caused by the evaporation of solvent or water adsorbed by the sample during storage. In addition, the mass decreased sharply and lost weight in the range of 248 to 575°C, with a mass loss of nearly 49wt%, mainly due to the thermal decomposition of carbon in the sample under an oxygen atmosphere. When the temperature is greater than 575°C, the weight of the sample stabilizes at a certain value. By calculation, the content of Bi in the Bi@C@CNFs composite material is about 31.7wt%.

[0053] Figure 6High-resolution C 1s spectrum of Bi@C@CNFs(Cu) prepared in Example 1. The three fitting peaks in the figure are located at 284.4, 284.8, and 285.8 eV, corresponding to C-C, C=N, and C-O bonds respectively. The presence of the C-N bond further proves the existence of N doping in the organic carbon framework. The two characteristic peaks at 286.3 eV and 288.2 eV represent C-H and C-Se bonds respectively.

[0054] Figure 7 High-resolution Bi 4f spectrum of Bi@C@CNFs(Cu) prepared in Example 1. It can be seen from the figure that the high-resolution spectrum of Bi 4f consists of two peaks at 158.1 and 163.3 eV, and the fitting shows the characteristic peaks corresponding to Bi 4f 5 / 2 and Bi4f 7 / 2 orbits, accurately corresponding to Bi.

[0055] Figure 8 High-resolution N 1s spectrum of Bi@C@CNFs(Cu) prepared in Example 1. It can be seen from the figure that there are three peaks at 54.7 eV, 55.6 eV, and 59.2 eV respectively, corresponding to pyridine N, pyrrole N, and graphitic N. These peak values fully prove that N1s hybrid atoms have been successfully doped into the carbon framework. Combining with the analysis results of XRD, it can be known that the process of in-situ substitution after carbonization with Cu-H3BTC@PAN as the precursor can achieve the complete substitution of Bi 3+ for Cu 2+ , and Bi@C@CNFs(Cu) has been successfully synthesized.

[0056] Figure 9 Cyclic voltammogram curves of the product prepared in Example 1 for the first 4 cycles. It can be seen from the figure that except for the first cycle, the curves of the remaining cycles have good coincidence with the increase of the number of cycles, indicating that the redox reaction of the target material electrode has good reversibility and stability.

[0057] Figure 10 Charge-discharge curves of the product prepared in Example 1 measured at a current density of 0.1 A g -1 . It can be seen by observation that the first cycle loses more capacity compared with the next few cycles, which can be attributed to the irreversible reaction of forming the SEI film during the first cycle. In the following several cycles, the capacity of the tested electrode material remains in a stable range (500 - 550 mAh g -1 ), the Coulomb efficiency is stable at about 98.6% - 99%, and the charge-discharge curves almost completely overlap, which is consistent with the conclusion obtained from the above CV curves.

[0058] Figure 11The rate performance of the Bi@C@CNFs(Cu) anode material is presented. It can be clearly observed that at 0.1, 0.2, 0.5, 1.0, 2.0 A g -1 At different current densities, the reversible discharge capacities of Bi@C@CNFs(Cu) are 601, 588, 502, 386, and 301.2 mAh g -1 , respectively. When the current density returns in the above pattern again, it still exhibits excellent rate performance and maintains a reversible capacity of 551 mAh g -1 when the current density returns to 0.1 A g -1 . The above results indicate that Bi@C@CNFs(Cu) has excellent rate performance.

[0059] Figure 12 The performance of the Bi@C@CNFs(Cu) anode of a sodium-ion battery after 200 cycles at 1.0 A g -1 is presented. At a current density of 1 A g -1 , the Bi@C@CNFs(Cu) anode can still maintain a discharge capacity of 388 mA h g -1 after 200 cycles, and there is a trend of gradually increasing discharge capacity. The above results prove that Bi@C@CNFs(Cu) has excellent cycling performance.

[0060] Figure 13 Cyclic voltammograms of Bi@C@CNFs(Cu) prepared in Example 1 at different scanning rates are shown. In the figure, the scanning rate increases from 0.1 mV s -1 to 0.8 mV s -1 in turn. According to the test results, its CV curves have the same redox potential, indicating that no other side reactions occur during the cycling of the Bi@C@CNFs(Cu) anode material, and this material has a fast sodiation / desodiation ability during the entire continuous charge-discharge process.

[0061] Figure 14 The fitting curve of the logarithm of the peak current versus the logarithm of the scanning rate of Bi@C@CNFs(Cu) prepared in Example 1 is shown. From the slope values b of the electrochemical curves tested for Bi@C@CNFs(Cu) being 0.52 and 0.88 respectively, both within the range (0.5 < b < 1), it can be seen that the electrochemical reaction process of this material is a process controlled by the combination of ion diffusion and capacitance.

[0062] Figure 15 The electrochemical impedance of the Bi@C@CNFs(Cu) anode of a sodium-ion battery before cycling is presented. As can be seen from the figure, the semicircle region of Bi@C@CNFs(Cu) is smaller, indicating that the charge transfer impedance of Bi@C@CNFs(Cu) is smaller, which is beneficial for shortening the electron diffusion path in the material to achieve more excellent electrochemical performance.

[0063] Figure 16 The electrochemical impedance of the Bi@C@CNFs(Cu) anode of the sodium-ion battery after 200 cycles is given. The results show that the semicircle diameter of Bi@C@CNFs(Cu) is smaller, further indicating that the charge transfer resistance of the Bi@C@CNFs(Cu) anode is lower, demonstrating that the Bi@C@CNFs(Cu) anode for sodium-ion batteries has excellent electrochemical performance.

[0064] Figure 17 The sectional view of Bi@C@CNFs(Cu) prepared in Example 1 before cycling. It shows that its thickness is 31.25 μm, and the material is tightly bonded to the electrode copper sheet.

[0065] Figure 18 For Bi@C@CNFs(Cu) prepared in Example 1 at a current density of 1 Ag -1 The sectional thickness image of cycling 200 times under the current density condition. Compared with Figure 17 this, the increased thickness of the material expansion is about 28.27 μm, and there is no situation where the material falls off from the surface of the electrode sheet.

[0066] Figure 19 The SEM image of Bi@C@CNFs(Cu) prepared in Example 1 before cycling. There is no cracking on the surface before cycling.

[0067] Figure 20 The SEM image of Bi@C@CNFs(Cu) prepared in Example 1 after 200 cycles. There is no cracking on the surface after 200 cycles, and the surface morphology of some of the original electrode materials can still be seen.

[0068] Example 2

[0069] The difference from Example 1 is that Co-H3BTC@PAN is used instead of Cu-H3BTC@PAN in the prepared material, and the prepared material is denoted as Example 2.

[0070] Figure 2 The SEM image of Bi@C@CNFs(Co) prepared in Example 2. It can be seen from the figure that the Bi@C@CNFs obtained using Co-H3BTC@PAN as the precursor is a fibrous structure, but there is a certain degree of adhesion phenomenon.

[0071] Figure 4The XRD pattern of Bi@C@CNFs(Co) is given. The results show that three obvious diffraction peaks of Bi@C@CNFs appear at 2θ = 27.2°, 38°, and 39.7°, corresponding to the (012), (104), and (110) crystal planes of metallic bismuth (Bi) (JCPDS: 44-1246). No characteristic peak of Cu appears, indicating that Bi 3+ Complete exchange of cobalt ions can be achieved through in-situ substitution to obtain the Bi@C@CNFs(Co) composite material.

[0072] Figure 11 It is a comparison chart of the rate performance of the Bi@C@CNFs(Cu), Bi@C@CNFs(Zn), and Bi@C@CNFs(Co) anode materials. Under the same conditions, Bi@C@CNFs(Co) is not sufficient to support the rate capacity performance test at different current densities, and its rate performance is far inferior to that of the Bi@C@CNFs(Cu) anode material. This is attributed to the fact that the structural stability of Co-H3BTC is inferior to that of Cu-H3BTC.

[0073] Figure 12 The cycling performance of Bi@C@CNFs(Co) is shown. At a current density of 1 A g -1 After 200 cycles, the discharge capacity of the Bi@C@CNFs(Co) anode material is only 163 mAh g -1 .

[0074] Figure 15 And Figure 18 It is the electrochemical impedance test (EIS) of Bi@C@CNFs(Co) before and after cycling. It can be seen that the EIS curve consists of a slant line in the low-frequency region and a semicircle in the high-frequency region, representing diffusion impedance and charge transfer impedance respectively. It can be seen from the figure that the charge transfer impedance of Bi@C@CNFs(Co) is relatively large, which is not conducive to electron diffusion. This is one of the reasons why the lithium storage performance of Bi@C@CNFs(Co) as the anode of a sodium-ion battery is weaker than that of Bi@C@CNFs(Cu).

[0075] Example 3

[0076] The difference from Example 1 is that Zn-H3BTC@PAN is used instead of Cu-H3BTC@PAN in the prepared material, and the prepared material is denoted as Example 3.

[0077] Figure 3 It is the SEM image of Bi@C@CNFs(Zn) prepared in Example 3. It can be seen from the figure that the obtained Bi@C@CNFs nanofibers are the same as those in Example 2, showing the phenomenon of mutual adhesion.

[0078] Figure 4The XRD pattern of Bi@C@CNFs(Zn) is given. The results show that three obvious diffraction peaks of Bi@C@CNFs appear at 2θ = 27.2°, 38°, and 39.7°, corresponding to the (012), (104), and (110) crystal planes of metallic bismuth (Bi) (JCPDS: 44 - 1246), and no characteristic peaks of Cu appear, indicating Bi 3+ Complete exchange of cobalt ions can be achieved through in-situ substitution to obtain the Bi@C@CNFs(Zn) composite material.

[0079] Figure 11 The rate performance of the Bi@C@CNFs(Zn) anode material is shown. Under the same conditions, the rate performance of Bi@C@CNFs(Zn) is far inferior to that of the Bi@C@CNFs(Cu) anode material, which is attributed to the fact that the structural stability of Zn-H3BTC is inferior to that of Cu-H3BTC.

[0080] Figure 12 The cycling performance of Bi@C@CNFs(Zn) as the anode of a sodium-ion battery is shown. At a current density of 1Ag -1 After 200 cycles, the discharge capacity of the Bi@C@CNFs(Zn) anode material is only 308 mAh g -1 .

[0081] Figure 15 and Figure 18 are the electrochemical impedance tests (EIS) of Bi@C@CNFs(Zn) before and after cycling. It can be seen that the EIS curve consists of an oblique line in the low-frequency region and a semicircle in the high-frequency region, representing diffusion impedance and charge transfer impedance respectively. It can be seen from the figure that the semicircle diameter of Bi@C@CNFs(Zn) is larger, indicating that the charge transfer resistance of its anode is larger.

[0082] Example 4

[0083] The difference from Example 1 is that in step (3), nitrogen is used to replace argon, and it is carbonized to 500 °C at a heating rate of 5 °C min -1 , and held for 2 h to obtain a black Bi@C@CNFs composite material. The prepared material is denoted as Example 4. Basic parameters: The SEM image shows that the product obtained in Example 4 has no agglomeration and adhesion phenomenon, but partial wrinkled carbon appears on the surface, and its sodium storage performance as the anode material of a sodium-ion battery is weaker than that of the Bi@C@CNFs(Cu) anode material.

[0084] Example 5

[0085] Differing from Example 1, the carbonization temperature for preparing the Bi@C@CNFs composite material was 400 °C, and the obtained black material was denoted as Example 5. This composite material has a three-dimensional fiber structure. As the anode material for a sodium-ion battery, its sodium storage performance is poor, which is mainly attributed to the relatively low carbonization temperature of 400 °C, resulting in a relatively low graphitization degree of the Bi@C@CNFs composite material.

[0086] Example 6

[0087] Differing from Example 1, the carbonization temperature for preparing the Bi@C@CNFs composite material was 600 °C, and the obtained black material was denoted as Example 6. This composite material has a three-dimensional fiber structure. As the anode material for a sodium-ion battery, its sodium storage performance is weaker than that of the Bi@C@CNFs composite material prepared by carbonization at 500 °C. Although the high carbonization temperature increases its graphitization degree, its porosity decreases.

[0088] The above shows and describes the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited by the above embodiments. What is described in the above embodiments and the specification only illustrates the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention will have various changes and improvements, and these changes and improvements all fall within the scope of the present invention claimed. The scope of protection claimed for the present invention is defined by the appended claims and their equivalents.

Claims

1. A three-dimensional Bi@C@CNFs composite material, characterized in that, First, an M-H3BTC precursor is prepared by mixing nitrate, polyvinylpyrrolidone, and 1,3,5-benzenetricarboxylic acid after dissolving them in methanol. Then, the M-H3BTC precursor is uniformly dispersed in polyacrylonitrile, and M-H3BTC@PAN is prepared by electrospinning technology. After carbonizing M-H3BTC@PAN, it is immersed in a mixed solution of glycerol and methanol of BiCl3 for bismuth substitution, and finally, a Bi@C@CNFs composite material is obtained by high-temperature calcination.

2. The three-dimensional Bi@C@CNFs composite material according to claim 1, wherein The nitrate is copper nitrate, zinc nitrate, or cobalt nitrate.

3. A method for preparing a three-dimensional Bi@C@CNFs composite material according to claim 1, characterized in that, It includes the following steps: (1) Preparation of M-H3BTC precursor: Add nitrate and polyvinylpyrrolidone to a methanol solvent and stir until completely dissolved, marked as solution A; weigh 1,3,5-benzenetricarboxylic acid and place it in methanol and stir until completely dissolved, marked as solution B; slowly add solution B to solution A, stir at room temperature for 30 min, let it stand for 24 h, then centrifuge and dry for 6 h to obtain the M-H3BTC precursor for standby. (2) Preparation of M-H3BTC@PAN precursor: Take the M-H3BTC precursor and disperse it in N,N-dimethylformamide, ultrasonically treat it for 20 min until the M-H3BTC precursor is uniformly dispersed, then add PAN, stir at 40 °C for 12 h to obtain an electrospinning dispersion liquid containing M-H3BTC, and then collect the M-H3BTC@PAN precursor on a receiver. The mass ratio of the M-H3BTC precursor to PAN is 1-2:

1. (3) Preparation of Bi@C@CNFs composite material: Pre-oxidize the M-H3BTC@PAN precursor, then carbonize it in a hydrogen-argon mixed atmosphere for 2 h. Take 1.89 g of BiCl3 and dissolve it in 50 mL of a mixed solvent of glycerol and methanol. Take 100 mg of the carbonized spinning material and immerse it therein, heat to 100 °C and react for 12 h, cool, wash, and dry, and then place it in an inert atmosphere again, carbonize at 400-600 °C, and keep it warm for 2 h to obtain a black Bi@C@CNFs composite material, where the volume ratio of glycerol to methanol is 6:

1.

4. The preparation method of a three-dimensional Bi@C@CNFs composite material according to claim 3, characterized in that, In step (2), the mass ratio of the M-H3BTC precursor to PAN is 1.25:1, and the volume of N,N-dimethylformamide is 5 mL.

5. The preparation method of a three-dimensional Bi@C@CNFs composite material according to claim 3, characterized in that, The advancing speed of the syringe described in step (2) is 0.5 mL / h -1 , the receiving distance is 11 cm, and the rotational speed of the swivel shaft receiver is 400 r / min -1 . After setting, a continuous high voltage of 16 kV is applied between the receiver and the needle tip 6. The preparation method of a three-dimensional Bi@C@CNFs composite material according to claim 2, characterized in that In step (3), the heating rate of pre-oxidation is 2 °C / min -1 , heat up to 250 °C and hold for 2 h.

7. The preparation method of a three-dimensional Bi@C@CNFs composite material according to claim 2, wherein, The heating rate during high-temperature carbonization in step (3) is 5 °C / min -1 , and it is heated to 500 °C.

8. The preparation method of a three-dimensional Bi@C@CNFs composite material according to claim 2, characterized in that, In step (3), the inert gas is argon.

9. Application of the three-dimensional Bi@C@CNFs composite material as described in claim 1 as a negative electrode material for a sodium-ion battery.

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