Preparation method of anode material for three-phase heterojunction sodium ion battery and the prepared material

Abstract

The invention discloses a preparation method of a negative electrode material for a three-phase heterojunction sodium ion battery. First, terephthalic acid, citric acid monohydrate and an iron salt are dissolved in N, N-dimethylformamide to form a solution. The solution is subjected to a hydrothermal reaction at 120-150 °C to obtain a prepolymer. Then, the prepolymer is calcined with dicyandiamide at 400-500 °C in an argon-hydrogen atmosphere. Then, the atmosphere is changed to an inert atmosphere and the temperature is raised to 750 °C for carbonization to obtain an intermediate. Finally, the intermediate is sulfided with sulfur powder at 350 °C to obtain a negative electrode material for a Fe3O4 / Fe / FeS three-phase heterojunction sodium ion battery. The negative electrode material for a sodium ion battery prepared by this method has good rate performance and cycle stability.

Description

Technical Field

[0001] The present invention relates to the field of anode materials for sodium-ion batteries, and specifically relates to a preparation method of an Fe3O4 / Fe / FeS three-phase heterojunction anode material for sodium-ion batteries and the prepared material. Background Art

[0002] As a form of energy storage, lithium-ion batteries have been widely used in people's lives since their commercialization, providing great help for social development. However, the earth's lithium resources are limited in reserves and unevenly distributed, which is not conducive to future sustainable development. It has been found that sodium-ion batteries have the advantages of cost, safety, and environmental protection, and are expected to replace some lithium-ion batteries, thus enriching the diversification of the energy storage field to meet different market demands. Developing sodium storage materials with high cycle efficiency, high energy density, power, and stable lifespan is the key to realizing the practical application of sodium-ion battery energy storage.

[0003] Iron-based oxides / sulfides are rich in variety, low in cost, green and environmentally friendly, and have a relatively high theoretical specific capacity. However, the obvious volume change during charge and discharge causes the material to break and fall off, resulting in poor cycle stability and reduced actual specific capacity. Summary of the Invention

[0004] One object of the present invention is to provide a preparation method of an Fe3O4 / Fe / FeS three-phase heterojunction anode material for sodium-ion batteries. The anode material for sodium-ion batteries prepared by this method has good rate performance and cycle stability.

[0005] To solve the above problems, the present invention adopts the following technical solutions:

[0006] A preparation method of a three-phase heterojunction anode material for sodium-ion batteries, comprising the following steps:

[0007] Step S1: Dissolve terephthalic acid, citric acid monohydrate, and an iron salt in N,N-dimethylformamide to form a solution, and perform a hydrothermal reaction on the solution at 120-150 °C to obtain a pre-polymer, wherein the mass ratio of terephthalic acid to citric acid monohydrate is 10:1, and the molar ratio of citric acid monohydrate to the iron salt is 0.1-0.15:1;

[0008] Step S2: Calcinate the pre-polymer with dicyandiamide at 400-500 °C in an argon-hydrogen atmosphere, and then change to an inert atmosphere and raise the temperature to 750 °C for carbonization to obtain an intermediate, wherein the mass ratio of the pre-polymer to dicyandiamide is 1:8-12;

[0009] Step S3: Sulfurize the intermediate with sulfur powder at 350 °C to obtain an Fe3O4 / Fe / FeS three-phase heterojunction anode material for sodium-ion batteries.

[0010] Preferably, in step S1, terephthalic acid and citric acid monohydrate are dissolved in N,N-dimethylformamide to obtain a first solution, and an iron salt is dissolved in N,N-dimethylformamide to obtain a second solution. The first solution and the second solution are stirred and then mixed. The iron salt is ferric chloride hexahydrate, ferric chloride, ferric nitrate or ferric sulfate.

[0011] Preferably, in step S1, the iron salt is ferric chloride hexahydrate, and the molar ratio of citric acid monohydrate to the iron salt is 0.12:1.

[0012] Preferably, in step S1, the solution is placed in a hydrothermal reaction kettle and reacted at 120 °C for 12 h.

[0013] Preferably, in step S2, the mass ratio of the prepolymer to dicyandiamide is 1:10. Under an argon-hydrogen atmosphere, the temperature is raised to 400 °C at a heating rate of 2 °C / min, and calcined and reduced for 2 h. Then, it is changed to nitrogen and the temperature is raised to 750 °C at a heating rate of 2 °C / min, and carbonized for 4 h to obtain the intermediate. -1 -1

[0014] Preferably, in step S3, the mass ratio of the intermediate to sulfur powder is 1:4. Under a nitrogen atmosphere, the temperature is raised to 350 °C at a heating rate of 5 °C / min, and sulfided for 2 h to obtain the Fe3O4 / Fe / FeS three-phase heterojunction sodium ion battery anode material. -1

[0015] Preferably, the inert atmosphere is nitrogen.

[0016] The second object of the present invention is to provide a three-phase heterojunction sodium ion battery anode material prepared by the above preparation method.

[0017] The method of the present invention first prepares a modified MIL-88B prepolymer, and then through reduction reaction, carbonization and sulfidation steps, a mixture phase of Fe3O4, Fe and FeS, namely Fe3O4 / Fe / FeS heterojunction, and a three-dimensional network structure of nitrogen-doped carbon nanotubes coating Fe3O4 / Fe / FeS spheres are generated. It has defects and relatively large internal free space. The open pores of the hollow nanotubes provide channels for the penetration of the electrolyte, increase the contact area with the material, provide more active sites for the redox reaction, shorten the diffusion path of Na + + ​​​​Diffusion Kinetics. Nitrogen-doped carbon nanotubes can increase the electrical conductivity of iron-based oxygen / sulfide, promote the rapid transport of electrons on the nanotubes, and the hollow nanotubes can reduce the weight of the material. The doping of N element plays an important role in maintaining the carbon tubes. The three-dimensional network structure gives good elasticity to volume changes. The nanotubes wrap the three-phase heterojunction material, which can relieve the volume expansion of metal oxygen / sulfide during cycling. Electrons can also be directly transported to the heterojunction through the nanotubes, and the Fe3O4 / Fe / FeS heterojunction can also promote the rapid transfer of electrons, improving the diffusion kinetics. In addition, Fe in the Fe3O4 / Fe heterojunction can adjust the d-band center in Fe3O4 to enhance the interaction between Fe3O4 and NaS2, thereby suppressing the shuttle effect, and thus the electrochemical reactions of Fe3O4, FeS, and Fe are improved. The cycle stability of the negative electrode material prepared by the method of the present invention is verified by a constant current charge / discharge test with a current density of 0.5 Ag -1 The rapid capacity decay in the first few cycles of FHNCS is due to the formation of the SEI film. The initial specific capacity is 466 mAh g -1 , and it can still maintain 390 mAh g -1 after 100 cycles. It can be seen from this that this material has good cycle stability. Description of the Drawings

[0018] Figure 1 SEM images of the negative electrode materials prepared in Example 1 and Comparative Examples 1-3, where a corresponds to the modified MIL-88B, b corresponds to the intermediate, c corresponds to FHNCS, d corresponds to FANCS, e corresponds to FCS, and f corresponds to FAP.

[0019] Figure 2 TEM image of FHNCS prepared in Example 1.

[0020] Figure 3 Mapping image of FHNCS prepared in Example 1.

[0021] Figure 4 HRTEM image of FHNCS prepared in Example 1.

[0022] Figure 5 SAED image of FHNCS prepared in Example 1.

[0023] Figure 6 XRD images of the negative electrode materials prepared in Example 1 and Comparative Examples 1-3, and XRD images of the Fe3O4, FeS2, and Fe standard cards.

[0024] Figure 7 Raman images of FHNCS in Example 1 and FANCS, FCS, and FAP in Comparative Examples 1-3.

[0025] Figure 8 XPS spectra of Fe2p, S2p, O1s, and N1s for FHNCS in Example 1 and FANCS, FCS, and FAP in Comparative Examples 1-3.

[0026] Figure 9 CV curves of the anode materials in Example 1 and Comparative Examples 1-3, where a corresponds to FHNCS, b corresponds to FANCS, c corresponds to FCS, and d corresponds to FAP.

[0027] Figure 10 Charge-discharge curves of the anode materials in Example 1 and Comparative Examples 1-3, where a corresponds to FHNCS, b corresponds to FANCS, c corresponds to FCS, and d corresponds to FAP.

[0028] Figure 11 Rate cycling curves of the anode materials in Example 1 and Comparative Examples 1-3, where a corresponds to FHNCS, b corresponds to FANCS, c corresponds to FCS, and d corresponds to FAP.

[0029] Figure 12 For the anode materials in Example 1 and Comparative Examples 1-3 at 0.5 A g -1 Current density cycling curves, where a corresponds to FHNCS, b corresponds to FANCS, c corresponds to FCS, and d corresponds to FAP. Detailed implementation methods

[0030] I. Preparation of anode materials

[0031] Example 1

[0032] Dissolve 0.498 g (0.0030 mol) of terephthalic acid and 0.050 g (0.00024 mol) of citric acid monohydrate in 15.0 mL of N,N-dimethylformamide, stir for 15 min to form the first solution. Dissolve 0.543 g (0.0020 mol) of ferric chloride hexahydrate in 15.0 mL of N,N-dimethylformamide, stir for 15 min to form the second solution. Mix the first solution and the second solution, continue stirring until completely miscible to form a yellowish-brown solution, then transfer it to a hydrothermal reaction kettle and react in an oven at 120 °C for 12 h. Separate by centrifuge (4000 r min -1 , 10 min), and obtain an orange-yellow pre-polymer after vacuum drying (this is modified MIL-88B).

[0033] Place the pre-polymer and dicyandiamide at both ends of a quartz boat according to a mass ratio of 1:10, calcine in a tube furnace, and set the program in an argon-hydrogen gas (where the volume fraction of hydrogen gas is 8%) atmosphere to rise to 400 °C at a heating rate of 2 °C min -1 The heating rate and hold for 2 h. Then replace the argon-hydrogen gas with nitrogen, and then at a rate of 2 °C min-1 Heat it up to 750 °C at a heating rate, hold for 4 h, and cool naturally to obtain an intermediate.

[0034] Place the intermediate and sublimed sulfur at both ends of a quartz boat according to a mass ratio of 1:4, and calcine in a tube furnace. Under a nitrogen atmosphere, heat it up to 350 °C at a heating rate of 5 °C / min -1 and hold for 2 h to obtain a negative electrode material for a three-phase heterojunction sodium-ion battery (denoted as FHNCS).

[0035] Example 2

[0036] Dissolve 0.499 g (0.003 mol) of terephthalic acid and 0.063 g (0.0003 mol) of citric acid monohydrate in 15.0 mL of N,N-dimethylformamide and stir for 15 min to prepare a solution. Dissolve 0.482 g (0.002 mol) of iron nitrate in 15.0 mL of N,N-dimethylformamide and stir for 15 min to prepare a solution. Mix the above solutions and continue to stir until they are completely miscible to form a light yellow solution. Transfer the pre-polymer solution to a hydrothermal reaction kettle, place it in an oven at 140 °C, react for 12 h, and separate by centrifuge (4000 r / min -1 , 10 min), and obtain an orange-yellow pre-polymer after vacuum drying.

[0037] Place the pre-polymer and dicyandiamide at both ends of a quartz boat according to a mass ratio of 1:8, and calcine in a tube furnace. Set the program under an argon-hydrogen gas (where the volume ratio of hydrogen gas is 7%) atmosphere to heat it up to 450 °C at a heating rate of 2 °C / min -1 and hold for 2 h. Then replace the argon-hydrogen gas with nitrogen, and then heat it up to 750 °C at a heating rate of 2 °C / min -1 and hold for 4 h, and cool naturally to obtain an intermediate.

[0038] Place the intermediate and sublimed sulfur at both ends of a quartz boat according to a mass ratio of 1:4, and calcine in a tube furnace. Under a nitrogen atmosphere, heat it up to 350 °C at a heating rate of 5 °C / min -1 and hold for 2 h to obtain a negative electrode material for a three-phase heterojunction sodium-ion battery (denoted as FHNCS)

[0039] Example 3

[0040] Dissolve 0.501 g (0.003 mol) of terephthalic acid and 0.042 g (0.0002 mol) of citric acid monohydrate in 15.0 mL of N,N-dimethylformamide and stir for 15 min to prepare a solution. Dissolve 0.799 g (0.002 mol) of ferric sulfate in 15.0 mL of N,N-dimethylformamide and stir for 15 min to prepare a solution. Mix the above solutions and continue to stir until they are completely miscible to form a brownish-yellow solution. Transfer the prepolymer solution to a hydrothermal reaction kettle and react at 130 °C in an oven for 12 h. Centrifuge (4000 r min -1 , 10 min), and obtain an orange-yellow prepolymer after vacuum drying.

[0041] Place the prepolymer and dicyandiamide at both ends of a quartz boat according to a mass ratio of 1:12, and calcine in a tube furnace. Set the program in an argon-hydrogen gas (where the volume fraction of hydrogen gas is 10%) atmosphere and increase the temperature at a rate of 2 °C min -1 to 500 °C and hold for 2 h. Then replace the argon-hydrogen gas with nitrogen, and then increase the temperature to 750 °C at a rate of 2 °C min -1 , hold for 4 h, and naturally cool to obtain an intermediate.

[0042] Place the intermediate and sublimed sulfur at both ends of a quartz boat according to a mass ratio of 1:4, and calcine in a tube furnace. Increase the temperature to 350 °C at a rate of 5 °C min -1 in a nitrogen atmosphere and hold for 2 h to obtain a ternary heterojunction sodium-ion battery anode material (denoted as FHNCS)

[0043] Comparative Example 1: Without citric acid monohydrate

[0044] Dissolve 0.498 g (0.003 mol) of terephthalic acid in 15.0 mL of N,N-dimethylformamide, stir for 15 min to prepare the first solution. Dissolve 0.543 g (0.002 mol) of ferric chloride hexahydrate in 15.0 mL of N,N-dimethylformamide, stir for 15 min to prepare the second solution. Mix the first solution and the second solution, continue to stir until they are completely miscible to form a brownish-yellow solution, and then transfer it to a hydrothermal reaction kettle. React at 120 °C in an oven for 12 h. Centrifuge (4000 r min -1 , 10 min), and obtain an orange-yellow prepolymer after vacuum drying.

[0045] Place the prepolymer and dicyandiamide at both ends of a quartz boat according to a mass ratio of 1:10, and calcine in a tube furnace. Set the program in an argon-hydrogen gas atmosphere and increase the temperature at a rate of 2 °C min -1 to 400 °C and hold for 2 h. Then replace the argon-hydrogen gas with nitrogen, and then increase the temperature to 750 °C at a rate of 2 °C min -1 , hold for 4 h, and naturally cool to obtain an intermediate.

[0046] Place the intermediate and sublimed sulfur at both ends of a quartz boat according to a mass ratio of 1:4, and calcine in a tube furnace. Under a nitrogen atmosphere, heat it at a heating rate of 5 °C / min -1 to 350 °C and hold for 2 h to obtain Comparative Product 1 (denoted as FANCS).

[0047] Comparative Example 2: Without dicyandiamide

[0048] Dissolve 0.498 g (0.003 mol) of terephthalic acid and 0.05 g (0.00024 mol) of citric acid monohydrate in 15.0 mL of N,N-dimethylformamide, stir for 15 min to prepare a first solution. Dissolve 0.543 g (0.002 mol) of ferric chloride hexahydrate in 15.0 mL of N,N-dimethylformamide, stir for 15 min to prepare a second solution. Mix the first solution and the second solution, and continue stirring until they are completely miscible to form a yellowish-brown solution. Then transfer it to a hydrothermal reaction kettle and react at 120 °C in an oven for 12 h. Centrifuge (4000 r / min -1 , 10 min), and obtain an orange-yellow prepolymer after vacuum drying.

[0049] Place the prepolymer in a quartz boat and calcine in a tube furnace. Under an argon-hydrogen gas atmosphere, set the program to heat at a rate of 2 °C / min -1 to 400 °C and hold for 2 h. Then replace the argon-hydrogen gas with nitrogen, and then heat to 750 °C at a heating rate of 2 °C / min -1 and hold for 4 h, and naturally cool to obtain an intermediate.

[0050] Place the intermediate and sublimed sulfur at both ends of a quartz boat according to a mass ratio of 1:4, and calcine in a tube furnace. Under a nitrogen atmosphere, heat it at a heating rate of 5 °C / min -1 to 350 °C and hold for 2 h to obtain Comparative Product 2 (denoted as FCS).

[0051] Comparative Example 3: Without citric acid monohydrate and without dicyandiamide

[0052] Dissolve 0.498 g (0.003 mol) of terephthalic acid in 15.0 mL of N,N-dimethylformamide, stir for 15 min to prepare a first solution. Dissolve 0.543 g (0.002 mol) of ferric chloride hexahydrate in 15.0 mL of N,N-dimethylformamide, stir for 15 min to prepare a second solution. Mix the first solution and the second solution, and continue stirring until they are completely miscible to form a yellowish-brown solution. Then transfer it to a hydrothermal reaction kettle and react at 120 °C in an oven for 12 h. Centrifuge (4000 r / min -1 , 10 min), and obtain an orange-yellow prepolymer after vacuum drying.

[0053] The precursor was placed in a quartz boat and calcined in a tube furnace. The program was set at 2 °C min under an argon-hydrogen atmosphere. -1 The heating rate was increased to 400 °C and kept at this temperature for 2 h. Then the argon and hydrogen were replaced with nitrogen and the temperature was increased at 2 °C min -1 The heating rate was raised to 750°C, kept at this temperature for 4 h, and naturally cooled to obtain an intermediate.

[0054] The intermediate and sublimed sulfur were placed at both ends of a quartz boat in a mass ratio of 1:4 and calcined in a tube furnace at 5 °C min under a nitrogen atmosphere. -1 The heating rate was increased to 350°C and kept at this temperature for 2 h to obtain comparative product 3 (denoted as FAP).

[0055] 2. Characterization of negative electrode materials

[0056] The negative electrode material FHNC prepared in Example 1 and the three comparative products FANCS, FCS and FAP prepared in Comparative Examples 1-3 were characterized by scanning electron microscopy. The results are as follows: Figure 1 .

[0057] Depend on Figure 1 It can be seen that in the process of preparing FHNCS in Example 1, the addition of MIL-88B regulated by citric acid monohydrate changes the shuttle-shaped nanostructure into a shuttle-like lamellar stacking structure ( Figure 1 a), proving that the addition of monohydrated citric acid can indeed regulate the morphology of the material. The precursor can be reduced to obtain iron element, which is then carbonized and calcined. The iron element acts as a catalyst to catalyze the formation of carbon nanotubes with a scaffold structure. During the initial calcination at this stage, the sheet-like stacked MIL-88B (Fe) is gradually broken to form carbon nanosheets embedded with Fe3O4 / Fe particles. Afterwards, nitrogen-doped carbon nanotubes begin to grow with the iron monomer as the catalytic center and cross-link with the Fe3O4 / Fe particles. At a calcination temperature of 750°C, there are obvious and longer nanotubes with a large number of defects on the surface of the nanotubes, and the intermediate ( Figure 1 b). The obtained intermediate was subjected to sulfurization treatment to obtain FHNCS ( Figure 1 c) In addition, the precursor without the addition of citric acid monohydrate was calcined by adding dicyandiamide, such as Figure 1 In d, it can be seen that there are large nanoparticles connected to the nanotubes. The precursor was calcined without adding dicyandiamide ( Figure 1 e), no carbon nanotubes were found, and the spindle shape was transformed into a spherical shape, exposing Fe3O4 nanoparticles. However, the comparative product 3 obtained by adding neither monohydrated citric acid nor dicyandiamide did not change the spindle structure and no nanotubes were found ( Figure 1 f).

[0058] The negative electrode material FHNCS prepared in Example 1 was characterized by transmission electron microscopy, and the results are as follows:Figure 2 As shown, it can be clearly seen the hollow nanotubes and the nanoparticles encapsulated therein. The nanotubes are connected to form a three-dimensional channel. The Fe3O4 / Fe / FeS core and the nitrogen-doped hollow carbon nanotube structure in the scaffold structure improve the mechanical stability and can provide more active sites for redox reactions.

[0059] For the FHNCS prepared in Example 1, through the elemental distribution map of TEM ( Figure 3 ), it can be proved that C and N elements are evenly distributed on the nanotubes, and the Fe element is evenly distributed on the spherical nanoparticles in the tubes. Surprisingly, after sulfidation, the S element is not dispersed on the nanotubes but evenly distributed on the surface of the nanospheres, indicating that the S element is not doped on the nanotubes in the form of a single substance but forms FeS on the nanospheres.

[0060] For the FHNCS prepared in Example 1, HRTEM was used to analyze the phase ( Figure 4 ). A clear lattice spacing of 0.330 nm can be seen, which is the (002) lattice plane of graphite. At the same time, through the local enlarged view, the (102) lattice plane of Fe with a lattice spacing of 0.274 nm, the (311) lattice plane of Fe3O4 with a lattice spacing of 0.196 nm, and the (102) lattice plane of FeS with a lattice spacing of 0.247 nm can be clearly observed. A triple heterojunction is generated at the intersection of these three lattices. The heterojunction interface can be clearly seen through the white dotted line marked in the figure, separating the lattice planes of Fe(102), FeS(102), and Fe3O4(311).

[0061] For the FHNCS prepared in Example 1, SAED was used to analyze the phase ( Figure 5 ). The lattice planes of Fe(102); Fe3O4(440), (400), and (331); FeS(102), (002) can be obtained, which can prove that the phase of FHNCS is the Fe3O4 / Fe / FeS heterojunction.

[0062] Furthermore, XRD, Raman, and XPS were used to analyze the FHNCS prepared in Example 1 and compare it with the FANCS, FCS, and FAP prepared in Comparative Examples 1-3. Through the XRD test ( Figure 6), was compared with the standard cards of PDF#88-0866-Fe3O4, PDF#03-1050-Fe and PDF#24-0080-FeS. It was found that the XRD curves of FHNCS and FCS showed three phases of Fe, Fe3O4 and FeS, while the XRD curves of FANCS and FAP showed two phases of Fe3O4 and FeS. This proved that adding dicyandiamide during the calcination process would reduce the content of Fe3O4 and generate elemental Fe. The addition of citric acid monohydrate had little effect on the phase, mainly affecting the morphology of the material, which was consistent with the results of HRTEM. The degree of carbonization of FHNCS, FANCS, FCS and FAP was studied by Raman spectroscopy ( Figure 7 ). The D and G bands can be divided into four peaks, and the ratio of ID1 / IG1 represents the content of defects in the carbon layer. FHNCS constructed an ordered three-dimensional network structure, with the largest ID1 / IG1 ratio and more defects than other more disordered materials. This means that due to the introduction of nitrogen elements, there are abundant defects in FHNCS, which promotes the transmission of electrons. Finally, through XPS spectroscopy ( Figure 8 ), the elemental composition and chemical valence states of FHNCS, FANCS, FCS and FAP were further analyzed. In the high-resolution spectral analysis of Fe2p, peaks corresponding to Fe 2+ and Fe 3+ were detected in all samples, which was consistent with the valence states of Fe in Fe3O4 and FeS. At the same time, Fe 0 with a binding energy of 707.3 eV was found in FHNCS and FANCS. Combining with morphology, it was proved that the Fe 0 produced by the reduction of dicyandiamide could catalyze the formation of nitrogen-doped carbon nanotubes, thus forming a scaffold structure. In addition, the binding energies of Fe-S and S 2- were analyzed from the high-resolution spectrum of S2p, and the Fe-O binding energy was analyzed from the high-resolution spectrum of O1s, which further proved the composition of the phase. The presence or absence of the peak corresponding to N1s was related to the addition of dicyandiamide, and only the peak corresponding to N1s was found in FHNCS and FANCS.

[0063] The above characterization results of the materials prepared in Example 2 and Example 3 were basically the same as those of FHNCS.

[0064] III. Electrochemical Performance Test of Anode Materials

[0065] By assembling 2032-type button cells in a glove box with oxygen and water content of 0.1 ppm, the electrochemical performances of the anode materials of FHNCS, FANCS, FCS and FAP were tested respectively. ( Figure 9 ) At 0.5 mV s -1The cyclic voltammetry test was carried out at a scanning rate of and a voltage window of 0.01 to 2.8 V. FHNCS has four pairs of obvious redox peaks at 0.52 V and 1.05 V, 1.08 V and 1.49 V, 1.48 V and 1.63 V, 1.84 V and 2.21 V. This is attributed to the redox reactions of Fe3O4, FeS and Fe. However, in FCS and FAP, there are only peaks corresponding to the redox reactions of Fe3O4 and FeS, which proves the difference in the phases from the side. Its charge-discharge diagram is as shown in Figure 10 shown. At a current density of 0.5 Ag -1 , the cyclic charge-discharge curve can be seen with four obvious charge-discharge platforms and a relatively high specific capacity, which is consistent with the positions of the redox peaks in the CV curve. The rate performance of different materials was also tested, and the results are as shown in Figure 11 shown. FHNCS has good rate performance. At current densities of 0.2, 0.5, 1.0, 2.0, 4.0 and 5.0 Ag -1 , after cycling 5 times, the specific capacities are 540, 454, 394, 294, 236 and 191 mAh g -1 respectively. When the current density returns to 0.2 Ag -1 , it still has a specific capacity of 485 mAh g -1 , proving its good cyclic reversibility. The excellent cyclic stability of FHNCS was demonstrated by a constant current charge / discharge test at a current density of 0.5 Ag -1 ( Figure 12 ). The capacity of FHNCS decays rapidly in the first few cycles because the SEI film is formed. The initial specific capacity is 466 mAh g -1 , and it can still maintain 390 mAh g -1 after 100 cycles.

[0066] The above embodiments are only illustrative of the concept and implementation of the present invention and do not limit it. Under the concept of the present invention, technical solutions without substantial transformation are still within the scope of protection.

Claims

1. A preparation method of a negative electrode material for a three-phase heterojunction sodium ion battery, characterized in that It includes the following steps: Step S1: Dissolve terephthalic acid, citric acid monohydrate and an iron salt in N,N-dimethylformamide to form a solution, and carry out a hydrothermal reaction on the solution at 120-150 °C to obtain a prepolymer, wherein the mass ratio of terephthalic acid to citric acid monohydrate is 10:1, and the molar ratio of citric acid monohydrate to the iron salt is 0.1-0.15:1; Step S2: Calcinate the prepolymer with dicyandiamide at 400-500 °C in an argon-hydrogen atmosphere, and then change to an inert atmosphere and heat up to 750 °C for carbonization to obtain an intermediate, and the mass ratio of the prepolymer to dicyandiamide is 1:8-12; Step S3: Sulfurize the intermediate with sulfur powder at 350 °C to obtain a Fe3O4 / Fe / FeS three-phase heterojunction negative electrode material for a sodium ion battery.

2. The preparation method according to claim 1, characterized in that, In the step S1, the terephthalic acid and citric acid monohydrate are dissolved in N,N-dimethylformamide to obtain a first solution, the iron salt is dissolved in N,N-dimethylformamide to obtain a second solution, and the first solution and the second solution are stirred and then mixed. The iron salt is ferric chloride hexahydrate, ferric chloride, ferric nitrate or ferric sulfate.

3. The preparation method according to claim 2, characterized in that, In the step S1, the iron salt is ferric chloride hexahydrate, and the molar ratio of citric acid monohydrate to the iron salt is 0.12:

1.

4. The preparation method according to claim 1, characterized in that, In the step S1, the solution is placed in a hydrothermal reaction kettle and reacted at 120 °C for 12 h.

5. The preparation method according to claim 1, wherein In the step S2, the mass ratio of the prepolymer to dicyandiamide is 1:10, and the temperature is raised to 400 °C at a heating rate of 2 °C / min in an argon-hydrogen atmosphere, and calcination reduction reaction is carried out for 2 h while keeping warm, then it is changed to nitrogen and the temperature is raised to 750 °C at a heating rate of 2 °C / min -1 while keeping warm, and carbonization is carried out for 4 h to obtain the intermediate. -1 ​ 6. The preparation method according to claim 1, characterized in that, In the step S3, the mass ratio of the intermediate to sulfur powder is 1:4, and the temperature is raised to 350 °C at a heating rate of 5 °C min -1 in a nitrogen atmosphere, and the temperature is maintained for 2 h for sulfidation to obtain the Fe3O4 / Fe / FeS three-phase heterogeneous nodular sodium ion battery anode material.

7. The preparation method according to claim 1, characterized in that In the step S2, the inert atmosphere is nitrogen.

8. A negative electrode material for a three-phase heterojunction sodium ion battery, characterized in that Prepared by the preparation method according to any one of claims 1 to 7.

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