Preparation method of anode material for sodium ion battery and prepared anode material

Abstract

The present invention discloses a preparation method of a negative electrode material for a sodium-ion battery. First, Fe2(MoO4)3 hollow single-shell or multi-shell spheres are sulfided with S powder in an inert atmosphere at a weight ratio of 1:4-6 at 330-370 °C to obtain FeS2 / MoS2 hollow single-shell or multi-shell spheres; then the FeS2 / MoS2 hollow single-shell or multi-shell spheres are placed in a ferric ion solution for reaction, so that FeS2 undergoes a chemical reaction with ferric ions to remove FeS2, the precipitate is separated and calcined in an inert atmosphere at 800 °C to obtain a sodium-ion battery negative electrode material of Fe single-atom doped porous MoS2 hollow single-shell or multi-shell spheres. This material has a high specific capacity, excellent rate performance and good cycle stability as the negative electrode material of a sodium-ion battery.

Description

Technical Field

[0001] The present invention relates to the technical field of sodium-ion batteries, and particularly to a preparation method of a negative electrode material for a sodium-ion battery and the prepared negative electrode material. Background Art

[0002] As a typical transition metal disulfide, MoS2 is bound together by van der Waals forces to have a suitable interlayer spacing of 0.62 nm, enabling the reversible insertion / extraction of Na + , with a theoretical specific capacity as high as 670 mAh g -1 . Therefore, MoS2 has received extensive attention as a negative electrode material for sodium-ion batteries due to its high specific capacity, abundant resources, environmental friendliness, and low cost. However, MoS2 as a negative electrode material still has the following four deficiencies: (1) MoS2 will gradually aggregate during the charge-discharge cycle, resulting in a loss of its active area; (2) the volume change during the charge-discharge process will cause the structure to collapse, greatly affecting its cycle stability; (3) at high current densities, the slow electron and Na + diffusion kinetics lead to low specific capacity; (4) the reaction of MoS2 during the sodiation process occurs in two steps, as shown in the following two equations. According to the second equation, MoS2 is finally converted to Na2S during discharge, and Na2S will be converted to polysulfide. The shuttle effect of polysulfide inhibits the electrochemical reversible reaction, resulting in poor rate performance. Therefore, it is necessary to further improve the electrochemical performance of MoS2 as a negative electrode material for sodium batteries.

[0003] MoS2 + xNa = NaxMoS2 (greater than 0.4 V, x < 2) (1)

[0004] Na x MoS2 + (4 - x)Na = Mo + 2Na2S (less than 0.4 V) (2)

[0005] Designing a reasonable nanostructure is highly effective in improving the structural stability and reaction kinetics of materials. Among them, the hollow multi-shell structure (HoMS) with multiple shells and inner cavities has great application prospects in fields such as rechargeable batteries, electromagnetic wave absorption, catalysis, sensors, and drug delivery. Especially as an electrode material for high-energy density batteries, HoMS can effectively regulate the volume change and stress change during the charge-discharge process, thereby improving the cycle stability. In addition, it can shorten the charge diffusion path, thereby improving the rate performance.

[0006] In addition to structural engineering, composition regulation also demonstrates great power in improving performance. Generally, strategies for modifying catalytic sites in electrode materials can significantly enhance reaction kinetics, thereby remarkably improving rate performance. For example, iron single-atom catalysts have strong chemical catalysis and can anchor Na2S, promoting the rapid and reversible conversion of Mo and Na2S into MoS2. Combining structural design with composition regulation is beneficial for improving cycle stability and rate performance.

[0007] Therefore, constructing a porous MoS2 multi-shell hollow spherical structure doped with iron single atoms is an important approach to improving the performance of MoS2 negative electrode materials for sodium-ion batteries. Summary of the Invention

[0008] An object of the present invention is to provide a preparation method for a negative electrode material of a sodium-ion battery, which can prepare a porous MoS2 hollow single-shell or multi-shell sphere doped with iron single atoms, and has a high specific capacity, excellent rate performance, and good cycle stability when used as a negative electrode material for a sodium-ion battery.

[0009] To achieve the above object, the present invention adopts the following technical solutions:

[0010] A preparation method for a negative electrode material of a sodium-ion battery includes the following steps:

[0011] Step S1: Sulfurize Fe2(MoO4)3 hollow single-shell or multi-shell spheres with sulfur powder in an inert atmosphere at a weight ratio of 1:4 - 6 at 330 - 370 °C to obtain FeS2 / MoS2 hollow single-shell or multi-shell spheres;

[0012] Step S2: Place the FeS2 / MoS2 hollow single-shell or multi-shell spheres in a ferric ion solution for reaction, enabling FeS2 to chemically react with ferric ions to remove FeS2, separate the precipitate, and calcine it in an inert atmosphere at 800 °C to obtain a porous MoS2 hollow single-shell or multi-shell sphere negative electrode material doped with iron single atoms for a sodium-ion battery. Preferably, it further includes step S0: The preparation steps of the Fe2(MoO4)3 hollow single-shell or multi-shell spheres are as follows:

[0013] Dissolve (NH4)2MoO4·4H2O, citric acid, and FeCl3·6H2O in water or a mixed solvent of water and ethanol to obtain a solution, then add carbon spheres to the solution and adsorb metal ions for several hours, separate the pre-polymer, and calcine the pre-polymer in a muffle furnace at 440 - 460 °C to obtain Fe2(MoO4)3 hollow single-shell or multi-shell spheres, and the number of shell layers of the Fe2(MoO4)3 hollow single-shell or multi-shell spheres is adjusted by the volume ratio of water to ethanol in the solution, as well as the time and temperature for adsorbing metal ions.

[0014] Preferably, the weight ratio of (NH4)2MoO4·4H2O, citric acid, and FeCl3·6H2O is 2.9 - 3:1.8 - 1.9:8.2 - 8.4. The adsorption is carried out at a temperature of 30 - 50°C, and the solution uses water or a mixed solvent of water and ethanol with a volume ratio of 3:1.

[0015] Preferably, in step S2, the ferric ion solution is an Fe2(SO4)3 solution.

[0016] The inert atmosphere is nitrogen or argon.

[0017] Another object of the present invention is to provide a negative electrode material for a sodium ion battery, which is prepared by the above preparation method.

[0018] The beneficial effects of the present invention are as follows: By using an improved sequential template method, through the combination of preparing single - shell or multi - shell Fe2(MoO4)3 hollow sphere templates, sulfidation, chemical etching with a ferric ion solution, and high - temperature calcination, a porous MoS2 hollow single - shell or multi - shell sphere doped with iron single atoms is synthesized. It not only has a unique single - shell or multi - shell structure, but also a large number of pores will be generated on the shell surface during the chemical etching process. This structure enables the electrolyte to easily penetrate from the surface inward, thereby promoting the contact between the material and the electrolyte, providing more active sites for redox reactions, and shortening the ion and electron transport paths. At the same time, during the charge - discharge process, the free space between the shells can release volume changes, the inner shell can support the outer shell, and the outer shell can also protect the inner shell, ensuring that the structure remains stable during the sodiation and desodiation reaction processes. When the porous MoS2 hollow multi - shell sphere doped with iron single atoms has four shells, as a negative electrode material for a sodium ion battery, at a current density of 0.1 A·g -1 after four cycles, it exhibits a high specific capacity of 462.6 mAh·g -1 Even at ultra - high current densities of 5, 10, 20, and 30 A·g -1 it still exhibits specific capacities of 300.0, 269.3, 239.8, and 213.3 mAh·g -1 respectively, showing excellent rate performance; after 100 cycles at a current density of 0.1 A·g -1 it can retain a specific capacity of 442 mAh·g -1 After 500 cycles at a high current density of 5 A·g -1 it can still maintain a specific capacity of 259.4 mAh·g -1 showing excellent cycle stability. Description of the Drawings

[0019] Figure 1(a) XRD patterns of Fe2(MoO4)3 hollow single- or multi-shell spheres (HoMS); (b) MoS2 / FeS with different shells x XRD patterns of HoMS.

[0020] Figure 2 XRD patterns of Fe-M-HoMS with different shells.

[0021] Figure 3 High-resolution transmission electron microscopy (HRTEM) images of Fe-M-HoMS-Q.

[0022] Figure 4 (a) Nitrogen adsorption-desorption isotherms and (b) pore size distribution curves of Fe-M-HoMS-Q, MF-HoMS-Q, and FMO-HoMS-Q.

[0023] Figure 5 (a) Scanning electron microscopy (SEM) images of Fe-M-HoMS-S, (b) Fe-M-HoMS-D, (c) Fe-M-HoMS-T, and (d) transmission electron microscopy (TEM) images of Fe-M-HoMS-S, (e) Fe-M-HoMS-D, (f) Fe-M-HoMS-T.

[0024] Figure 6 Energy dispersive spectroscopy analysis of Fe-M-HoMS-Q.

[0025] Figure 7 Morphology and electronic structure of Fe-M-HoMS-Q, (a) scanning electron microscopy (SEM), (b) transmission electron microscopy (TEM), (c) elemental distribution, (d) low-pass filtered atomic resolution STEM-ADF, (e) Figure 7 Enlarged view of the boxed area in (d); Mo k-edge (f) XANES and (h) EXAFS analysis spectra of Fe-M-HoMS-Q, Fe k-edge (g) XANES and (i) EXAFS analysis spectra.

[0026] Figure 8 Atomic resolution electron energy loss spectroscopy of Fe-M-HoMS-Q.

[0027] Figure 9Fitting results of the experimental EXAFS function (solid line) and the calculated Mo-Mo and Mo-S contributions (dotted line). (a) Mo EXAFS spectrum of Mo foil in K-space, (b) Mo EXAFS spectrum of MoS2 in K-space, (c) Mo EXAFS spectrum of Fe-M-HoMS-Q in K-space, (d) Mo EXAFS spectrum of Mo foil in K-space, (e) Mo EXAFS spectrum of MoS2 in R-space, (f) Mo EXAFS spectrum of Fe-M-HoMS-Q in R-space.

[0028] Figure 10 Fitting results of the experimental EXAFS function (solid line) and the calculated Fe-Fe, Fe-O and Fe-S contributions (dotted line). (a) Fe EXAFS spectrum of Fe foil in K-space, (b) Fe EXAFS spectrum of FeS2 in K-space, (c) Fe EXAFS spectrum of Fe-M-HoMS-Q in K-space, (d) EXAFS spectrum of Fe foil in R-space, (e) Fe EXAFS spectrum of FeS2 in R-space, (f) Fe EXAFS spectrum of Fe-M-HoMS-Q in R-space.

[0029] Figure 11 Electrochemical impedance analysis of Fe-M-HoMS.

[0030] Figure 12 Electrochemical performance of Fe-M-HoMS in the half-cell system (a-g) and the Fe-M-HoMS / / NVP@C full-cell system (h-j). (a) Rate performance of MoS2, MF-HoMS-Q and Fe-M-HoMS-Q at current densities of 0.1, 0.2, 0.5, 1.0, 2.0, 5.0, 10.0, 20.0, 30.0 and 0.1 A g -1 and (b) cycling stability at a current density of 0.1 A g -1 . (c) Differences in peak current density and voltage between the anodic and cathodic peaks in the CV curve. (d) Cycling stability at a current density of 0.1 A g -1 . (e) Rate performance of different shell Fe-M-HoMS at current densities of 0.1, 0.2, 0.5, 1.0, 2.0, 5.0, 10.0, 20.0, 30.0 and 0.1 A g -1 . (f) Comparison of rate performance with other MoS2 composites. (g) Cycling stability of Fe-M-HoMS-Q at 5 A g -1 current density. (h) Schematic diagram of the Fe-M-HoMS / / NVP@C full cell. (i) CV curves of the full cell at different scan rates. (j) Rate performance of the full cell.

[0031] Figure 13 CV curves of different shell Fe-M-HoMS in the third cycle.

[0032] Figure 14 Fe-M-HoMS-Q at 0.1 A·g -1 Galvanostatic charge-discharge test at the current density. Specific implementation mode

[0033] The present invention will be further described below through specific examples and electrochemical effect tests.

[0034] Example 1

[0035] Step S0: Preparation of Fe2(MoO4)3 hollow four-shell spheres (FMO-HoMS-Q):

[0036] Dissolve 2.94 g (0.00238 mol) of (NH4)2MoO4·4H2O, 1.8 g (0.00856 mol) of citric acid and 8.2 g (0.03034 mol) of FeCl3·6H2O in 22.5 mL of deionized water in sequence. After complete dissolution, add 7.5 mL of absolute ethanol and stir evenly. Then add 0.6 g of carbon spheres into the above solution and ultrasonicate for 15 min. Subsequently, adsorb metal ions at a constant temperature in a water bath at 50 °C for 24 hours. After the reaction is completed, separate the material by suction filtration and wash it alternately with water and ethanol. Dry the separated material and then grind it to obtain a brown powder precursor. Calcinate the collected precursor in a muffle furnace at a heating rate of 1 °C min -1 to 450 °C for 1 h to obtain Fe2(MoO4)3 hollow four-shell spheres (FMO-HoMS-Q).

[0037] Step S1: Place Fe2(MoO4)3 hollow four-shell spheres and S powder on both sides of a quartz boat at a weight ratio of 1:4. Subsequently, heat up to 350 °C in a nitrogen atmosphere and calcine for 2 h for sulfidation. The heating rate is 5 °C min -1 to finally obtain FeS2 / MoS2 hollow four-shell spheres (MF-HoMS-Q).

[0038] Step S2: Place the FeS2 / MoS2 hollow four-shell spheres in an Fe2(SO4)3 solution and react at a constant temperature in a water bath at 30 °C for 24 h to make FeS2 chemically react with ferric ions to generate FeSO4 and dissolve it in the aqueous solution to remove FeS2. After the reaction is completed, separate it with a centrifuge and wash it twice with water (9000 r min -1 , 10 min), and dry it in an oven. Subsequently, at a rate of 5 °C min -1The negative electrode material of sodium-ion battery, Fe single-atom doped porous MoS2 hollow four-shell spheres (Fe-M-HoMS-Q), was obtained by calcining at a heating rate of 800 °C for 2 h in a nitrogen atmosphere. The material is in the form of black powder.

[0039] Example 2

[0040] Step S0: Preparation of Fe2(MoO4)3 hollow four-shell spheres (FMO-HoMS-Q):

[0041] 2.9 g (0.00234 mol) of (NH4)2MoO4·4H2O, 1.9 g (0.00904 mol) of citric acid and 8 g (0.02959 mol) of FeCl3·6H2O were successively dissolved in 22.5 mL of deionized water. After complete dissolution, 7.5 mL of absolute ethanol was added and stirred evenly. Then, 0.6 g of carbon spheres was added to the above solution and ultrasonicated for 15 min. Subsequently, the metal ions were adsorbed at a constant temperature of 50 °C for 24 hours. After the reaction was completed, the material was separated by suction filtration and washed alternately with water and ethanol. The separated material was dried and then ground to obtain a brown powder precursor. The collected precursor was placed in a muffle furnace and calcined at a heating rate of 1 °C / min -1 to 440 °C for 1 h.

[0042] Step S1: Fe2(MoO4)3 hollow four-shell spheres and S powder were placed on both sides of a quartz boat at a weight ratio of 1:5. Subsequently, the temperature was raised to 330 °C in a nitrogen atmosphere and calcined for 2 h for sulfidation. The heating rate was 4 °C / min -1 to finally obtain FeS2 / MoS2 hollow four-shell spheres (MF-HoMS-Q).

[0043] Step S2: The FeS2 / MoS2 hollow four-shell spheres were placed in an Fe2(SO4)3 solution and reacted at a constant temperature of 30 °C in a water bath for 24 h to cause a chemical reaction between FeS2 and ferric ions to generate FeSO4, which was dissolved in the aqueous solution to remove FeS2. After the reaction was completed, it was separated by a centrifuge and washed twice with water (9000 r / min -1 , 10 min), and then dried in an oven. Subsequently, it was calcined at a heating rate of 5 °C / min -1 to 800 °C for 2 h in a nitrogen atmosphere to obtain the negative electrode material of sodium-ion battery, Fe single-atom doped porous MoS2 hollow four-shell spheres (Fe-M-HoMS-Q). The material is in the form of black powder.

[0044] Example 3

[0045] Step S0: Preparation of Fe2(MoO4)3 hollow four-shell spheres (FMO-HoMS-Q):

[0046] Dissolve 3 g (0.00242 mol) of (NH4)2MoO4·4H2O, 1.8 g (0.00856 mol) of citric acid, and 8.4 g (0.03107 mol) of FeCl3·6H2O successively in 22.5 mL of deionized water. After complete dissolution, add 7.5 mL of absolute ethanol and stir evenly. Then add 0.6 g of carbon spheres to the above solution and sonicate for 15 min. Subsequently, adsorb metal ions at a constant temperature of 50 °C for 24 hours. After the reaction is completed, separate the material by suction filtration and wash it alternately with water and ethanol. Dry the separated material and grind it to obtain a brown powder-like pre-polymer. The collected pre-polymer is calcined in a muffle furnace at a heating rate of 1 °C min -1 to 460 °C for 1 h.

[0047] Step S1: Place Fe2(MoO4)3 hollow multi-shell spheres and S powder on both sides of a quartz boat at a weight ratio of 1:6. Subsequently, heat to 370 °C in a nitrogen atmosphere and calcine for 2 h for sulfidation. The heating rate is 6 °C min -1 , to obtain FeS2 / MoS2 hollow four-shell spheres (MF-HoMS-Q).

[0048] Step S2: Place the FeS2 / MoS2 hollow four-shell spheres in an Fe2(SO4)3 solution and react at a constant temperature of 30 °C in a water bath for 24 h to cause a chemical reaction between FeS2 and ferric ions to generate FeSO4, which dissolves in the aqueous solution to remove FeS2. After the reaction is completed, separate by centrifuge and wash twice with water (9000 r min -1 , 10 min), and dry in an oven. Subsequently, calcine in a nitrogen atmosphere at 800 °C at a heating rate of 5 °C min -1 for 2 h to obtain a sodium-ion battery anode material of Fe single-atom doped porous MoS2 hollow four-shell spheres (Fe-M-HoMS-Q), and the material is in the form of black powder.

[0049] Example 4

[0050] Step S0: Preparation of Fe2(MoO4)3 hollow single-shell spheres (FMO-HoMS-S):

[0051] Dissolve 2.94 g (0.00238 mol) of (NH4)2MoO4·4H2O, 1.8 g (0.00856 mol) of citric acid, and 8.2 g (0.03034 mol) of FeCl3·6H2O successively in 30 mL of deionized water. Then add 0.6 g of carbon spheres to the above solution and sonicate for 15 min. Subsequently, adsorb metal ions at a constant temperature of 30 °C in a water bath for 6 hours. After the reaction is completed, separate the material by suction filtration and wash with water. Dry the separated material and grind it to obtain a brown powder-like pre-polymer. The collected pre-polymer is calcined in a muffle furnace at a heating rate of 1 °C min -1Heat it up to 450 °C at a heating rate and calcine for 1 h.

[0052] Step S1: Place the Fe2(MoO4)3 hollow single-shell spheres and S powder on both sides of a quartz boat at a weight ratio of 1:4, and then heat it up to 350 °C in a nitrogen atmosphere and calcine for 2 h for sulfidation. The heating rate is 5 °C / min -1 , and finally obtain FeS2 / MoS2 hollow single-shell spheres (MF-HoMS-S).

[0053] Step S2: Place the FeS2 / MoS2 hollow single-shell spheres in an Fe2(SO4)3 solution, and react at a constant temperature of 30 °C for 24 h to cause a chemical reaction between FeS2 and ferric ions to generate FeSO4 and dissolve it in the aqueous solution to remove FeS2. After the reaction is completed, centrifuge and wash twice with water (9000 r / min -1 , 10 min), and dry in an oven. Then, calcine in a nitrogen atmosphere at 800 °C for 2 h at a heating rate of 5 °C / min -1 to obtain a sodium-ion battery anode material of Fe single-atom doped porous MoS2 hollow single-shell spheres (Fe-M-HoMS-S), and the material is in the form of black powder.

[0054] Example 5

[0055] Step S0: Preparation of Fe2(MoO4)3 hollow double-shell spheres (FMO-HoMS-D):

[0056] Dissolve 2.94 g (0.00238 mol) of (NH4)2MoO4·4H2O, 1.8 g (0.00856 mol) of citric acid, and 8.2 g (0.03034 mol) of FeCl3·6H2O in 30 mL of deionized water in sequence. Then add 0.6 g of carbon spheres to the above solution and ultrasonicate for 15 min. After adsorbing metal ions at a constant temperature of 30 °C in a water bath for 24 h, filter by suction and wash with water for separation. Dry the separated material and grind it to obtain a brown powder-like pre-polymer. Calcine the collected pre-polymer in a muffle furnace at a heating rate of 1 °C / min -1 to 450 °C and calcine for 1 h.

[0057] Step S1: Place the Fe2(MoO4)3 hollow double-shell spheres and S powder on both sides of a quartz boat at a weight ratio of 1:4, and then heat it up to 350 °C in a nitrogen atmosphere and calcine for 2 h for sulfidation. The heating rate is 5 °C / min -1 , to obtain FeS2 / MoS2 hollow double-shell spheres (MF-HoMS-D).

[0058] Step S2: Place the FeS2 / MoS2 hollow double-shell spheres in an Fe2(SO4)3 solution and react at a constant temperature of 30 °C for 24 h to cause a chemical reaction between FeS2 and ferric ions to generate FeSO4, which dissolves in the aqueous solution to remove FeS2. After the reaction is completed, centrifuge and wash twice with water (9000 r min -1 , 10 min), and dry in an oven. Subsequently, calcine at a heating rate of 5 °C min -1 in a nitrogen atmosphere at 800 °C for 2 h to obtain a porous MoS2 hollow double-shell sphere (Fe-M-HoMS-D) anode material for sodium-ion batteries doped with Fe single atoms. The material is in the form of black powder.

[0059] Example 6

[0060] Step S0: Preparation of Fe2(MoO4)3 hollow triple-shell spheres (FMO-HoMS-T):

[0061] Dissolve 2.94 g (0.00238 mol) of (NH4)2MoO4·4H2O, 1.8 g (0.00856 mol) of citric acid, and 8.2 g (0.03034 mol) of FeCl3·6H2O in 30 mL of deionized water in sequence. Then add 0.6 g of carbon spheres to the above solution and ultrasonicate for 15 min. Subsequently, adsorb metal ions at a constant temperature of 50 °C for 24 h. After the reaction is completed, separate the material by suction filtration and wash with water. Dry the separated material and grind it to obtain a brown powder-like pre-polymer. Calcinate the collected pre-polymer in a muffle furnace at a heating rate of 1 °C min -1 to 450 °C for 1 h.

[0062] Step S1: Place the Fe2(MoO4)3 hollow triple-shell spheres and S powder on both sides of a quartz boat at a weight ratio of 1:4. Subsequently, heat to 350 °C in a nitrogen atmosphere and calcine for 2 h for sulfidation. The heating rate is 5 °C min -1 , to obtain FeS2 / MoS2 hollow triple-shell spheres (MF-HoMS-T).

[0063] Step S2: Place the FeS2 / MoS2 hollow triple-shell spheres in an Fe2(SO4)3 solution and react at a constant temperature of 30 °C for 24 h to cause a chemical reaction between FeS2 and ferric ions to generate FeSO4, which dissolves in the aqueous solution to remove FeS2. After the reaction is completed, centrifuge and wash twice with water (9000 r min -1 , 10 min), and dry in an oven. Subsequently, calcine at a heating rate of 5 °C min -1 in a nitrogen atmosphere at 800 °C for 2 h to obtain a porous MoS2 hollow triple-shell sphere (Fe-M-HoMS-T) anode material for sodium-ion batteries. The material is in the form of black powder.

[0064] XRD tests were carried out on FMO-HoMS-Q, MF-HoMS-Q, and Fe-M-HoMS-Q in Example 1 and FMO-HoMS-(S, D, T), MF-HoMS-(S, D, T), and Fe-M-HoMS-(S, D, T) in Examples 4-6. The test results are shown in Figure 1 , Figure 2 . From Figure 1 the XRD pattern of a, it can be seen that the characteristic peaks of FMO-HoMS with different shells match the standard card of Fe2(MoO4)3, proving the successful synthesis of Fe2(MoO4)3. In addition, for MoS2 / FeS in Example 1 x The XRD pattern of HoMS (MF-HoMS-Q) is as shown in Figure 1 b, proving the successful synthesis of MoS2 and FeS after sulfidation x . Figure 2 is the XRD pattern of Fe-M-HoMS. It can be seen from the figure that its characteristic peaks match the standard card (PDF#83-1701) of 2H-MoS2 and are confirmed by high-resolution transmission electron microscopy (HRTEM). As shown in Figure 3 , its lattice spacing of 0.613 nm corresponds to the (002) crystal plane of 2H-MoS2. In addition, no characteristic peaks of Fe and its compounds are observed in the spectrum. Therefore, it can be proved that after the redox reaction between FeS x and Fe2(SO4)3, FeS x is removed and 2H-MoS2 is retained.

[0065] The porous multi-shell structure obtained after chemical etching can be confirmed by nitrogen adsorption-desorption curves, pore size distribution curves, scanning electron microscopy, and transmission electron microscopy. Figure 4 a, 4b are the nitrogen adsorption and desorption isotherm curves and pore size distribution curves of Fe-M-HoMS-Q, MF-HoMS-Q, and FMO-HoMS-Q in Example 1. It can be seen from the figure that the specific surface area and pore volume of Fe-M-HoMS-Q are 48.95 m 2 g -1 and 0.173 cm 3 g -1 , which is greater than MF-HoMS-Q. This result proves that more pores and exposed active sites are generated through chemical etching. Scanning (SEM) and transmission (TEM) characterizations were carried out on Fe-M-HoMS-Q in Example 1 and Fe-M-HoMS-(S, D, T) in Examples 4-6. The characterization results are shown in Figure 5 , 7a, 7b. SEM and TEM images demonstrate the successful synthesis of hollow spheres with different shell structures. It can also be seen from the images that the HoMS structure is not damaged after chemical etching. In addition, due to the loss of FeS x , abundant pores are formed on the shell surface, which also proves the formation of a porous multi-layer structure. Interestingly, energy-dispersive spectroscopy (EDS), the elemental distribution shown in 7c, and the elemental composition shown in Table 1 demonstrate that in addition to molybdenum and sulfur elements, iron elements are also evenly dispersed in porous MoS2. Figure 6

[0066] Table 1 Elemental composition of Fe-M-HoMS-Q

[0067]

[0068] To further prove the doping of Fe single atoms and explore the specific occupancy positions of iron single atoms in MoS2, atomic-resolution STEM-ADF imaging ( Figure 7 d) analysis was carried out. No particles or clusters belonging to metallic iron or iron compounds were detected, which proves the existence of iron single atoms. In addition, the single-substitution defects observed in the images are caused by the doping of Fe single atoms into MoS2. Figure 7 e is Figure 7 a partial enlarged view of the boxed part in Figure 7 d. As shown in Figure 7 e, according to the contrast of atomic number (Z), S (darker) and Mo (brighter) atoms in the MoS2 lattice are clearly observed. However, the originally ordered Mo atoms in the honeycomb atomic structure of MoS2 become disordered. This phenomenon can be attributed to the doping of Fe single atoms. In the STEM-EELS analysis at the atomic scale, the L2,3 edge characteristics of Fe atoms further confirm this phenomenon ( Figure 8 ). The electronic structure information of Fe and Mo atoms was detected using extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES), respectively. As shown in Figure 7 f, the Mo k-edge XANES spectrum of Fe-M-HoMS-Q is located between that of Mo foil and standard MoS2, indicating that the valence state of Mo decreases due to the formation of defects caused by Fe atom doping. The Fe k-edge XANES spectrum of Fe-M-HoMS-Q ( Figure 7 g) is close to that of standard FeS2, indicating that the valence states of Fe include +2 and +3. At the same time, the R-space EXAFS spectrum of Fe-M-HoMS-Q ( Figure 7 h and Figure 7i) The intensities of Mo-S and Mo-Mo in it are reduced compared with those of pure MoS₂, indicating the formation of defects and edges. In particular, no Fe-Fe bond is found, confirming that the Fe atoms in Fe-M-HoMS exist in the form of single atoms. In addition, taking metal foil and metal sulfide as the standard states ( Figure 9 and Figure 10 ), the specific coordination numbers are further proved by fitting the EXAFS spectra in R space and K space. Among them, it is further proved that the coordination ratio of single Fe atoms to S and O is 1 / 2 (marked as FeO₄), as shown in Table 2. Therefore, it can be concluded that Fe single atoms are doped into the MoS₂ lattice of Fe-M-HoMS, which leads to the formation of defects and the change of coordination environment, promoting the electrochemical reaction to obtain excellent rate performance.

[0069] Table 2 Fitting results of EXAFS spectra of Fe-M-HoMS-Q in R space and K space

[0070]

[0071] By assembling 2032-type coin cells in a glove box with oxygen and water content of 0.1 ppm, and taking pure MoS₂ and MF-HoMS-Q as comparisons, the influence of Fe single atoms on the electrochemical performance was explored. As Figure 12 shown in -1 a, through constant current charge-discharge experiments with current densities of 0.1, 0.2, 0.5, 1.0, 2.0, 5.0, 10.0, 20.0, 30.0 and 0.1 A g -1 , the rate performance of the material was verified. Obviously, the specific capacities of Fe-M-HoMS-Q at different current densities are 498.9, 414.4, 383.6, 357.7, 333.1, 300.0, 269.3, 239.8, 213.3 and 417.1 mAh g -1 respectively, which are higher than those of MoS₂ and MF-HoMS-Q, showing better rate performance. In addition, Fe-M-HoMS-Q obtained an initial specific capacity of 462.6 mAh g -1 at the 4th cycle under the condition of a current density of 0.1 A g -1 , and had a lower attenuation rate during the cycling process. After 100 cycles, a higher capacity was retained ( Figure 12 b), indicating better cycle stability of Fe-M-HoMS-Q.

[0072] In addition, the influence of structural design on the electrochemical performance of SIB negative electrodes was studied. First, electrochemical impedance analysis of the materials was carried out as Figure 11 shown, and Fe-M-HoMS-Q showed the smallest charge transfer resistance (Rct) between the electrolyte and the electrode material.Figure 12 c is the difference in peak current density and voltage between the anodic peak and the cathodic peak (obtained by summarizing the CV curves of different shells, with a voltage window of 0.1 - 2.8 V). It can be seen from the figure that the potential difference between the oxidation peak and the reduction peak decreases with the increase in the number of shells, that is, the polarization becomes smaller. The peak current increases with the increase in the number of shells, indicating an increase in specific capacity. In addition, the constant current charge - discharge test confirms that Fe - M - HoMS - Q has a higher specific capacity than other shells ( Figure 13 ). In addition, Fe - M - HoMS - Q has good cycle stability. At a current density of 0.1 A g Figure 14 ), the specific capacity at the fourth cycle is 462.6 mAh g -1 , and the specific capacity after 100 cycles is 442 mAh g -1 ( -1 ( Figure 12 d). It is better than other shells. Even at a high current density of 5 A g -1 , 259.4 mAh g -1 (83.68%) is still retained after 500 cycles ( Figure 12 g). In addition, as shown in Figure 12 e, all Fe - M - HoMS materials exhibit excellent rate performance. In particular, Fe - M - HoMS - Q has a specific capacity as high as 498.9 mAh g -1 at 0.1 A g -1 , and at an ultra - high current density of 30 A g -1 , the specific capacity is 213.3 mAh g -1 . When the current density is restored to 0.1 A g -1 , a specific capacity of 417.1 mAh g -1 is restored, showing excellent rate performance, which is better than most reported MoS2 - based anode materials ( Figure 12 f). Therefore, Fe - M - HoMS - Q has a short Na + diffusion path and excellent rate performance. In addition, due to the unique HoMS structure, Fe - M - HoMS - Q has the characteristic of buffering volume expansion and excellent cycle stability.

[0073] Figure 12 To further explore the practical value of Fe - M - HoMS - Q, a full cell of Fe - M - HoMS - Q / / NVP@C was assembled with the self - made NVP@C as the cathode ( Figure 12 h). The charge - discharge rate performance of the battery was examined by CV tests at different scan rates and constant current charge - discharge tests at different current densities. As shown in Figure 12 i, as the scan rate increases from 0.1 mV s -1 to 0.3 mV s-1 , the slight change in the peak position proves excellent rate performance. In addition, at a current density of 0.05 A g -1 , the specific capacity of the cathode reaches 129.4 mAh g -1 ( Figure 12 j), at a high current density of 2 A g -1 , the specific capacity still remains 55.0 mAh g -1 . In particular, when the current density returns to 0.05 A g -1 , it can still reach 128.4 mAh g -1 , which means excellent rate performance.

[0074] In summary, Fe-M-HoMS-Q, as the anode material for sodium-ion batteries, has a high specific capacity, excellent cycle stability, and rate performance.

Claims

1. A preparation method of a negative electrode material for a sodium ion battery, characterized in that It includes the following steps: Step S1: Sulfurize Fe2(MoO4)3 hollow single-shell or multi-shell spheres with S powder in an inert atmosphere at a weight ratio of 1:4 - 6 at 330 - 370 °C to obtain FeS2 / MoS2 hollow single-shell or multi-shell spheres; Step S2: Place the FeS2 / MoS2 hollow single-shell or multi-shell spheres in a ferric ion solution for reaction, so that FeS2 undergoes a chemical reaction with ferric ions to remove FeS2, separate the precipitate and calcine it in an inert atmosphere at 800 °C to obtain a sodium-ion battery anode material of Fe single-atom doped porous MoS2 hollow single-shell or multi-shell spheres; It also includes step S0: The preparation steps of the Fe2(MoO4)3 hollow single-shell or multi-shell spheres are as follows: Dissolve (NH4)2MoO4·4H2O, citric acid and FeCl3·6H2O in water or a mixed solvent of water and ethanol to obtain a solution, then add carbon spheres to the solution and adsorb metal ions for several hours, separate the pre-polymer and calcine the pre-polymer in a muffle furnace at 440 - 460 °C to obtain Fe2(MoO4)3 hollow single-shell or multi-shell spheres, and the number of shell layers of the Fe2(MoO4)3 hollow single-shell or multi-shell spheres is adjusted by the volume ratio of water to ethanol in the solution and the time and temperature of adsorbing metal ions; The weight ratio of (NH4)2MoO4·4H2O, citric acid and FeCl3·6H2O is 2.9 - 3:1.8 - 1.9:8.2 - 8.4, the adsorption is carried out at a temperature of 30 - 50 °C, and the solution uses water or a mixed solvent with a volume ratio of water to ethanol of 3:

1.

2. The preparation method of the sodium-ion battery anode material according to claim 1, wherein: In step S2, the ferric ion solution is an Fe2(SO4)3 solution.

3. The preparation method of the sodium-ion battery anode material according to claim 1, wherein: The inert atmosphere is nitrogen or argon.

4. A negative electrode material for a sodium-ion battery, characterized in that Prepared by the preparation method according to any one of claims 1 - 3.

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