Preparation method of MoO3QDs / Zn / NC nano composite material and positive electrode material of lithium-sulfur battery prepared from MoO3QDs / Zn / NC nano composite material

Abstract

The invention relates to a preparation method of a MoO3QDs / Zn / NC nano composite material and a positive electrode material of a lithium-sulfur battery prepared from the MoO3QDs / Zn / NC nano composite material. The preparation method specifically comprises the following steps: synthesizing a dodecahedral precursor ZIF-8 through a room temperature aging method, then carrying out high-temperature calcination in an Ar atmosphere to prepare ZIF-8-derived nitrogen-doped carbon-coated zinc Zn / NC, then taking ammonium molybdate as a molybdenum source, adding polyvinylpyrrolidone as an auxiliary agent, and preparing molybdenum trioxide quantum dots MoO3QDs through a hot melting method. Finally, MoO3QDs are loaded on Zn / NC through a room-temperature stirring method to obtain the MoO3QDs / Zn / NC nano composite material, and the MoO3QDs / Zn / NC nano composite material is applied to a positive electrode material of a lithium-sulfur battery. According to the MoO3QDs / Zn / NC nano composite material prepared by the preparation method disclosed by the invention, the amorphous MoO3QDs have a strong adsorption effect on polysulfide, a high specific surface area and a large number of exposed active sites, the polysulfide is effectively locked through physical confinement of a zinc simple substance in porous carbon, catalytic conversion of the polysulfide is realized, and meanwhile, a nitrogen-doped carbon framework can relieve volume expansion and provide structural support, so that the composite material has a good application prospect. And the overall performance of the lithium-sulfur battery is synergistically improved by multiple functions of adsorption, range limitation and catalysis.

Description

Technical Field

[0001] This invention relates to the field of battery materials technology, and in particular to a method for preparing a nanocomposite material and a cathode material for lithium-sulfur batteries. Background Technology

[0002] In current technologies, the highest energy density of lithium-ion batteries is nearing its limit, but it still cannot meet the increasingly demanding range requirements of emerging electric vehicles, hybrid vehicles, and next-generation portable electronic devices. To achieve higher energy densities, advanced battery systems utilizing new electrochemistry and novel cathode materials have received widespread attention in recent years.

[0003] Lithium-sulfur (Li-S) batteries, using abundant elemental sulfur as the cathode material, have become one of the most promising candidates for next-generation energy storage devices due to their high theoretical energy density and low cost. Unlike intercalation electrochemistry, Li-S batteries involve multi-electron transfer electrochemistry, based on S₈⁺¹⁶Li. + + 16e - The conversion reaction of →8Li₂S has a theoretical specific capacity of 1675 mAh / g, with a discharge plateau of approximately 2.3 V (sulfur converts to long-chain polysulfides) and 2.1 V (long-chain polysulfides convert to short-chain polysulfides). The theoretical specific energy density of lithium-sulfur batteries is 2567 Wh / kg. However, the development of lithium-sulfur batteries is hindered by the following factors:

[0004] (1) Sulfur and the discharge product lithium sulfide have low electrical conductivity; (2) During charging and discharging, sulfur is converted into lithium polysulfides (LiPSs), resulting in a significant volume change (expansion of approximately 80%); (3) Most importantly, soluble lithium polysulfides (Li2S) are present during charging and discharging. x The (4 ≤ x ≤ 8) shuttle effect ultimately leads to the loss of active materials and instability of the positive electrode. Summary of the Invention

[0005] This invention addresses the problems of low conductivity and unstable structure in existing lithium-sulfur battery cathode materials by providing a low-cost method for preparing MoO3QDs / Zn / NC nanocomposite materials. This provides a nitrogen-doped carbon matrix material with nitrogen-loaded amorphous molybdenum trioxide quantum dots and good electrochemical performance, which has high conductivity and good electrochemical performance.

[0006] The objective of this invention is achieved as follows: a method for preparing MoO3QDs / Zn / NC nanocomposite materials, characterized by the following preparation method: First, using anhydrous zinc acetate or zinc nitrate hexahydrate as the zinc source, a dodecahedral zinc-based zeolite imidazole ester framework material precursor ZIF-8 is synthesized by room temperature aging. Then, ZIF-8 is calcined at high temperature under an Ar atmosphere to obtain nitrogen-doped carbon-coated zinc Zn / NC derived from ZIF-8. Next, using ammonium molybdate tetrahydrate as the molybdenum source, polyvinylpyrrolidone (PVP) is added as an additive to obtain amorphous molybdenum trioxide quantum dots (MoO3QDs) by a solvothermal method. Finally, MoO3QDs are loaded onto Zn / NC by room temperature stirring to obtain the MoO3QDs / Zn / NC nanocomposite material.

[0007] Further, the MoO3QDs / Zn / NC nanocomposite material of the present invention is prepared by the following steps: Step 1, prepare dodecahedral ZIF-8, mix zinc source and dimethylimidazole, then add methanol to make a mixed solution, after magnetic stirring evenly, age at 25°C for 20-24h, then wash with methanol and vacuum dry to obtain dodecahedral ZIF-8; Step 2, prepare Zn / NC, calcine the dodecahedral ZIF-8 obtained in Step 1 at 700-900°C for 2-3h under Ar protective atmosphere to obtain Zn / NC; Step 3, prepare MoO3QDs, disperse ammonium molybdate tetrahydrate in a mixed solution of deionized water and hydrogen peroxide aqueous solution, then add polyvinylpyrrolidone and stir, carry out solvothermal reaction at 150-200°C and 2-5 MPa for 2-6h, then wash the reactants with ethanol and acetone respectively, and vacuum dry to obtain MoO3QDs; (4) Preparation of MoO3QDs / Zn / NC: The Zn / NC obtained in step 2 and the MoO3QDs obtained in step 3 were dispersed in methanol and reacted with magnetic stirring at room temperature for 4-6 hours. The reactants were separated by centrifugation, and the precipitate separated by methanol was then dried under vacuum to obtain MoO3QDs / Zn / NC.

[0008] Furthermore, in step 1, the molar ratio of the zinc source to dimethylimidazole is 1:1-5, and the proportion of anhydrous zinc acetate in the mixed solution after adding methanol is 0.2-0.25 mol / L.

[0009] Furthermore, in step 2, the heating rate of the combustion furnace during calcination is 2-3℃ / min.

[0010] Furthermore, in step 3, the molar ratio of water to hydrogen peroxide in the mixed solution of deionized water and hydrogen peroxide solution is 1-3:1, and the mass concentration of the hydrogen peroxide solution is 30%.

[0011] Furthermore, in step 3, the mass ratio of ammonium molybdate tetrahydrate to polyvinylpyrrolidone is 1:(3~7).

[0012] In step 4, the mass ratio of Zn / NC to MoO3QDs is 1:(0.05 ~ 0.25), and the amount of methanol used is 0.5-0.6 g / L based on the concentration of Zn / NC.

[0013] Furthermore, in step 4, the mass ratio of Zn / NC to MoO3QDs is 1:0.15.

[0014] Furthermore, in steps 1, 3, and 4, the vacuum drying temperature is 60-70℃, and the drying time is 10-12 hours.

[0015] Compared with the prior art, the above-mentioned preparation method of MoO3QDs / Zn / NC of the present invention has the following beneficial effects: (1) The present invention uses ZIF-8 as a template to prepare nitrogen-doped carbon framework nanomaterials with regular structure by high-temperature calcination. Zinc ions are reduced to amorphous elemental zinc at high temperature and are highly dispersed in the form of nanoparticles and coated in the porous carbon matrix. At the same time, amorphous molybdenum trioxide quantum dots prepared by solvothermal method are loaded onto the carbon framework coated with elemental zinc by stirring at room temperature. The large specific surface area and a large number of exposed active sites provide more reaction interfaces, enhance the adsorption of polysulfides. Due to the physical confinement of elemental zinc in the porous carbon, it can more effectively "lock" polysulfides, suppress the shuttle effect, and realize the catalytic conversion of polysulfides. At the same time, the nitrogen-doped carbon framework can alleviate volume expansion and provide structural support, realizing the multifunctional synergistic improvement of the overall performance of lithium-sulfur batteries through "adsorption-confinement-catalysis". (2) Using anhydrous zinc acetate or zinc nitrate hexahydrate as the zinc source and dimethylimidazolium as the organic ligand, uniform dodecahedral ZIF-8 can be prepared by aging at room temperature for 24 h. Compared with the traditional method of synthesizing dihedral ZIF-8, the particle size is moderate and the uniformity is higher, which greatly helps the morphology uniformity of the composite material in the subsequent synthesis process; (3) With the assistance of appropriate polyvinylpyrrolidone, amorphous MoO3QDs were prepared by a simple solvothermal method. Compared with conventional crystalline MoO3QDs, the surface has more defects and unsaturated coordination sites. These abundant active sites can achieve stronger adsorption capacity, thereby effectively inhibiting the shuttle of polysulfides; (4) The raw materials used in the preparation method of this invention are abundant and inexpensive. The preparation process is simple and the synthesis cycle is short, which is suitable for industrial production.

[0016] The present invention also provides a positive electrode material for lithium-sulfur batteries prepared using the MoO3QDs / Zn / NC nanomaterials obtained by the above preparation method. The above MoO3QDs / Zn / NC and sublimed sulfur are mixed at a mass ratio of 3:(6~7) and kept at 155°C for 10~12 h to obtain S / MoO3QDs / Zn / NC. The positive electrode material for lithium-sulfur batteries is then prepared by combining S / MoO3QDs / Zn / NC, conductive carbon black and PVDF binder. The mass ratio of S / MoO3QDs / Zn / NC, conductive carbon black and PVDF binder is (8~10):1:1.

[0017] In the above-mentioned cathode material of lithium-sulfur battery, the adsorption of polysulfides by amorphous molybdenum trioxide quantum dots (MoO3QDs / Zn / NC) and nitrogen-doped carbon framework is utilized to load sulfur onto molybdenum trioxide quantum dots / zinc / nitrogen-doped carbon dodecahedrons. The structure is stable, the size is uniform, and the loose and porous carbon framework stores sulfur, effectively adsorbs and converts polysulfides, improves conductivity, and enhances battery performance. Attached Figure Description

[0018] Figure 1 The dodecahedral ZIF-8 prepared for Example 2 ( Figure 1 a) Zn / NC ( Figure 1 b) MoO3QDs / Zn / NC Figure 1 c) and S / MoO3QDs / Zn / NC ( Figure 1 d) Scanning electron microscope image.

[0019] Figure 2 The dodecahedral ZIF-8 prepared for Example 2 ( Figure 2 a) Zn / NC ( Figure 2 b) MoO3QDs / Zn / NC Figure 2 c) and S / MoO3QDs / Zn / NC ( Figure 2 Transmission electron microscopy image of d).

[0020] Figure 3 Transmission electron microscopy image and size distribution histogram of MoO3 QDs prepared in Example 2.

[0021] Figure 4 EDS mapping of the MoO3QDs / Zn / NC composite material prepared in Example 2 ( Figure 3 ab).

[0022] Figure 5 The dodecahedral ZIF-8 and Zn / NC (prepared for Example 2) Figure 4 a), MoO3QDs and MoO3QDs / Zn / NC( Figure 4 b) XRD pattern of the composite material.

[0023] Figure 6 Scanning electron microscope image of the octahedral ZIF-8 prepared for comparison.

[0024] Figure 7 The electrochemical impedance spectroscopy (EIS) spectra of the S / MoO3QDs / Zn / NC composite materials prepared in Examples 1, 2, and 3, and the Zn / NC composite material prepared in the comparative example, are shown in Example 4, which are used as the positive electrodes of the battery.

[0025] Figure 8 The graph shows the long-cycle performance of the S / MoO3QDs / Zn / NC composite material used as the positive electrode in Example 4, after 500 cycles at a current density of 0.1 C (1 C = 1675 mA / g) and 0.5 C. Detailed Implementation

[0026] To better illustrate the technical process and purpose of the present invention, the experimental process of the present invention will be further described in detail below with reference to specific embodiments.

[0027] Example 1 The dodecahedral MoO3QDs / Zn / NC composite material prepared in this example is as follows: (1) Preparation of dodecahedral ZIF-8: First, weigh 0.85 g (4.63 mmol) of anhydrous zinc acetate and 0.38 g (4.63 mmol) of dimethylimidazole and mix them evenly in a beaker. Then add 200 mL of methanol and stir magnetically for 5 min to mix evenly. Place it at 25℃ for 24 h of aging. After the reaction is completed, centrifuge and wash the precipitate with methanol 3 times. Place the sample in a vacuum oven and dry at 60℃ for 12 h to obtain dodecahedral ZIF-8. (2) Preparation of Zn / NC: Place the dodecahedral ZIF-8 sample prepared in (1) in a tube furnace and calcine at 700℃ for 2 h under argon protection at a heating rate of 2℃ / min to obtain Zn / NC. (3) Preparation of MoO3QDs: Add 0.1 g of ammonium molybdate tetrahydrate to 40 mL of methanol. A mixture of 1 mL deionized water and 30% hydrogen peroxide aqueous solution in a 1:1 molar ratio was prepared, and then 0.3 g of polyvinylpyrrolidone was added. The mixture was magnetically stirred for 1 h, and then transferred to a 100 mL stainless steel high-pressure reactor lined with polytetrafluoroethylene. The reaction temperature was 150 °C and the reaction time was 2 h. After the reaction was completed, the product was allowed to cool naturally to room temperature. The precipitate was washed 4 times with anhydrous ethanol and 2 times with acetone. After centrifugation, the sample was vacuum dried at 60 °C to obtain MoO3QDs. (4) Preparation of MoO3QDs / Zn / NC: Take 0.08 g of Zn / NC prepared in step (2) and 0.004 g of MoO3QDs prepared in step (3) into a beaker, then add 150 mL of methanol, stir magnetically for 6 h at room temperature, centrifuge to separate the precipitates after the reaction, wash the precipitates 3 times with methanol, and then place the washed precipitate samples in a vacuum oven at 60 °C for 12 h to obtain the MoO3QDs / Zn / NC composite material.

[0028] Example 2

[0029] The dodecahedral MoO3QDs / Zn / NC composite material prepared in this embodiment is prepared by the following steps: (1) Preparation of dodecahedral ZIF-8: First, weigh 0.85 g (4.63 mmol) of anhydrous zinc acetate and 1.32 g (16.08 mmol) of dimethylimidazole and mix them evenly in a beaker. Then add 200 mL of methanol and stir magnetically for 5 min to mix evenly. Place it at 25℃ for 24 h of aging. After the reaction is completed, centrifuge and wash the precipitate three times with methanol. Place the sample in a vacuum oven and dry at 60℃ for 12 h to obtain dodecahedral ZIF-8. (2) Preparation of Zn / NC: Place 0.10 g of the dodecahedral ZIF-8 sample prepared in (1) in a tube furnace and calcine at 800℃ for 2 h under argon protection at a heating rate of 2℃ / min to obtain Zn / NC. (3) Preparation of MoO3QDs: 0.1 g of ammonium molybdate tetrahydrate was added to a mixed solution of 40 mL of deionized water and 30% hydrogen peroxide aqueous solution in a molar ratio of 2:1, and then 0.4 g of polyvinylpyrrolidone was added. The mixture was magnetically stirred for 1 h and then transferred to a 100 mL stainless steel high-pressure reactor lined with polytetrafluoroethylene. The reaction temperature was 170 °C and the reaction time was 4 h. After the reaction was completed, the product was allowed to cool naturally to room temperature. The precipitate was washed 4 times with anhydrous ethanol and 2 times with acetone. After centrifugation, the sample was vacuum dried at 60 °C to obtain MoO3QDs. (4) Preparation of MoO3QDs / Zn / NC: Weigh 0.08 g (2) and 0.012 g (3) into a beaker, then add 150 mL of methanol, stir magnetically for 6 h at room temperature, centrifuge after the reaction is finished, wash the precipitate with methanol 3 times, place the sample in a vacuum oven and dry at 60℃ for 12 h to obtain MoO3QDs / Zn / NC composite material.

[0030] like Figure 1 The image shown is a scanning electron microscope (SEM) image of the dodecahedral ZIF-8, Zn / NC, MoO3QDs / Zn / NC, and S / MoO3QDs / Zn / NC composite materials prepared in this embodiment. Figure 1 (a) It can be clearly seen that the ZIF-8 precursor has a standard dodecahedral structure, is uniformly dispersed and has a smooth surface, with a diameter of about 400 nm. Figure 1 (b) is a SEM image of the Zn / NC nanomaterial. The image clearly shows that ZIF-8, after high-temperature calcination, transforms into a loose, porous carbon framework while still maintaining a regular dodecahedral structure. From Figure 1 (c) It can be seen that the composite material loaded with amorphous molybdenum trioxide quantum dots does not show significant changes in morphology, and the molybdenum trioxide quantum dots, after being combined with the conductive matrix, become part of the conductive network. From Figure 1(d) It can be seen that the overall structure and morphology of the S / MoO3QDs / Zn / NC composite material obtained after sulfur loading are not damaged, and there are no obvious large sulfur blocks on the outside, which proves that sulfur was successfully loaded into the MoO3QDs / Zn / NC composite material.

[0031] like Figure 2 Transmission electron microscopy (TEM) images of the dodecahedral ZIF-8, Zn / NC, MoO3QDs / Zn / NC, and S / MoO3QDs / Zn / NC composite materials prepared in this embodiment. Figure 2 (a) It can be clearly seen that the prepared dodecahedral ZIF-8 precursor is a solid dodecahedral structure. Figure 2 (b) is a TEM image of the Zn / NC nanomaterial, clearly showing the porous carbon framework structure of Zn / NC, with a smaller dodecahedral particle size. From Figure 2 (c) clearly shows that the MoO3QDs / Zn / NC composite material obtained after being loaded with amorphous molybdenum trioxide quantum dots still perfectly maintains its three-dimensional structure. Figure 2 As can be seen in (d), in the S / MoO3QDs / Zn / NC composite material obtained after sulfur loading, sulfur is loaded into the interior of the material from the porous structure, and there are no obvious large sulfur blocks on the outside, which proves that sulfur was successfully loaded.

[0032] like Figure 3 Transmission electron microscopy (TEM) images and size distribution cubic plots of the MoO3 quantum dots prepared using this embodiment are shown. Typical sizes of molybdenum trioxide quantum dots range from 0.5 to 6 nm. The size distribution histograms show an average lateral size of 3.25 nm, with 80% of the quantum dots falling within the 1.5–4.5 nm range.

[0033] like Figure 4 This is the EDS mapping image of the MoO3QDs / Zn / NC composite material prepared using this embodiment. The elemental distribution in MoO3QDs / Zn / NC was determined by energy-dispersive X-ray spectroscopy (EDS). It can be seen that the outer layer of the carbon framework is N-doped carbon, the interior is encapsulated with zinc, and the surface has small amounts of molybdenum and oxygen, demonstrating the successful loading and uniform distribution of amorphous molybdenum trioxide quantum dots. The absence of lattice diffraction fringes in the overall material confirms the amorphous structure of the molybdenum trioxide quantum dots, elemental zinc, and nitrogen-doped carbon layer.

[0034] Example 3

[0035] The preparation of dodecahedral MoO3QDs / Zn / NC composite material in this embodiment is as follows: (1) Preparation of dodecahedral ZIF-8: First, weigh 0.85 g (4.63 mmol) of anhydrous zinc acetate and 1.90 g (23.14 mmol) of dimethylimidazole and mix them evenly in a beaker. Then add 200 mL of methanol and stir magnetically for 5 min to mix evenly. Place it at 25℃ for aging for 24 h. After the reaction is completed, centrifuge and wash the precipitate with methanol 3 times. Place the sample in a vacuum oven and dry at 60℃ for 12 h to obtain dodecahedral ZIF-8. (2) Preparation of Zn / NC: Place 0.10 g of the dodecahedral ZIF-8 sample prepared in (1) in a tube furnace and calcine at 900℃ for 2 h under argon protection at a heating rate of 2℃ / min to obtain Zn / NC. (3) Preparation of MoO3QDs: 0.1 g of ammonium molybdate tetrahydrate was added to a mixed solution of 40 mL of deionized water and 30% hydrogen peroxide aqueous solution in a molar ratio of 3:1, and then 0.5 g of polyvinylpyrrolidone was added. The mixture was magnetically stirred for 1 h and then transferred to a 100 mL stainless steel high-pressure reactor lined with polytetrafluoroethylene. The reaction temperature was 200 °C and the reaction time was 6 h. After the reaction was completed, the product was allowed to cool naturally to room temperature. The precipitate was washed 4 times with anhydrous ethanol and 2 times with acetone. After centrifugation, the sample was vacuum dried at 60 °C to obtain MoO3QDs. (4) Preparation of MoO3QDs / Zn / NC: Weigh 0.08 g of the product prepared in step (2) and 0.02 g of the product prepared in step (3) into a beaker, then add 150 mL of methanol, stir magnetically for 6 h at room temperature, and centrifuge to separate the precipitate after the reaction is completed. Wash the precipitate with methanol 3 times, and place the washed precipitate sample in a vacuum oven at 60 °C for 12 h to obtain the MoO3QDs / Zn / NC composite material.

[0036] Figure 5 XRD patterns of the dodecahedral ZIF-8, Zn / NC, MoO3QDs, and MoO3QDs / Zn / NC composite materials prepared in Examples 1, 2, and 3. Figure 5 (a) The successful synthesis of the precursors ZIF-8 and Zn / NC was carried out in Example 2. The Zn / NC nanomaterials in the figure correspond to the (101) crystal plane in the standard PDF card of zinc (PDF#00-004-0831). Figure 5(b) Successful preparation of amorphous molybdenum trioxide quantum dots and MoO3QDs / Zn / NC nanocomposite materials. Taking the material of Example 2 as an example, the diffraction peaks of molybdenum trioxide quantum dots at 2θ=23.010° ​​and 2θ=25.014° in the figure correspond to the (011) and (200) crystal planes in the standard PDF card (PDF#00-037-1445).

[0037] Comparative Example 1

[0038] This comparative study investigated the performance of ZIF-8 precursor materials prepared at different aging temperatures: First, 0.85 g (4.63 mmol) of anhydrous zinc acetate and 1.32 g (16.08 mmol) of dimethylimidazole were weighed and mixed evenly in a beaker. Then, 200 mL of methanol was added, and the mixture was stirred magnetically for 5 min to achieve uniform mixing. The mixture was then aged at 15 °C for 24 h. After the reaction was completed, the mixture was centrifuged, and the precipitate was washed three times with methanol. The sample was then dried in a vacuum oven at 60 °C for 12 h to obtain ZIF-8 material.

[0039] like Figure 6 The image shown is a scanning electron microscope (SEM) image of the ZIF-8 precursor material prepared in this comparative example. Figure 6 It is clear that the ZIF-8 precursor is not a standard dodecahedral structure, but an octahedral structure, with some surface damage and a diameter of approximately 1 μm. Compared to the dodecahedral ZIF-8 with a diameter of approximately 400 nm, the particles are more prone to agglomeration, leading to a reduction in effective specific surface area and active sites. Therefore, the effect of aging temperature on the ZIF-8 precursor will ultimately affect the performance of the target product. Furthermore, the inventors conducted aging temperature experiments at 20°C and 30°C under the conditions of the examples, obtaining ZIF-8 structures with intact dodecahedral structures in both cases. Therefore, the simplest determination is that an aging temperature of 20-30°C is the suitable temperature.

[0040] Example 4

[0041] In this embodiment, the MoO3QDs / Zn / NC composite materials prepared in Examples 1, 2, and 3, and the Zn / NC prepared in the comparative example, were used as positive electrodes for lithium-sulfur battery assembly. Specifically, the nanomaterials MoO3QDs / Zn / NC from each example and the comparative example were mixed with sublimed sulfur at a mass ratio of 3:7 and kept at 155°C for 10-12 h to obtain sulfur / molybdenum trioxide quantum dots / zinc / nitrogen-doped carbon (S / MoO3QDs / Zn / NC). Using NMP (N-methylpyrrolidone) as a solvent, the positive electrode material for lithium-sulfur batteries was prepared by composite composition of S / MoO3QDs / Zn / NC, conductive carbon black, and polyvinylidene fluoride (PVDF) binder. The mass ratio of S / MoO3QDs / Zn / NC, conductive carbon black, and PVDF binder was 8:1:1. A slurry was prepared by magnetic stirring for 8 hours. The prepared slurry was then uniformly coated onto copper foil using a coating machine and held at 80℃ for 10 hours. After drying, the slurry was cut into electrode sheets using a slicing machine. The electrode sheets were weighed and directly used as the positive electrode of a lithium-sulfur battery. In an argon-filled glove box, 2032-type coin cells were assembled. The assembly sequence from top to bottom was: negative electrode shell, spring, gasket, lithium sheet, separator, S / MoO3QDs / Zn / NC electrode sheet, and positive electrode shell. The resulting coin cells were further tested using the Xinwei Battery Testing System. 5 The AC impedance spectrum of the test electrode was measured in the frequency range of ~0.01 Hz. The calculation of the charge and discharge current was based on the mass of S / MoO3QDs / Zn / NC. The prepared coin cells were charged and discharged at current densities of 0.1 C and 0.5 C, respectively, with a voltage range of 1.7 ~ 2.8 V. The S / MoO3QDs / Zn / NC cells were tested for 500 cycles at a current density of 0.5 C to examine their cycle stability.

[0042] Figure 7 Electrochemical impedance spectroscopy (EIS) spectra of S / MoO3QDs / Zn / NC composite materials obtained from the nanomaterials in Examples 1, 2, 3, and the comparative example, used as cathodes in lithium-sulfur batteries. In Example 2, the Nyquist curve for the nanomaterial (mass ratio Zn / NC:MoO3QDs = 1:0.15) shows the semicircle and slope line corresponding to the charge transfer resistance and lithium-ion transfer diffusion process, respectively. The charge transfer resistance (Rct) of the semicircle in the high-frequency region of the S / MoO3QDs / Zn / NC composite material is significantly lower than that of the comparative material and the corresponding nanomaterials in other examples, indicating faster charge transfer. Simultaneously, the slope of the S / MoO3QDs / Zn / NC cathode with this ratio is steeper than that of the comparative material in the low-frequency range, indicating a faster lithium-ion diffusion rate.

[0043] Figure 8This is a test graph showing the long-term cycling performance of the S / MoO3QDs / Zn / NC composite material obtained from the nanomaterials in Examples 1, 2, 3, and the comparative examples as the positive electrode of a lithium-sulfur battery in the voltage range of 1.7 ~ 2.8 V. First, after activation at a current density of 0.1 C for 5 cycles, the S / MoO3QDs / Zn / NC composite material corresponding to the electrode in Example 2 provides a high initial capacity of 1188.96 mAh / g. After cycling for 500 cycles at a current density of 0.5 C, the discharge capacity remains at 564.21 mAh / g, and the coulombic efficiency (CE) remains stable at 98.95%, indicating that side reactions are effectively suppressed. Throughout the cycling process, the discharge specific capacity is significantly higher than that of the material electrodes in other examples and the comparative examples, and it exhibits a lower capacity decay rate. This demonstrates that the S / MoO3QDs / Zn / NC composite material of this formulation has good long-term cycling stability and excellent capacity retention.

Claims

1. A preparation method of a MoO3 QDs / Zn / NC nanocomposite, characterized in that, The preparation is carried out through the following methods: firstly, a dodecahedral zinc-based zeolitic imidazolate framework structure material precursor ZIF-8 is synthesized by using anhydrous zinc acetate or zinc nitrate hexahydrate as a zinc source and through a room temperature aging method, then the ZIF-8 is calcined at a high temperature under an Ar atmosphere to prepare ZIF-8 derived nitrogen-doped carbon coated zinc Zn / NC; secondly, amorphous molybdenum trioxide quantum dots (MoO3QDs) are prepared by using ammonium molybdate tetrahydrate as a molybdenum source and adding polyvinylpyrrolidone (PVP) as an assistant through a solvothermal method; finally, the MoO3QDs are loaded onto the Zn / NC through a room temperature stirring method to obtain a MoO3QDs / Zn / NC nanocomposite.

2. The method for preparing MoO3 QDs / Zn / NC nanocomposites according to claim 1, characterized in that, The preparation is carried out through the following steps: Step 1, preparation of dodecahedral ZIF-8, a zinc source and dimethyl imidazole are mixed, then methanol is added to prepare a mixed solution, after being uniformly stirred by a magnetic force, the mixed solution is aged at 20-25℃ for 20-24h, then the dodecahedral ZIF-8 is obtained after being washed with methanol and vacuum dried; Step 2, preparation of Zn / NC, the dodecahedral ZIF-8 prepared in step 1 is calcined at 700-900℃ for 2-3h under an Ar protective atmosphere to obtain Zn / NC; Step 3, preparation of MoO3QDs, ammonium molybdate tetrahydrate is dispersed in a mixed solution of deionized water and hydrogen peroxide aqueous solution, then polyvinylpyrrolidone is added and stirred, a solvothermal reaction is carried out at a temperature of 150-200℃ and a pressure of 2-5Mpa for 2-6h, then the reaction product is washed with ethanol and acetone respectively, and vacuum dried to obtain MoO3QDs; Step 4, preparation of MoO3QDs / Zn / NC, the Zn / NC prepared in step 2 and the MoO3QDs prepared in step 3 are dispersed in methanol, a magnetic force stirring reaction is carried out at room temperature for 4-6h, the reaction product is centrifuged and separated, the precipitated product separated with methanol is obtained, and then vacuum dried to obtain MoO3QDs / Zn / NC.

3. The method for preparing MoO3 QDs / Zn / NC nanocomposites according to claim 2, characterized in that, In step 1, the molar ratio of the zinc source to dimethyl imidazole is 1:1-5, and the proportion of anhydrous zinc acetate in the mixed solution after adding methanol is 0.2-0.25mol / L.

4. The method for preparing MoO3 QDs / Zn / NC nanocomposites according to claim 2, characterized in that, In step 2, the heating rate of the combustion furnace during calcination is 2-3℃ / min. 5.The method for preparing MoO 3 QDs / Zn / NC nanocomposites according to claim 2, characterized in that, In step 3, the molar ratio of water to hydrogen peroxide in the mixed solution of deionized water and hydrogen peroxide aqueous solution is 1-3:1, and the mass concentration of the hydrogen peroxide aqueous solution is 30%.

6. The method for preparing MoO3 QDs / Zn / NC nanocomposites according to claim 5, characterized in that, In step 3, the mass ratio of ammonium molybdate tetrahydrate to polyvinylpyrrolidone is 1:(3-7).

7. The method for preparing MoO3 QDs / Zn / NC nanocomposites according to claim 5, characterized in that, In step 4, the mass ratio of Zn / NC to MoO3QDs is 1:(0.05-0.25), and the amount of methanol is 0.5-0.6g / L based on the concentration of Zn / NC.

8. The method for preparing Mo03 QDs / Zn / NC nanocomposites according to claim 7, characterized in that, In step 4, the mass ratio of Zn / NC to MoO3QDs is 1:0.

15.

9. The method for preparing the MoO3QDs / Zn / NC nanocomposite material according to claim 2, characterized in that, In steps 1, 3 and 4, the temperature for vacuum drying is 60-70℃, and the drying time is 10-12h.

10. A positive electrode material for a lithium-sulfur battery, characterized by comprising: The MoO3 QDs / Zn / NC according to any one of 1-9 is mixed with sublimed sulfur at a mass ratio of 3:(6-7), and then kept at 155 DEG C for 10-12 h to obtain S / MoO3 QDs / Zn / NC; and then the S / MoO3 QDs / Zn / NC, conductive carbon black and PVDF binder are compounded to prepare a positive electrode material of a lithium-sulfur battery, and the mass ratio of the S / MoO3 QDs / Zn / NC, conductive carbon black and PVDF binder is (8-10):1:1.

Images