A method for preparing a few-layer molybdenum disulfide / reduced graphene composite material for sodium ion batteries based on crystallization heat treatment

Abstract

The application discloses a method for preparing a few-layer molybdenum disulfide / reduced graphene composite material for a sodium ion battery based on crystallization heat treatment and belongs to the field of sodium ion batteries. The application aims at solving the problems of the existing few-layer MoS2-based sodium ion battery electrode material preparation process, such as complexity, finished product transfer difficulty and uneven crystal quality. In the application, lower solvent heat temperature, solvent being ethylene glycol or isopropyl alcohol and high molybdenum-sulfur ratio are adopted to prepare amorphous MoS2 / rGO as a precursor, and then the amorphous MoS2 / rGO is vacuum sealed in a quartz tube and subjected to vacuum high-temperature crystallization heat treatment operation, so that the few-layer MoS2 / rGO composite material with high crystal purity, uniform quality and fewer layers is obtained at a proper heating rate. The MoS2 obtained by the method has very few layers, which is beneficial to exposing more active sites, promoting fast ion transfer, shortening the sodium ion diffusion path, enhancing the reaction kinetics and maintaining the structural integrity.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion batteries, and specifically relates to a method for preparing a few-layer molybdenum disulfide / reduced graphene composite material for sodium-ion batteries based on crystallization heat treatment. Background Technology

[0002] Molybdenum disulfide (MoS2) has become a typical anode material for sodium-ion batteries due to its unique layered structure, high theoretical specific capacity (670 mAh / g), and large interlayer spacing (0.62 nm). However, multilayer MoS2, with its limited reaction sites, fixed ion transport pathways, and slow redox kinetics, is prone to aggregation of MoS2 nanosheets and instability of the electrode structure during long-term charge-discharge processes, even pulverization of the electrode material, ultimately leading to rapid capacity decay. Currently, the main measures to improve the limited number of active sites and fixed ion transport pathways in multilayer MoS2 include heteroatom doping, defect engineering, interlayer spacing engineering, and the preparation of few-layer structures. Among these, the preparation of few-layer MoS2 has attracted widespread attention in recent years, mainly due to its advantages of large exposed active sites, effectively shortening the diffusion distance of sodium ions, thus facilitating sodium insertion and extraction, ultimately achieving high discharge specific capacity and cycle stability. Furthermore, this few-layer material not only shows great application potential in electrochemical energy storage but also has broad application prospects in electronics, sensing, and catalysis.

[0003] However, existing methods for preparing few-layer MoS2 mainly include mechanical exfoliation, vapor deposition, and electrochemical synthesis. However, these strategies have the following main problems: 1) These methods have disadvantages such as requiring additional post-processing or cumbersome preparation processes, making it difficult to balance cost and large-scale preparation; 2) The size, number of layers, and position of the finished product are highly random, and it is difficult to transfer the finished product to a specific location or device; 3) The crystal quality is not uniform, and the impurities introduced during the preparation process are difficult to separate and purify at the end. Summary of the Invention

[0004] The present invention aims to solve the technical problems of complex preparation process, difficult transfer of finished product and non-uniform crystal quality of existing few-layer MoS2-based sodium-ion battery electrode materials, and provides a method for preparing a few-layer molybdenum disulfide / reduced graphene composite material for sodium-ion batteries based on crystallization heat treatment.

[0005] To address the prominent issues of existing MoS2-based electrode materials, which are mostly multilayered structures and suffer from complex preparation processes, the need for additional post-processing, and inconsistent crystal quality, this invention proposes a solvothermal-assisted high-temperature crystallization heat treatment synergistic control strategy. A fully amorphous MoS2 / rGO precursor is prepared in a low-temperature solvothermal stage, and a vacuum-sealed system is further introduced to effectively suppress sulfur volatilization during subsequent high-temperature heat treatment. This allows for precise control of MoS2 grain growth and nucleus repair rates, ultimately achieving simultaneous optimization of the microstructure while maintaining the few-layered MoS2 structure. Compared with existing methods, this composite strategy combining low-temperature solvothermal and high-temperature crystallization heat treatment offers significant advantages such as simplified operation steps, shorter preparation time, and stronger process controllability. It provides a general approach for the structural engineering of transition metal sulfides / layered materials and is expected to offer new insights and guidance for the development and application of electrode materials based on the structural control and design of layered materials.

[0006] To address the aforementioned technical problems, the present invention adopts the following technical solution: The purpose of this invention is to provide a method for preparing a few-layer molybdenum disulfide / reduced graphene composite material for sodium-ion batteries based on crystallization heat treatment, comprising the following steps: Step 1: Add graphene oxide to ethylene glycol or isopropanol, sonicate at low temperature, and stir at room temperature until homogeneous to obtain graphene oxide dispersion. Step 2: Then add the sulfur source and molybdenum source, place in a water bath environment and heat, stirring continuously until completely dissolved; Step 3: Then carry out a solvothermal reaction. After the reaction is complete, cool and wash with deionized water and anhydrous ethanol at least 3 times in sequence. Dry and grind to obtain amorphous MoS2 / rGO composite material. Step 4: Then place the amorphous MoS2 / rGO composite material inside the quartz tube; Step 5: Evacuate the inside of the quartz tube, then fill it with high-purity argon gas. Repeat this operation 2-3 times, then fill it with high-purity argon gas again, and finally seal the quartz tube. Step 6: Perform crystallization heat treatment, then cool to room temperature by air cooling. After the quartz tube has completely cooled, remove the product from the quartz tube, grind it, and obtain the few-layer molybdenum disulfide / reduced graphene composite material.

[0007] Further specifying, in step 1, the ultrasonic treatment is performed at 0-20℃, with an ultrasonic power of 500-700 W, an ultrasonic frequency of 30-60 kHz, and stirring at a speed of 400-700 rpm.

[0008] Further specifying, in step 2, the sulfur source is sodium sulfide or thiourea; the molybdenum source is ammonium molybdate or sodium molybdate; the water bath heating temperature is 40-70℃; and the magnetic stirring speed is 600-800 rpm.

[0009] Further specifying, in step 1, the concentration of the graphene oxide dispersion is 1-4 mg / ml.

[0010] Further specifying, in step 1, the molar ratio of sulfur source to molybdenum source is 10:1-20:1, and the ratio of the mass of sulfur source to the volume of graphene oxide dispersion is (0.01-0.02) mol: (50-70) ml.

[0011] Further specifying, in step 3, a solvothermal reaction is carried out at 150-160℃, and the powder is ground to a particle size of 50-100 micrometers.

[0012] Furthermore, the diameter of the quartz tube is specified as 12-18 mm.

[0013] Further specifying the method for pretreatment of quartz tubes, the following steps are taken: first, rinse with tap water 3-5 times, then sonicate with deionized water for 5-10 minutes, then immerse in a 5% dilute nitric acid solution for 30 minutes of sonic cleaning, then continue to place the quartz tube in dilute nitric acid and let it stand overnight, then take it out and rinse with distilled water 3-5 times, then place it in a forced-air drying oven and dry it at 80℃ for 48 hours.

[0014] Further specifying, in step 5, the vacuum is evacuated to 1~5×10 -4 Pa.

[0015] Further specifying, in step 6, the crystallization heat treatment involves heating from room temperature to 800°C at a rate of 5-15°C / min and holding at that temperature for 5-15 hours.

[0016] Another object of the present invention is to provide a few-layer molybdenum disulfide / reduced graphene composite material prepared by any of the above methods.

[0017] Based on specific implementation methods, the composite preparation strategy of few-layer MoS2 in this invention is carried out according to the following steps: Step 1: Preparation of amorphous MoS2 / rGO: 1. Pretreatment of graphene oxide dispersion: (1) The graphene oxide powder was subjected to low-temperature ultrasonication in an ethylene glycol or isopropanol dispersion to ensure uniform distribution of the graphene oxide in the ethylene glycol or isopropanol dispersion. The final concentration of the graphene oxide dispersion was 1-4 mg / ml, the total volume of the solution was 25-100 ml, the ultrasonic temperature was 0-20℃, the ultrasonic time was 5-10 min, the ultrasonic power was 500-700 W, and the ultrasonic frequency was 30-60 kHz. (2) Disperse the ultrasonic graphene oxide at room temperature and stir for 10-20 h to ensure that the graphene oxide is uniformly dispersed in the dispersion at a speed of 400-700 rpm.

[0018] 2. Low-temperature solvothermal treatment of amorphous MoS2 / rGO precursor: (1) Add the sulfur source (sodium sulfide, thiourea, etc.) and the molybdenum source (ammonium molybdate, sodium molybdate, etc.) to the graphene oxide dispersion in ethylene glycol or isopropanol after thorough stirring according to the molar mass ratio. Heat in a water bath and stir magnetically until completely dissolved. The molar ratio of sulfur source to molybdenum source is 10:1-20:1, the water bath heating temperature is 40-70℃, the magnetic stirring speed is 600-800 rpm, and the volume of graphene oxide dispersion is 50-70 ml; (2) Transfer the graphene oxide ethylene glycol or isopropanol dispersion containing sulfur source and molybdenum source to a polytetrafluoroethylene-lined hydrothermal reactor, seal it, and place it in a forced-air drying oven at 150-160℃ for 18-22 h.

[0019] Step 2: Cleaning and drying of amorphous MoS2 / rGO: (1) Take the gray-black columnar product out of the completely cooled hydrothermal reactor and wash it 3-5 times with deionized water and anhydrous ethanol to remove the sulfur source, molybdenum source and excess graphene oxide dispersion that did not react completely during the solvothermal process. (2) The washed product is placed in a forced-air drying oven and dried for 20-30 h at a temperature of 50-80℃. The dried product is the amorphous MoS2 / rGO composite material. MoS2 is in an amorphous state, while rGO maintains the crystalline state of the raw material. Amorphous MoS2 is distributed on the surface of rGO.

[0020] Step 3: Grinding of amorphous MoS2 / rGO: The dried amorphous MoS2 / rGO was placed in a mortar and ground thoroughly for 0.5-1 hour to produce powder particles with a diameter of 50-100 micrometers.

[0021] Step 4: Cleaning, drying, and pretreatment of the quartz tube: Take a quartz tube with a diameter of Φ12-18mm×450mm and a through hole at one end, rinse it with tap water 3-5 times, then sonicate it with deionized water for 5-10 minutes, then immerse it in a 5% dilute nitric acid solution for 30 minutes of sonic cleaning, and then place the quartz tube in dilute nitric acid and let it stand overnight. After taking it out, rinse it with distilled water 3-5 times, and put it in a forced-air drying oven to dry it at 80℃ for 48 hours. Step 5: Vacuum sealing of the amorphous MoS2 / rGO composite material: 0.1-0.3 g of dried amorphous MoS2 / rGO composite material was placed in a quartz glass tube. The quartz tube was evacuated to 1-5 × 10⁻⁴ Pa and then filled with high-purity argon gas at a volume concentration of 80-100 kPa. The evacuation was repeated 2-3 times, with the final filling using high-purity argon gas at a volume concentration of 0-20 kPa. One open end of the quartz tube was then sealed using an oxyacetylene flame. The purpose of the final filling with high-purity argon gas is to protect the material from oxidation.

[0022] Step Six: High-Temperature Crystallization Heat Treatment of Amorphous MoS2 / rGO Composite Material: A sealed quartz tube containing the amorphous MoS2 / rGO composite material was placed in a high-temperature tube furnace for high-temperature crystallization heat treatment. The crystallization heat treatment involved heating from room temperature to 800°C at a heating rate of 5-15°C / min, holding at that temperature for 5-15 hours, and finally cooling to room temperature using air cooling. This process yielded either a few-layer MoS2 / rGO composite material (F-MoS2 / rGO) or a multi-layer MoS2 / rGO composite material (P-MoS2 / rGO). Air cooling was used to preserve the 1T metastable phase of the sample.

[0023] Step 7: Post-processing of few-layer MoS2 / rGO composites: The product was removed from a fully cooled quartz tube and ground for 0.5–1 h to ensure uniform particle size, thus obtaining a few-layer MoS2 / rGO composite material that can be used in sodium-ion battery electrodes.

[0024] Compared with the prior art, the present invention has the following beneficial effects: This invention addresses the problems of complex preparation processes, inconsistent product crystal quality, and difficulty in precisely controlling the number of layers in existing few-layer MoS2 preparation methods. It proposes a method using a relatively low solvothermal temperature (160℃), ethylene glycol as the solvent, and a high molybdenum-to-sulfur ratio (10:1) to prepare amorphous MoS2 / rGO as a precursor. The amorphous MoS2 / rGO is then vacuum-sealed within a quartz tube and subjected to vacuum high-temperature crystallization heat treatment. At an appropriate heating rate (10℃ / min), a few-layer MoS2 / rGO composite material with high crystal purity, uniform quality, and a small number of layers is obtained. This solves a series of problems associated with existing preparation methods, such as difficulties in product transfer and complex preparation processes.

[0025] The method of this invention yields a very small number of MoS2 layers; this facilitates the exposure of more active sites, promotes rapid ion transfer, shortens the sodium ion diffusion path, enhances reaction kinetics, and maintains structural integrity.

[0026] For a deeper understanding of the features and technical content of this invention, please refer to the accompanying detailed description and drawings. It should be noted that the drawings are provided for illustrative purposes only and are not intended to limit the scope of the invention. Attached Figure Description

[0027] Figure 1 The image shows the microstructure of Example 1 after high-temperature crystallization heat treatment. Figure 2 The image shows the microstructure of Comparative Example 1 after high-temperature crystallization heat treatment. Figure 3 The XRD patterns of Example 1 and Comparative Example 1 after high-temperature crystallization heat treatment are shown. The vertical lines represent the standard PDF card of MoS2. Figure 4 The images show the Raman spectra of Examples 1 and 1 after high-temperature crystallization heat treatment. Figure 5 This is a high-resolution spectrum obtained after high-temperature crystallization heat treatment in Example 1; Figure 6 This is an AFM scan cross-sectional image obtained after high-temperature crystallization heat treatment in Example 1; Figure 7 The charge-discharge long-cycle spectrum obtained by assembling 2032 coin cells in Example 1 and Comparative Example 1; Figure 8 The AC impedance spectrum is obtained by assembling 2032 coin cells in Example 1 and Comparative Example 1. Detailed Implementation

[0028] The present invention will be described in detail below with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but should not be considered as limiting the present invention. It should be noted that those skilled in the art can make several modifications and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0029] Example 1: The preparation of a few-layer molybdenum disulfide / reduced graphene composite material for sodium-ion batteries based on crystallization heat treatment was carried out according to the following steps: I. Preparation of amorphous precursor MoS2 / rGO 50 ml of ethylene glycol (EG) solvent and 0.1 g of graphene oxide powder were mixed and first subjected to low-temperature sonication at 10 °C for 10 min at a power of 600 W and a frequency of 50 kHz. The mixture was then magnetically stirred at room temperature for 8 h to obtain a stable and homogeneous graphene oxide-ethylene glycol dispersion (GO-EG). Subsequently, 0.5 mmol of ammonium molybdate ((NH4)6Mo7O) was added. 24The ammonium molybdate was stirred at 50°C and 600 rpm for 2 hours to ensure complete dissolution. Then, 10 mmol of thiourea (H₂NCSNH₂) was added to the dispersion, and stirring continued until completely dissolved. The ammonium molybdate and thiourea were added in portions to reduce the formation of the byproduct molybdenum trioxide (MoO₃). Finally, the solution was transferred to a 100 ml polytetrafluoroethylene hydrothermal reactor and placed in a forced-air drying oven for a solvothermal reaction. The temperature was raised to 160°C and held for 20 hours to obtain the amorphous precursor MoS₂ / rGO.

[0030] II. Cleaning and Drying of Amorphous Precursor MoS2 / rGO The grayish-black cylindrical amorphous precursor MoS2 / rGO was soaked in 25% ammonia water for 10 hours, followed by washing three times sequentially with deionized water and anhydrous ethanol. The soaking in ammonia water, washing with deionized water and anhydrous ethanol were to remove byproducts from the solvothermal process. The washed amorphous precursor MoS2 / rGO was placed in a forced-air drying oven, heated to 80℃, and held at that temperature for 30 hours to obtain dried amorphous precursor MoS2 / rGO. In this configuration, MoS2 is in an amorphous state, rGO is in a crystalline state, and MoS2 is attached to rGO.

[0031] III. Grinding of amorphous MoS2 / rGO: The dried amorphous MoS2 / rGO was placed in a mortar and ground thoroughly for 1 hour to produce powder particles with a diameter of 50-100 micrometers.

[0032] IV. Cleaning, Drying and Pretreatment of Quartz Tubes: Take a quartz tube with a diameter of Φ14×450 mm and a hole at one end, rinse it with tap water 3 times, then sonicate it with deionized water for 10 minutes, then immerse it in a 5% dilute nitric acid solution and sonicate it for 30 minutes. Then, place the quartz tube in dilute nitric acid and let it stand overnight. After taking it out, rinse it with distilled water 3 times, and put it in a forced-air drying oven to dry it at 80℃ for 48 hours. V. Vacuum sealing of amorphous MoS2 / rGO composite materials: 0.3 g of dried amorphous MoS2 / rGO composite material was placed in a quartz glass tube, and the quartz tube was evacuated to 1×10⁻⁶. -4 After Pa, high-purity argon gas is introduced, with a volume content of 100 kPa, and then vacuumed to 1×10⁻⁶ kPa. -4 The process is repeated 2-3 times, with the final injection of high-purity argon gas at a volume concentration of 1 kPa. Then, an oxyacetylene flame is used to seal one end of the quartz tube opening. The purpose of this final injection of high-purity argon gas is to protect the material from oxidation.

[0033] VI. High-temperature crystallization heat treatment of amorphous MoS2 / rGO composite materials: A sealed quartz tube containing amorphous MoS2 / rGO composite material was placed in a high-temperature tube furnace for high-temperature crystallization heat treatment. The crystallization heat treatment involved heating from room temperature to 800°C at a heating rate of 10°C / min, holding at that temperature for 10 hours, and finally cooling to room temperature using air cooling. This process yielded either a few-layer MoS2 / rGO composite material (F-MoS2 / rGO) or a multi-layer MoS2 / rGO composite material (P-MoS2 / rGO). Air cooling was used to preserve the 1T metastable phase of the sample.

[0034] VII. Post-processing of few-layer MoS2 / rGO composites: The product was removed from a fully cooled quartz tube and ground for 1 h to make the particle size uniform, thus obtaining a few-layer MoS2 / rGO composite material that can be applied to sodium-ion battery electrodes.

[0035] Comparative Example 1 I. Preparation of amorphous precursor MoS2 / rGO 50 ml of ethylene glycol (EG) solvent and 0.1 g of graphene oxide powder were mixed and first subjected to low-temperature sonication at 10 °C for 10 min at a power of 600 W and a frequency of 50 kHz. The mixture was then magnetically stirred at room temperature for 8 h to obtain a stable and homogeneous graphene oxide-ethylene glycol dispersion (GO-EG). Subsequently, 0.5 mmol of ammonium molybdate ((NH4)6Mo7O) was added. 24 The ammonium molybdate was stirred at 50°C and 600 rpm for 2 hours to ensure complete dissolution. Then, 10 mmol of thiourea (H₂NCSNH₂) was added to the dispersion, and stirring continued until completely dissolved. The ammonium molybdate and thiourea were added in portions to reduce the formation of the byproduct molybdenum trioxide (MoO₃). Finally, the solution was transferred to a 100 ml polytetrafluoroethylene hydrothermal reactor and placed in a forced-air drying oven for a solvothermal reaction. The temperature was raised to 160°C and held for 20 hours to obtain the amorphous precursor MoS₂ / rGO.

[0036] II. Cleaning and Drying of Amorphous Precursor MoS2 / rGO The grayish-black cylindrical amorphous precursor MoS2 / rGO was soaked in 25% ammonia water for 10 hours, followed by washing three times sequentially with deionized water and anhydrous ethanol. The soaking in ammonia water, washing with deionized water and anhydrous ethanol were to remove byproducts from the solvothermal process. The washed amorphous precursor MoS2 / rGO was placed in a forced-air drying oven, heated to 80℃, and held at that temperature for 30 hours to obtain dried amorphous precursor MoS2 / rGO. In this configuration, MoS2 is in an amorphous state, rGO is in a crystalline state, and MoS2 is attached to rGO.

[0037] III. Grinding of amorphous MoS2 / rGO: The dried amorphous MoS2 / rGO was placed in a mortar and ground thoroughly for 1 hour to produce powder particles with a diameter of 50-100 micrometers.

[0038] IV. Cleaning, Drying and Pretreatment of Quartz Tubes: Take a quartz tube with a diameter of Φ14×450 mm and a hole at one end, rinse it with tap water 3 times, then sonicate it with deionized water for 10 minutes, then immerse it in a 5% dilute nitric acid solution and sonicate it for 30 minutes. Then, place the quartz tube in dilute nitric acid and let it stand overnight. After taking it out, rinse it with distilled water 3 times, and put it in a forced-air drying oven to dry it at 80℃ for 48 hours. V. Vacuum sealing of amorphous MoS2 / rGO composite materials: 0.3 g of dried amorphous MoS2 / rGO composite material was placed in a quartz glass tube, and the quartz tube was evacuated to 1×10⁻⁶. -4 After Pa, high-purity argon gas is introduced, with a volume content of 100 kPa, and then vacuumed to 1×10⁻⁶ kPa. -4 The process is repeated 2-3 times, with the final injection of high-purity argon gas at a volume concentration of 1 kPa. Then, an oxyacetylene flame is used to seal one end of the quartz tube opening. The purpose of this final injection of high-purity argon gas is to protect the material from oxidation.

[0039] VI. High-temperature crystallization heat treatment of amorphous MoS2 / rGO composite materials: A sealed quartz tube containing the amorphous MoS2 / rGO composite material was placed in a high-temperature tube furnace for high-temperature crystallization heat treatment. The crystallization heat treatment involved heating from room temperature to 800°C at a heating rate of 5°C / min, holding at that temperature for 10 hours, and finally cooling to room temperature using air cooling. This process yielded the multilayer MoS2 / rGO composite material (P-MoS2 / rGO). The purpose of using air cooling was to preserve the 1T metastable phase of the sample.

[0040] VII. Post-processing of multilayer MoS2 / rGO composite materials: The product was removed from a fully cooled quartz tube and ground for 1 h to make the particle size uniform, thus obtaining a multilayer MoS2 / rGO composite material that can be used in sodium-ion battery electrodes.

[0041] Assemble the 2032 button cell according to the following steps: I. Electrode preparation First, prepare the slurry required for coating. Weigh 0.07 g of the ground few-layer / multi-layer MoS2 / rGO composite material, 0.02 g of conductive carbon (super P), and 0.01 g of PVDF into a 5 ml weighing bottle according to a mass ratio of 7:2:1. Then add 1 ml of NMP (as a solvent) and stir magnetically for 10 hours at 700 rpm to form a viscous and uniformly distributed slurry.

[0042] Clean the aluminum foil. Cut aluminum foil to 15cm in length and 6cm in width, ensuring the surface is wrinkle-free. Wipe the surface of the aluminum foil three times with anhydrous ethanol to ensure it is clean and dust-free.

[0043] Coating. The mixed slurry is evenly coated onto a smooth aluminum foil using a four-sided coater to a thickness of 100 μm.

[0044] Vacuum drying. The coated electrode sheet is placed in a vacuum drying oven, heated to 80°C, and kept at that temperature for 12 hours to obtain a dried electrode sheet.

[0045] Electrode cutting. The dried electrode sheets are cut into small round pieces with a diameter of 12mm using a battery slicer.

[0046] II. Battery Assembly A sodium sheet was used as the counter electrode, and aluminum foil coated with F-MoS2 / rGO (P-MoS2 / rGO) was used as the working electrode. GF / A glass fiber was used as the separator, and 1.0M NaPF6 in EC:DMC+5%FEC was used as the electrolyte. The batteries were assembled in an argon-filled glove box and sealed using a hydraulic button cell sealing machine to form 2032 coin cells. After complete encapsulation, the Na||F-MoS2 / rGO (Na||P-MoS2 / rGO) cells were horizontally stabilized for 12 hours. The water and oxygen content in the glove box was <0.01ppm. Electrochemical performance tests were then conducted.

[0047] Figure 1 These are microscopic images of the few-layer MoS2 / rGO composite material (F-MoS2 / rGO) obtained in Example 1 after high-temperature crystallization heat treatment at an appropriate heating rate (10℃ / min) using this method. Figure 2The images show the microstructure of the comparative example 1 multilayer MoS2 / rGO composite material (P-MoS2 / rGO) obtained after high-temperature crystallization heat treatment at an adjusted heating rate (15℃ / min). The microstructures of the two are not significantly different, both exhibiting irregular granular structures. However, compared to the P-MoS2 / rGO, the P-MoS2 / rGO particles exhibit a "harder" microstructure. This pronounced particle size can lead to poor contact between the electrode material and the electrolyte or current collector during long-term sodium ion insertion and extraction, resulting in a "dead sodium" phenomenon and ultimately rapid capacity decay. In contrast, the few-layer MoS2 / rGO composite material (F-MoS2 / rGO) exhibits a mixed state of flexible lamellar and granular morphologies, which is beneficial for structural stability during cycling.

[0048] Figure 3 The XRD patterns of Example 1 (F-MoS2 / rGO) and Comparative Example 1 (P-MoS2 / rGO) obtained after heat treatment at different heating rates (10 and 15 °C / min) using this method are shown. The positions of the diffraction peaks of P-MoS2 / rGO correspond to the positions of the peaks in the standard card of MoS2, indicating that no other impurity phases were introduced during the preparation of P-MoS2 / rGO, and MoS2 was successfully prepared. In addition, the (002) peak of MoS2 represents the interlayer spacing of adjacent MoS2, but compared with P-MoS2 / rGO, the diffraction peak of the (002) crystal plane representing the interlayer spacing of MoS2 in F-MoS2 / rGO disappears, while other diffraction peaks all correspond to the standard card of MoS2. This initially indicates that the number of MoS2 layers in F-MoS2 / rGO is extremely small. This number of MoS2 layers can provide more active sites and a larger specific surface area, and can promote rapid ion transfer. The disappearance of the (002) crystal plane, representing the interlayer spacing of MoS2, can be attributed to the spatial confinement effect caused by the enclosed, tiny volume within the quartz tube. This confined space limits the total concentration of gaseous molecules, and under high-temperature vacuum conditions, the crystal growth mode tends towards two-dimensional layered growth. When the first layer of MoS2 is formed, due to the inert van der Waals forces connecting the MoS2 layers, the potential barrier for adsorbing new atoms and nucleating on them is high. Under the constraints of raw materials and gas pressure, a few layers of MoS2 are ultimately formed, specifically manifested as the disappearance of the (002) crystal plane representing the layer in the XRD pattern.

[0049] Figure 4 The Raman spectra of Example 1 (F-MoS2 / rGO) and Comparative Example 1 (P-MoS2 / rGO) obtained after heat treatment at different heating rates (10 and 15 °C / min) using this method are shown. Both exhibit A, representing MoS2. 1g and E 2gThe presence of peak 1, along with peaks D and G representing carbon materials, confirms the successful preparation of MoS2 / rGO without the introduction of impurity phases. Furthermore, related studies indicate that A... 1g and E 2g The vibrational mode of peak 1 significantly alters the distance between MoS2 layers, therefore A 1g and E 2g The difference (Δk) between peaks can be used to determine the number of layers in MoS2. The Δk of an ideal monolayer MoS2 is approximately 20 cm⁻¹. -1 The calculated Δk for P-MoS2 / rGO is 22.71 cm. -1 The Δk of F-MoS2 / rGO is 20.82 cm. -1 This further demonstrates the relatively small number of MoS2 layers in F-MoS2 / rGO.

[0050] Figure 5 TEM tests were performed on Example 1 (F-MoS2 / rGO) obtained after heat treatment at an appropriate heating rate (10 °C / min) using this method. Its HRTEM spectrum showed a regular, periodically arranged lattice pattern, demonstrating the high crystallinity of F-MoS2 / rGO. Its interplanar spacing was 0.23 nm, corresponding to the (103) crystal plane of MoS2. Only two stripes representing layer stacking appeared at its edges (within the red box), with an interlayer spacing of 0.62 nm, consistent with the interlayer spacing of MoS2, proving the successful preparation of few-layer MoS2. This few-layer structure exposes more active sites, effectively shortening the ion diffusion path and reducing the migration distance of sodium ions within the material.

[0051] Figure 6AFM testing was performed on Example 1 (F-MoS2 / rGO) obtained after heat treatment at an appropriate heating rate (10 °C / min) using this method. Related studies indicate that the theoretical atomic layer thickness of a single MoS2 layer is approximately 0.6 nm, while the atomic layer thickness of graphene oxide prepared using the Hummers method is approximately 1.0 nm. In the MoS2 / rGO composite, rGO and MoS2 are connected by van der Waals forces, and their interlayer spacing is typically greater than 0.6 nm, meaning the layer thickness of the MoS2 / rGO composite with a single MoS2 layer is approximately 2.2 nm. The AFM scan profile of F-MoS2 / rGO shows an average distance of approximately 2.2 nm from the substrate, further confirming the relatively small number of MoS2 layers in F-MoS2 / rGO. Furthermore, the results of XRD, Raman, and TEM spectroscopy further validated the limited number of MoS2 layers in F-MoS2 / rGO. This F-MoS2 / rGO composite material with few layers of MoS2 not only provides abundant active sites, but also provides a shorter diffusion path for sodium ion transport, which is beneficial for achieving high reversible specific capacity and long cycle life.

[0052] Figure 7To conduct long-term charge-discharge cycle tests on 2032 coin cells assembled with sodium metal as the counter electrode, the P and F-MoS2 / rGO electrode materials obtained by heat treatment at different heating rates (10 and 15 °C / min) using this method were tested. Using sodium metal as the counter electrode, P and F-MoS2 / rGO electrode materials were assembled into Na||F-MoS2 / rGO and Na||P-MoS2 / rGO coin cells. After standing at room temperature for 15 h, long-term charge-discharge cycle tests were conducted on MoS2 / rGO electrode materials with different crystallinities at a current density of 1.0 A / g. Compared with the P-MoS2 / rGO electrode, F-MoS2 / rGO showed the highest initial discharge specific capacity (416.59 mAh / g), and the discharge specific capacity after 300 cycles was 362.39 mAh / g, with a capacity retention of 86.99%, exhibiting excellent cycle stability. After 300 cycles, the discharge specific capacity of P-MoS2 / rGO was 83.82 mAh / g, with a capacity retention of only 28.06%. This is attributed to the fact that the few-layer MoS2 provides a fast ion transport channel and abundant active sites, significantly enhancing reaction kinetics and thus maintaining structural integrity during long-term sodium intercalation and deintercalation, ultimately resulting in the excellent cycling stability of the P-MoS2 / rGO electrode. Furthermore, the microstructure of the electrode material also affects its electrochemical performance. The "harder" microstructure of the P-MoS2 / rGO electrode material leads to greater volume expansion during long-term sodium intercalation and deintercalation, ultimately resulting in rapid capacity decay.

[0053] Figure 8 AC impedance testing was conducted on 2032 coin cells assembled with sodium metal as the counter electrode for Example 1 (F-MoS2 / rGO) and Comparative Example 1 (P-MoS2 / rGO) obtained by heat treatment at different heating rates (10 and 15 °C / min) using this method. AC impedance testing was performed on the assembled and rested Na||F-MoS2 / rGO and Na||P-MoS2 / rGO coin cells, and the results are as follows. Figure 8As shown, the two curves exhibit similar characteristics, both consisting of a semicircle in the high-frequency region and a straight line in the low-frequency region. Charge transfer impedance was calculated for both coin cells. The charge transfer impedance (Rct) of Na||F-MoS2 / rGO was 160.1 Ω, while that of Na||P-MoS2 / rGO was 409.4 Ω. Furthermore, the slope of the F-MoS2 / rGO electrode is steeper, indicating that F-MoS2 / rGO has faster ion transport kinetics. This significant difference is because, compared to multilayered MoS2, the fewer layers of MoS2 in F-MoS2 / rGO have shorter ion diffusion pathways, thus effectively reducing the charge transfer resistance.

[0054] The specific embodiments of the present invention have been described in detail above. It should be noted that the present invention is not limited to the specific embodiments described above. Various modifications or alterations can be made by those skilled in the art without departing from the scope of protection defined by the claims, and all such modifications or alterations fall within the scope of the present invention.

Claims

1. A method for preparing few-layer molybdenum disulfide / reduced graphene composite materials for sodium-ion batteries based on crystallization heat treatment, characterized in that, Includes the following steps: Step 1: Add graphene oxide to ethylene glycol or isopropanol, sonicate at low temperature, and stir at room temperature until homogeneous to obtain graphene oxide dispersion. Step 2: Then add the sulfur source and molybdenum source, place in a water bath environment and heat, stirring continuously until completely dissolved; Step 3: Then carry out a solvothermal reaction. After the reaction is complete, cool and wash with deionized water and anhydrous ethanol at least 3 times in sequence. Dry and grind to obtain amorphous MoS2 / rGO composite material. Step 4: Then place the amorphous MoS2 / rGO composite material into the pretreated quartz tube; Step 5: Evacuate the inside of the quartz tube, then fill it with high-purity argon gas. Repeat this operation 2-3 times, then fill it with high-purity argon gas again, and finally seal the quartz tube. Step 6: Perform crystallization heat treatment, then cool to room temperature by air cooling. After the quartz tube has completely cooled, remove the product from the quartz tube, grind it, and obtain the few-layer molybdenum disulfide / reduced graphene composite material.

2. The method according to claim 1, characterized in that, In step 1, the ultrasonic waves are set at 0-20℃ with an ultrasonic power of 500-700W and an ultrasonic frequency of 30-60 kHz, and stirred at a speed of 400-700 rpm.

3. The method according to claim 1, characterized in that, In step 2, the sulfur source is sodium sulfide or thiourea; the molybdenum source is ammonium molybdate or sodium molybdate; the water bath heating temperature is 40-70℃; and the magnetic stirring speed is 600-800 rpm.

4. The method according to claim 1, characterized in that, In step 1, the concentration of the graphene oxide dispersion is 1-4 mg / ml.

5. The method according to claim 4, characterized in that, In step 1, the molar ratio of sulfur source to molybdenum source is 10:1-20:1, and the ratio of the mass of sulfur source to the volume of graphene oxide dispersion is (0.01-0.02) mol: (50-70) ml.

6. The method according to claim 1, characterized in that, In step 3, a solvothermal reaction is carried out at 150-160℃, and the powder is ground to a particle size of 50-100 micrometers.

7. The method according to claim 1, characterized in that, In step 4, the diameter of the quartz tube is 12-18 mm. The pretreatment method of the quartz tube is as follows: first rinse with tap water 3-5 times, then sonicate with deionized water for 5-10 minutes, then immerse in a 5% dilute nitric acid solution for ultrasonic cleaning for 30 minutes, then continue to place the quartz tube in dilute nitric acid and let it stand overnight. After taking it out, rinse it with distilled water 3-5 times, and put it in a forced-air drying oven to dry at 80℃ for 48 hours.

8. The method according to claim 1, characterized in that, In step 5, a vacuum is drawn to 1~5×10⁻⁶. -4 Pa.

9. The method according to claim 1, characterized in that, In step 6, the crystallization heat treatment involves heating from room temperature to 800°C at a rate of 5-15°C / min and holding at that temperature for 5-15 hours.

10. A few-layer molybdenum disulfide / reduced graphene composite material prepared by the method of any one of claims 1-9.

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