Method for preparing molybdenum dioxide-molybdenum carbide heterostructure material through in-situ carbonization method and application of molybdenum dioxide-molybdenum carbide heterostructure material

Abstract

The invention discloses a method for preparing a molybdenum dioxide-molybdenum carbide heterostructure material through an in-situ carbonization method and application of the molybdenum dioxide-molybdenum carbide heterostructure material. A molybdenum source and dopamine hydrochloride are self-assembled in a trihydroxymethyl aminomethane buffer solution with the pH being 8.5 to form a precursor, after centrifugation, cleaning and drying, gradient heating carbonization is conducted in an inert atmosphere, and the molybdenum dioxide-molybdenum carbide heterostructure material is obtained. Molybdenum dioxide and molybdenum carbide in the obtained material are tightly compounded in a nanocrystalline form to form a hierarchical porous structure, the material can be used as a multifunctional modification layer of a polypropylene (PP) diaphragm in a lithium-sulfur battery, and the specific structure and interface electron enhancement effect of the material are utilized to improve the performance of the lithium-sulfur battery. The adsorption and catalytic conversion kinetics of the lithium polysulfide intermediate can be effectively improved, so that the comprehensive electrochemical performance of the lithium-sulfur battery is improved. The method is simple in process and low in cost, and a new strategy is provided for design of a high-performance energy storage material.

Description

[0001] The present invention relates to the technical field of energy catalytic materials, in particular to a method for preparing a molybdenum dioxide-molybdenum carbide heterostructure material by an in-situ carbonization method, and using the same in a lithium-sulfur battery. Background Art

[0002] Polar MoO with rutile structure 2 MoO has been widely studied in the field of lithium-ion batteries due to its relatively high lithium storage capacity and high chemical stability. 2 Although it has strong chemical adsorption capacity for lithium polysulfide, its low intrinsic conductivity will slow down the redox kinetics. 2 The long-distance diffusion paths on the surface may lead to the accumulation of intermediates, which in turn may lead to the formation of solid-state Li 2 S 2 / Li 2 The deposition of S hinders the further contact of lithium polysulfide with the polar surface. 2 C ensures a fast electron pathway and acts as a polar fixative to bond the liquid phase lithium polysulfide intermediates to the solid phase S and Li 2 S 2 / Li 2 The transition between MoO and S provides more active sites. 2 , Mo 2 Compared with MoO 2 -Mo 2 C heterostructures not only have faster electrons and Li + conductivity, and presents a lower lithium polysulfide conversion barrier during the oxidation process. Although there are some reports on the synthesis of molybdenum dioxide-molybdenum carbide composite materials (patent number: CN109499592 A), these methods require relatively complex conditions and it is difficult to easily obtain molybdenum dioxide-molybdenum carbide composite materials. In addition, the existing methods require high-temperature treatment, and the product morphology is closely related to the preparation environment, such as gas type, gas flow rate, reaction temperature, furnace pressure and many other factors. Therefore, it is difficult to accurately control the reaction, the steps are relatively cumbersome, and the efficiency is low. Summary of the invention

[0003] To achieve the above purpose, the present invention provides a method and application of preparing molybdenum dioxide-molybdenum carbide heterostructure material by in-situ carbonization, which solves the above technical problems.

[0004] The present invention provides a method for preparing a molybdenum dioxide-molybdenum carbide heterostructure material by an in-situ carbonization method, comprising the following steps: mixing a molybdenum source and an organic carbon source in a buffer solution to form a precursor composite; and preparing the molybdenum dioxide-molybdenum carbide heterostructure material after heat treatment.

[0005] Furthermore, the method further comprises the following steps: cleaning and drying the precursor composite before heat treatment.

[0006] Furthermore, the molybdenum source is selected from at least one of ammonium molybdate, potassium molybdate and sodium molybdate.

[0007] Furthermore, the organic carbon source is polydopamine hydrochloride.

[0008] Furthermore, the buffer solution is a tris(hydroxymethyl)-aminomethane solution with a pH value of 8.5 and a concentration of 10 mM.

[0009] Furthermore, the mass ratio of the molybdenum source to the organic carbon source is 1:3-1:5.

[0010] Furthermore, the heat treatment temperature is 700-900° C. and the time is 2-3 hours.

[0011] Furthermore, the heat treatment heating rate is 5°C / min.

[0012] Furthermore, the molybdenum dioxide-molybdenum carbide heterostructure material has a layered microsphere morphology and is composed of stacked nanosheets.

[0013] Furthermore, the grain size of the molybdenum dioxide-molybdenum carbide heterostructure material is 3-10 nm.

[0014] Furthermore, the molybdenum dioxide-molybdenum carbide heterostructure material is applied to the separator of lithium-sulfur batteries.

[0015] The method and application of preparing molybdenum dioxide-molybdenum carbide heterostructure material by in-situ carbonization proposed in the present invention have the following beneficial effects:

[0016] 1. The present invention provides a method for conveniently preparing molybdenum dioxide-molybdenum carbide heterostructure materials. The method synthesizes MoO by in-situ carbonization. 2 -Mo 2 C microspheres, which are used as modified diaphragm materials for lithium-sulfur batteries. 4 ) 6 Mo 7 O 24 ·4H 2 O was polymerized in a Tris buffer solution. Dopamine was selected as the carbon precursor because it can chelate and adsorb a variety of metal ions during the self-polymerization process. Then, dopamine was converted into carbon during the annealing process combined with a certain temperature heat treatment, and (NH 4 ) 6 Mo 7 O 24 ·4H 2 O decomposes into MoO at high temperature2 In addition, carbon and MoO 2 The reaction produced Mo 2 C. This method has the advantages of mild reaction conditions, easy scale-up and regulation, and is expected to be used for the industrial preparation of molybdenum dioxide-molybdenum carbide composite materials, which can be applied in the energy field, especially in the field of lithium-sulfur batteries;

[0017] 2. Polydopamine (PDA) is generated by self-polymerization of dopamine hydrochloride in alkaline Tris buffer (pH 8.5). It is rich in amino and hydroxyl functional groups. Mo ions coordinate with the functional groups in PDA to form a molybdenum-polydopamine complex, which provides a uniformly dispersed molybdenum source for subsequent carbonization and reduction; polydopamine is carbonized during high-temperature annealing to generate a nitrogen-doped porous carbon matrix (NC), which improves the conductivity and stability of the material;

[0018] 3. Microstructure Mo 2 C nanoparticles (5-50nm) are uniformly embedded in the nitrogen-doped carbon matrix to form a core-shell structure. The porous carbon matrix provides active site exposure and has a high specific surface area. 2 The catalytic activity of C and the electrical conductivity and chemical stability of NC are combined to synergistically enhance the performance of high-performance molybdenum-based carbon composites, molybdenum dioxide (MoO 2 ) and molybdenum carbide (Mo 2 C) Coexistence to form a heterogeneous interface, improving battery performance through synergistic catalysis and adsorption;

[0019] 4. The present application constructs a layered microsphere structure through self-assembly + in-situ carbonization. The process is simple and does not require a reducing atmosphere. The reaction time is shortened and there is no need to use a cationic surfactant. The nitrogen-doped carbon matrix and the heterogeneous interface of the present application synergistically enhance adsorption and catalysis, which is superior to ordinary physical adsorption. The molybdenum dioxide-molybdenum carbide heterostructure material of the present application is a spherical or quasi-spherical structure with a high specific surface area, a smooth surface, and uniform dispersion. It is suitable for adsorption as a diaphragm modification layer, and is superior to other nanoflower structures that are commonly used in positive electrode materials but are not beneficial as diaphragm materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 For the MoO in Example 1 2 -Mo 2 C.MoO 2 and Mo 2 Cycling performance curves of the battery prepared by C at current densities of 0.5C and 1C;

[0021] Figure 2 (a) is a SEM image of the precursor in Example 1;

[0022] Figure 2 (b) is MoO in Example 12 -Mo 2 SEM image of C.

[0023] Figure 2 (c) is MoO in Example 1 2 -Mo 2 TEM image of C.

[0024] Figure 2 (d) is MoO in Example 1 2 -Mo 2 HRTEM image of C;

[0025] Figure 3 is MoO in Example 1 2 -Mo 2 C.MoO 2 and Mo 2 XRD spectrum of C. DETAILED DESCRIPTION

[0026] The following describes the embodiments of the present invention through specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention.

[0027] Unless otherwise specified, the examples and comparative examples are parallel tests with the same components, component contents, preparation steps and preparation parameters. The experimental methods in the following examples are conventional methods unless otherwise specified. The experimental materials used in the following examples are purchased from commercial channels unless otherwise specified.

[0028] Unless otherwise stated, all reagents were used as received without further purification.

[0029] In the preparation examples and embodiments of the present invention, the “parts” are parts by weight unless otherwise specified, and the concentration percentages are concentrations by weight unless otherwise specified.

[0030] Embodiment 1:

[0031] Step 1: Add 0.1 g of ammonium molybdate and 0.3 g of dopamine hydrochloride into 50 mL of tris(hydroxymethyl)-aminomethane buffer solution (10 mM, pH=8.5). Stir for 12 hours;

[0032] Step 2, centrifuging the above solution to obtain a precursor, washing it with distilled water and ethanol, and then drying it in a vacuum drying oven at 70°C;

[0033] Step 3: The above precursor was heated at 5°C min -1 The annealing was carried out at 800 °C for 3 h at a heating rate of .

[0034] Embodiment 2:

[0035] When other steps remain unchanged, the ammonium molybdate in step 1 can be replaced by potassium molybdate.

[0036] Embodiment 3:

[0037] When other steps remain unchanged, the ammonium molybdate in step 1 can be replaced by sodium molybdate.

[0038] Embodiment 4:

[0039] While other steps remain unchanged, the weight ratio of ammonium molybdate to dopamine hydrochloride in step 1 is replaced with 1:4.

[0040] Embodiment 5:

[0041] When other steps remain unchanged, the heat treatment temperature in step 3 can be set to 700° C. and the time range is 3 hours.

[0042] Embodiment 6:

[0043] When other steps remain unchanged, the heat treatment temperature in step 3 can be set to 900° C. and the time range is 2 hours.

[0044] In terms of application, the material prepared in Example 1 was mixed with CNT and PVDF in NMP at a mass ratio of 7:2:1, and a uniform slurry was formed by vigorous stirring, and then coated on a Celgard 2500 polypropylene (PP) diaphragm, and cut into discs of about 19 mm after vacuum drying; the sulfur positive electrode material (CNT / S) was mixed with Super P and PVDF in NMP at a weight ratio of 8:1:1, stirred to obtain a uniform slurry, and then coated on aluminum foil and vacuum dried overnight, and cut into discs of 12 mm in diameter to form a sulfur electrode. Metallic lithium and the above-mentioned modified diaphragm were used as the negative electrode and diaphragm, respectively. 1.0M LiTFSI in DOL and DME (v / v=1:1) and 1wt% LiNO3 additive were used as electrolytes, and the battery was assembled in an argon-protected glove box with water and oxygen contents below 1ppm.

[0045] Comparative Example 1

[0046] The difference from Example 1 is that dopamine is replaced by polyvinyl pyrrolidone in equal parts by weight, and the other parts are the same.

[0047] Comparative Example 2

[0048] The difference from Example 1 is that the Tris buffer is adjusted to pH=7.0 (weakly acidic), and the other conditions remain unchanged.

[0049] Comparative Example 3

[0050] The difference from Example 1 is that the Tris buffer is missing, and ammonium molybdate and dopamine are directly mixed in deionized water, and the other conditions are the same.

[0051] Performance Testing

[0052] The materials prepared in the examples and comparative examples were mixed with CNT and PVDF in NMP at a mass ratio of 7:2:1, and a uniform slurry was formed by vigorous stirring, and then coated on a Celgard 2500 polypropylene (PP) diaphragm, and cut into discs of about 19 mm after vacuum drying; the sulfur positive electrode material (CNT / S) was mixed with Super P and PVDF in NMP at a weight ratio of 8:1:1, stirred to obtain a uniform slurry, and then coated on aluminum foil and vacuum dried overnight, and cut into discs of 12 mm in diameter to form a sulfur electrode. Metal lithium and the above-mentioned modified diaphragm were used as the negative electrode and diaphragm, respectively. The battery was assembled in an argon-protected glove box with 1.0 M LiTFSI in DOL and DME (v / v=1:1) and 1 wt% LiNO3 additive as the electrolyte, and the following tests were performed: 1. Constant current discharge / charge and cycle stability performance test: The assembled battery was tested using a LANDCT2001A system (China) in different groups (1C=1675 mA g -1 ) were used for constant current discharge / charge measurements with a voltage window of 1.7-2.8 V. The improved separator showed excellent performance in button cell applications, see Figure 1 As shown in Table 1, after 500 cycles at 0.5C and 1C rates, they can still provide 717 and 506 mAh g, respectively. -1 2. BET specific surface area test: The conditions are liquid nitrogen temperature (-196°C), relative pressure (P / P 0 ) range 0.05-0.3 (BET calculation area), and the BET specific surface area (m 2 / g), see Table 1; 3. The formation mechanism of the catalytic active center of Example 1 of the present invention is analyzed. Through DFT theoretical calculation, the heterogeneous interface adsorption energy of Example 1 is -2.3 eV, and Li 2 S decomposition energy barrier is reduced by 0.15 eV; 4. Polysulfide adsorption experiment is carried out on Example 1 of the present invention. Example 1 has a positive effect on Li 2 S 4 The adsorption amount is 120 mg / g; 5. Use SEM electron microscope to observe the surface morphology and microstructure of the material in Example 1; 6. Perform XRD detection. The XRD results are as follows Figure 3 shown.

[0053] Table 1

[0054]

[0055] As shown in Table 1, Example 1 has a high specific surface area, more active sites, and a higher cycle retention rate; while the material prepared in Comparative Example 1 using polyvinyl pyrrolidone (PVP) as a carbon source and template is a solid microsphere, and the solid structure without nanosheet stratification leads to a low specific surface area; Comparative Example 2 is a disordered nanosheet stack, lacking the tight interlayer bonding of the microsphere structure, and the loose structure leads to poor battery performance; Comparative Example 3 is an irregular block particle, without ordered stratification, resulting in poor specific surface area and cycle retention rate.

[0056] Figure 1 As shown, it is shown that Example 1 has good practical application prospects in the field of Li-S batteries.

[0057] The SEM was used to observe Example 1, and the results are shown in Figure 2 , Figure 2 It can be seen that Figure 2 (a) The scale bar in the SEM image is 10 μm, showing that the precursor is a micron-sized particle with an overall size of about 10 μm. The precursor presents a uniformly dispersed spherical or quasi-spherical structure with a smooth surface (no nanosheet features). Figure 2 (b): The scale in the SEM image is 10 μm, showing that the material still maintains a micron-scale microsphere structure (-10 μm) after carbonization, proving that the heat treatment process did not destroy the overall morphology of the precursor, and the surface roughness of the microspheres increased (comparison Figure 2 (a)), suggesting the formation of internal nanostructures. Figure 2 (d) The interior of the microsphere is composed of Mo 2 C (lattice spacing 0.23nm) and MoO 2 (lattice spacing 0.34nm) nanocrystals, the grain size is about 3-10nm, the nanocrystals are further assembled into layered nanosheets ( Figure 2 (c) shows stacked flakes with a thickness of about 5-20 nm. 2 -Mo 2 C is a 10 μm microsphere aggregate, Mo 2 C / MoO 2 The grain size is 3-10nm.

[0058] The 10 μm microspheres of the precursor retain their macroscopic morphology after carbonization ( Figure 2 (b)), but the interior is reconstructed into 1 μm microspheres ( Figure 2 (c)), further composed of nanosheets and grains ( Figure 2(d)) is consistent with the typical characteristics of the "micrometer-meso-nano" multi-level structure. It can be seen that after carbonization, MoO 2 -Mo 2 The morphology of C material perfectly retains the hierarchical microspheres, and it can be clearly observed that the microspheres are composed of many hierarchical nanosheets.

[0059] The specific embodiments of the present invention are described in detail above, but they are only examples, and the present invention is not limited to the specific embodiments described above. For those skilled in the art, any equivalent modifications and substitutions made to the present invention are also within the scope of the present invention. Therefore, the equalization changes and modifications made without departing from the spirit and scope of the present invention should be included in the scope of the present invention.

Claims

1. A method for preparing molybdenum dioxide-molybdenum carbide heterostructure materials by in-situ carbonization, characterized in that: The following steps are involved: mixing a molybdenum source and an organic carbon source in a buffer solution to form a precursor complex; The precursor composite is heat treated to prepare a molybdenum dioxide-molybdenum carbide heterostructure material.

2. The method according to claim 1, characterized in that The following steps are also included: The precursor composite is cleaned and dried before heat treatment.

3. The method according to claim 1, characterized in that: The molybdenum source is selected from at least one of ammonium molybdate, potassium molybdate and sodium molybdate.

4. The method according to claim 1, characterized in that: The organic carbon source is polydopamine hydrochloride.

5. The method according to claim 1, characterized in that The buffer solution is a tris(hydroxymethyl)-aminomethane solution with a pH value of 8.5 and a concentration of 10 mM.

6. The method according to claim 1, characterized in that The mass ratio of the molybdenum source to the organic carbon source is 1:3-1:

5.

7. The method according to claim 1, characterized in that The heat treatment temperature is 700-900° C. and the time is 2-3 hours.

8. The method according to claim 7, characterized in that The heating rate of the heat treatment is 5°C / min.

9. The method according to claim 1, characterized in that: The molybdenum dioxide-molybdenum carbide heterostructure material has a layered microsphere morphology and is composed of stacked nanosheets; the grain size of the molybdenum dioxide-molybdenum carbide heterostructure material is 3-10nm.

10. The method according to claim 1, characterized in that The molybdenum dioxide-molybdenum carbide heterostructure material is applied to the separator of lithium-sulfur battery.

Images