Li2S / MoS2 / RGO composite material as well as preparation method and application thereof

Abstract

The invention discloses a Li2S / MoS2 / RGO composite material as well as a preparation method and application thereof. The preparation method comprises the following steps: (1) uniformly stirring graphite powder and potassium permanganate to prepare a first mixture; adding an acid solution into the first mixture under the conditions of ice-water bath and continuous stirring, and then sequentially carrying out heating reaction, cooling, adding hydrogen peroxide and water, washing and dialyzing to obtain a graphene oxide dispersion liquid; (2) uniformly stirring a molybdenum source, a sulfur source, a surfactant and water to obtain a second mixture, dropwise adding the graphene oxide dispersion liquid into the second mixture, continuously stirring, and sequentially performing hydrothermal reaction, washing and freeze drying to obtain a MoS2 / RGO composite material; and (3) the MoS2 / RGO composite material and lithium sulfide are subjected to mechanical ball milling and mixing in an inert atmosphere to obtain the Li2S / MoS2 / RGO composite material, the rotating speed of mechanical ball milling is 400-600 revolutions per minute, the time is 6-10 hours, the ball milling treatment is performed for 0.5-1 hour, and the stop time is 5-15 minutes. By adopting the method, the Li2S / MoS2 / RGO composite material with high conductivity and rich active sites can be prepared.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-sulfur batteries, and in particular relates to a Li2S / MoS2 / RGO composite material and a preparation method and application thereof. Background Art

[0002] As the global energy transition accelerates, the development of low-cost, high-energy-density and environmentally friendly sustainable energy technologies has become a strategic priority for all countries. Lithium-sulfur batteries are considered to be the most promising next-generation secondary battery system due to their extremely high theoretical specific energy density (2600 Wh / kg) and environmentally friendly characteristics. However, the practical application of lithium-sulfur batteries still faces multiple technical bottlenecks.

[0003] The active material of the positive electrode of lithium-sulfur batteries, elemental sulfur (S8), is insulating and requires a large amount of conductive additives to improve the efficiency of electron transfer. In addition, during the discharge process, the sulfur electrode will undergo a significant volume expansion (about 180%), reducing from high-density S8 to low-density Li2S, resulting in instability of the electrode structure and shedding of active materials. More seriously, polysulfides (Li2S x , 3≤x≤8) as an intermediate product is easily soluble in aprotic electrolytes, triggering a "shuttle effect" - that is, polysulfides migrate between the positive and negative electrodes and deposit on the surface of lithium metal, resulting in loss of active materials, growth of lithium dendrites and decreased Coulomb efficiency, ultimately leading to rapid decay of battery capacity and shortened cycle life. These problems make it difficult for the energy density and cycle stability of lithium-sulfur batteries to meet the needs of practical applications. In order to break through the limitations of traditional elemental sulfur positive electrodes, lithium sulfide (Li2S) has become a new type of positive electrode material due to its stable crystal structure and controllable volume change. However, lithium sulfide still faces the problem of low conductivity, the need to apply a high overpotential of more than ~3.6 V to activate lithium sulfide during the initial charging process, and the irreversible loss of active materials caused by the dissolution and shuttle effect of polysulfides during the battery cycle.

[0004] Therefore, there is an urgent need for a Li2S / MoS2 / RGO composite material and a preparation method and application thereof to solve the deficiencies of the existing technical problems. Summary of the invention

[0005] In order to solve the problem of low conductivity of lithium sulfide and irreversible loss of active materials caused by dissolution and shuttle effect of polysulfides during battery cycling, the inventors of this application adopted a strategy of confining lithium sulfide in reduced graphene oxide (RGO) and introducing two-dimensional layered transition metal disulfide MoS2. In theory, two-dimensional MoS2 nanosheets have abundant active sites, which can bind soluble polysulfides to the positive electrode side through chemical adsorption to prevent the dissolution of polysulfides; at the same time, the layered structure of two-dimensional MoS2 nanosheets facilitates the rapid transmission of electron ions; and RGO can enhance the electrochemical performance of MoS2 nanosheets due to its excellent conductivity and high charge mobility. At the same time, the large surface area and inherent flexibility of RGO reduce the stacking of MoS2 layers to achieve high active material loading. However, the inventors of this application found in the experimental process that the MoS2 / RGO composite material with high conductivity and abundant active sites cannot be directly obtained by hydrothermal reaction of reduced graphene oxide, molybdenum source and sulfur source and then thermal drying. After repeated studies, the inventors found several key reasons for the poor performance of MoS2 / RGO composite materials: during the traditional thermal drying process, the capillary force generated by the evaporation of the liquid solvent squeezes and stacks the graphene sheets, causing RGO to easily agglomerate, which in turn makes the loading of lithium sulfide uneven and insufficient, thereby reducing the conductivity of the composite material and reducing the active sites; at the same time, MoS2 is also prone to aggregation during the preparation process due to its two-dimensional layered structure, further exacerbating the problem of reduced conductivity and reduced active sites. Therefore, only by solving the agglomeration problem of RGO and MoS2 during the preparation process can a MoS2 / RGO composite material with uniform morphology be obtained, thereby obtaining a Li2S / MoS2 / RGO composite material with high conductivity and abundant active sites.

[0006] In view of the above problems, the purpose of the present invention is to provide a Li2S / MoS2 / RGO composite material, a preparation method and application thereof. The preparation method of the present invention achieves uniform distribution of RGO, MoS2 and Li2S in the composite material by introducing a surfactant into the hydrothermal reaction, freeze-drying after the hydrothermal reaction, controlling the ball milling speed and using intermittent ball milling, and successfully prepares a Li2S / MoS2 / RGO composite material with uniform morphology. The material not only has high electrical conductivity but also is rich in active sites.

[0007] To achieve the above objectives, the first aspect of the present invention provides a method for preparing a Li2S / MoS2 / RGO composite material, comprising the following steps: (1) stirring graphite powder and potassium permanganate to obtain a first mixture; adding an acid solution to the first mixture in an ice-water bath under continuous stirring, and then sequentially performing a temperature reaction, cooling, adding hydrogen peroxide and water, washing, and dialyzing to obtain a graphene oxide dispersion, wherein the acid solution is a mixture of concentrated sulfuric acid and concentrated phosphoric acid; (2) stirring a molybdenum source, a sulfur source, a surfactant and water to obtain a second mixture, then dropping the graphene oxide dispersion into the second mixture and continuously stirring, and then sequentially subjecting the mixture to hydrothermal reaction, washing and freeze drying to obtain a MoS2 / RGO composite material, wherein the surfactant is selected from at least one of polyvinyl pyrrolidone and hexadecyltrimethylammonium bromide; (3) Under an inert atmosphere, the MoS2 / RGO composite material and lithium sulfide are mechanically ball-milled to obtain a Li2S / MoS2 / RGO composite material, wherein the rotation speed of the mechanical ball milling is 400-600 rpm, the time is 6-10 h, and each ball milling treatment is 0.5-1 h and rested for 5-15 min.

[0008] Compared with the prior art, the present invention can guide the uniform growth of MoS2 on the RGO surface and prevent agglomeration by introducing a surfactant (at least one of polyvinyl pyrrolidone and hexadecyl trimethyl ammonium bromide) in the hydrothermal reaction, thereby significantly improving the conductivity and the number of active sites of the MoS2 / RGO composite material. At the same time, freeze-drying technology is used after the hydrothermal reaction to avoid the presence of liquid solvents through direct sublimation of ice crystals (solid → gaseous), eliminate capillary forces, and use porous ice crystal skeletons to physically isolate graphene sheets, weaken the influence of van der Waals forces, maintain the dispersed state of graphene sheets, increase the specific surface area and form a uniform conductive network layer. In addition, the present invention also controls the ball milling speed to 400-600 rpm in step (3) to avoid the problem of excessive mechanical energy due to excessive speed and agglomeration of lithium sulfide powder; and uses intermittent ball milling to avoid heat accumulation and agglomeration of lithium sulfide powder. Therefore, the present invention introduces a surfactant into the hydrothermal reaction, adopts freeze-drying after the hydrothermal reaction, controls the speed of ball milling and adopts intermittent ball milling, thereby achieving uniform distribution of RGO, MoS2 and Li2S in the composite material, and successfully prepares a MoS2 / RGO composite material with uniform morphology. The material not only has high electrical conductivity but also is rich in active sites.

[0009] Specifically, the long-chain molecules of polyvinyl pyrrolidone (PVP) are adsorbed on the surfaces of MoS2 nanosheets and graphene oxide through van der Waals forces and hydrogen bonds, forming a steric barrier layer, which inhibits the stacking and agglomeration of MoS2 nanosheets and ensures that MoS2 is evenly dispersed on RGO. In addition, PVP is mainly adsorbed on the edges or defect sites of graphene oxide, and does not cover the conductive plane of graphene oxide over a large area, so it has little effect on electron transport. Therefore, polyvinyl pyrrolidone can guide MoS2 to grow evenly on the surface of RGO and prevent agglomeration.

[0010] Specifically, hexadecyltrimethylammonium bromide (CTAB) is an amphiphilic molecule composed of a hydrophobic hexadecyl chain and a hydrophilic quaternary ammonium group. The hexadecyl chain of CTAB is adsorbed on the surface of MoS2 nanosheets through hydrophobic action, increasing the spatial distance between MoS2 nanosheets, thereby reducing the van der Waals force and inhibiting agglomeration; at the same time, the hydrophilic quaternary ammonium group of CTAB faces the water phase, and the quaternary ammonium group will ionize in water. The positively charged quaternary ammonium group enhances the dispersibility of MoS2 nanosheets in polar solvent water through electrostatic repulsion, further inhibiting agglomeration. Therefore, CTAB can guide MoS2 to grow uniformly on the surface of RGO and prevent agglomeration.

[0011] Furthermore, the mass ratio of graphite powder to potassium permanganate of the present invention is 1:6-7. The mass ratio of the two can be, but is not limited to, 1:6, 1:6.5, and 1:7. Potassium permanganate, as a strong oxidant, can effectively peel off the graphite layer at an appropriate ratio to form graphene oxide (GO). Too high a dosage of potassium permanganate may lead to excessive oxidation and damage the structure of graphene; while too low a dosage may lead to insufficient oxidation and failure to form high-quality GO; therefore, a mass ratio of graphite powder to potassium permanganate of 1:6-7 is sufficient to fully oxidize the graphite powder without excessively damaging the structure of graphene.

[0012] Furthermore, the volume ratio of the concentrated sulfuric acid to the concentrated phosphoric acid of the present invention is 8 to 10:1. The volume ratio of the two can be, but is not limited to, 8:1, 9:1, or 10:1. Concentrated sulfuric acid is a strong acid that can provide protons (H + ) promotes the oxidation reaction, while concentrated phosphoric acid plays a buffering role, slowing down the reaction rate and preventing the reaction from being too violent and causing structural damage to the graphite. Therefore, the volume ratio of concentrated sulfuric acid to concentrated phosphoric acid is 8~10:1, which can ensure that the acidity of the reaction system is strong enough without destroying the structure of the graphite.

[0013] Furthermore, the concentration of the graphene oxide dispersion obtained in step (1) of the present invention is 4-10 mg / mL. For example, the concentration of the graphene oxide dispersion may be, but is not limited to, 4 mg / mL, 6 mg / mL, 7 mg / mL, 8 mg / mL, 9 mg / mL, and 10 mg / mL.

[0014] Furthermore, the temperature-raising reaction in step (1) of the present invention includes: stirring the reaction at 40-50°C for 10-12h. For example, the temperature of the temperature-raising reaction may be, but not limited to, 40°C, 42°C, 44°C, 46°C, 48°C, and 50°C; the time of the temperature-raising reaction may be, but not limited to, 10h, 10.5h, 11h, 11.5h, and 12h. The temperature range of 40-50°C can effectively promote the oxidation reaction of graphite powder by potassium permanganate to form graphene oxide rich in oxygen-containing functional groups (such as hydroxyl, carboxyl, epoxy, etc.), while avoiding excessively high temperature that causes excessively violent reactions or increased side reactions.

[0015] Furthermore, in step (2) of the present invention, the mass ratio of the molybdenum source, the sulfur source, the graphene oxide dispersion, the surfactant and water is 1~1.4:0.9~1.3:0.4~0.6:0.3~0.5:80~120. Too high a content of the molybdenum source may lead to excessive generation of MoS2. Too high a content of the sulfur source may lead to residual sulfur source affecting the purity of the product. Too high a content of the graphene oxide dispersion leads to excessive RGO, affecting the crystallization quality of MoS2; too low a dosage may lead to uneven growth of MoS2. A moderate amount of surfactant can effectively disperse MoS2. Too high a water content may lead to too low a reaction concentration, affecting the growth rate of MoS2; too low a water content may cause the reaction system to be too dense, affecting the uniformity of MoS2 growth. Therefore, the mass ratio of molybdenum source, sulfur source, graphene oxide dispersion, surfactant and water is 1~1.4:0.9~1.3:0.4~0.6:0.3~0.5:80~120, which can ensure the uniform growth of MoS2 on the RGO surface.

[0016] Furthermore, the molybdenum source of the present invention is ammonium molybdate tetrahydrate, and the sulfur source is thiourea. The combination of ammonium molybdate tetrahydrate and thiourea can achieve uniform release of molybdenum ions and sulfur ions, promote uniform growth of MoS2 on the surface of RGO, and form a MoS2 / RGO composite material with high conductivity and abundant active sites.

[0017] Further, the temperature of the hydrothermal reaction of the present invention is 180-200°C, and the time is 20-24h. The temperature of the hydrothermal reaction may be, but not limited to, 180°C, 190°C, 195°C, and 290°C; the time may be, but not limited to, 20h, 22h, 23h, and 24h. This temperature range can provide sufficient energy to promote the full reaction of the molybdenum source and the sulfur source to generate MoS2 with excellent crystallization quality; at the same time, the high temperature environment also helps MoS2 to nucleate and grow uniformly on the surface of RGO to form a composite material with stable structure. In addition, the reaction time of 20-24h ensures the full crystallization and growth of MoS2, avoiding incomplete crystallization or structural defects caused by insufficient reaction time, thereby obtaining a high-performance MoS2 / RGO composite material.

[0018] Furthermore, the continuous stirring time in step (2) of the present invention is 2.5 to 3.5 hours. The continuous stirring time may be, but is not limited to, 2.5 hours, 3 hours, or 3.5 hours. Preferably, the continuous stirring time is 3 hours. Continuous stirring ensures that the molybdenum source, sulfur source, graphene oxide dispersion, and surfactant are uniformly mixed in the reaction system, avoiding excessively high or low local concentrations.

[0019] Furthermore, the freeze-drying time in step (2) of the present invention is 40 to 60 hours. For example, the freeze-drying time may be, but is not limited to, 40 hours, 45 hours, 50 hours, 55 hours, and 60 hours. Preferably, the freeze-drying time is 48 hours.

[0020] Furthermore, in step (1) of the present invention, the concentration of hydrogen peroxide is 30%; the washing in step (1) includes washing with deionized water; and the deionized water is replaced every 48 hours during dialysis.

[0021] Furthermore, the washing in step (2) of the present invention includes performing multiple centrifugal washings by alternating deionized water and anhydrous ethanol as washing solvents. Deionized water can remove water-soluble impurities, and anhydrous ethanol can remove organic residues. Combined with the efficient separation effect of centrifugal force, it is ensured that the reaction residues and impurities are completely removed, thereby obtaining a high-purity MoS2 / RGO composite material.

[0022] Furthermore, in step (3) of the present invention, the mass ratio of the MoS2 / RGO composite material to lithium sulfide is 1:3-5. The mass ratio of the two can be, but is not limited to, 1:3, 1:4, 1:5. Preferably, the mass ratio of the MoS2 / RGO composite material to lithium sulfide is 1:4. When the mass ratio of the MoS2 / RGO composite material to lithium sulfide is 1:3-5, it can ensure the appropriate loading of lithium sulfide on the MoS2 / RGO composite material, which not only provides sufficient active material lithium sulfide, but also avoids the agglomeration of lithium sulfide caused by excessive loading.

[0023] Correspondingly, the second aspect of the present invention also provides a Li2S / MoS2 / RGO composite material, which is prepared by the preparation method of the Li2S / MoS2 / RGO composite material mentioned above. The morphology of the Li2S / MoS2 / RGO composite material is uniform, and RGO, MoS2 and Li2S are evenly distributed in the composite material. The two-dimensional MoS2 nanosheets have abundant active sites, which can bind soluble polysulfides to the positive electrode side through chemical adsorption to prevent the dissolution of polysulfides; at the same time, the layered structure of the two-dimensional MoS2 nanosheets is conducive to the rapid transmission of electronic ions; and RGO can enhance the electrochemical properties of MoS2 nanosheets due to its excellent conductivity and high charge mobility. Therefore, the Li2S / MoS2 / RGO composite material of the present invention can effectively solve the problem of low conductivity of lithium sulfide, as well as the problem of irreversible loss of active materials caused by the dissolution and shuttle effect of polysulfides during battery cycling.

[0024] Correspondingly, the third aspect of the present invention provides the application of Li2S / MoS2 / RGO composite material, wherein the Li2S / MoS2 / RGO composite material prepared by the above-mentioned preparation method of Li2S / MoS2 / RGO composite material is used as the positive electrode material of lithium-sulfur battery.

[0025] Compared with the existing technology, since the Li2S / MoS2 / RGO composite material not only has high electrical conductivity but also is rich in active sites, using the Li2S / MoS2 / RGO composite material as the positive electrode material of the lithium-sulfur battery can make the lithium-sulfur battery have excellent cycle stability and high coulombic efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a transmission electron microscopy image of RGO in MoS2 / RGO prepared in step (2) of Example 1.

[0027] Figure 2 This is a transmission electron microscopy image of the Li2S / MoS2 / RGO composite material prepared in step (3) of Example 1.

[0028] Figure 3 This is a scanning electron microscope image of MoS2 in MoS2 / RGO prepared in step (2) of Comparative Example 1.

[0029] Figure 4 The long cycle curve of the lithium-sulfur button cell assembled with the Li2S / MoS2 / RGO composite material prepared in Example 1 as the positive electrode material is shown. DETAILED DESCRIPTION

[0030] In order to better illustrate the purpose, technical scheme and beneficial effects of the present invention, the present invention will be further described below in conjunction with specific embodiments. It should be noted that the following implementation method is a further explanation of the present invention and should not be used as a limitation of the present invention.

[0031] Example 1 This embodiment provides a method for preparing a Li2S / MoS2 / RGO composite material, the steps comprising: (1) 3 g of graphite powder and 18 g of potassium permanganate were stirred evenly to obtain a first mixture; 400 mL of acid solution was slowly added to the first mixture under ice water bath and continuous stirring, and then stirred and reacted at 50° C. for 12 h. After cooling to room temperature, 30% hydrogen peroxide and deionized water were added and stirred continuously. After standing, the supernatant was removed, and the mixture was washed with deionized water for multiple times, and then dialyzed with a dialysis bag, and the deionized water was replaced every 48 h to obtain a graphene oxide dispersion, wherein the acid solution was a mixture of concentrated sulfuric acid and concentrated phosphoric acid in a volume ratio of 9:1; (2) 0.5 g of ammonium molybdate tetrahydrate, 0.53 g of thiourea, 0.2 g of polyvinyl pyrrolidone and 40 mL of deionized water were mixed and stirred for 3 h to obtain a second mixture, and then 20 mL of 10 mg / mL graphene oxide dispersion was added dropwise to the second mixture under stirring and stirred for 3 h. The mixture was then hydrothermally reacted at 180 °C for 24 h. After cooling to room temperature, deionized water and anhydrous ethanol were used alternately as washing solvents for multiple centrifugal washings, and freeze-dried for 48 h to obtain a MoS2 / RGO composite material. (3) The MoS2 / RGO composite material was mixed with lithium sulfide in a mass ratio of 1:4, and then ball-milled at 400 rpm for 8 h in a nitrogen atmosphere to obtain the Li2S / MoS2 / RGO composite material, wherein each ball-milling treatment was 0.5 h and rested for 10 min. Example 2 This embodiment provides a method for preparing a Li2S / MoS2 / RGO composite material, the steps comprising: (1) 3 g of graphite powder and 20 g of potassium permanganate are uniformly stirred to obtain a first mixture; 400 mL of acid solution is slowly added to the first mixture under ice water bath and continuous stirring conditions, and then stirred and reacted at 45° C. for 11 hours, and then 30% hydrogen peroxide and deionized water are added and continuously stirred after cooling to room temperature. After standing, the supernatant is removed, and the mixture is washed with deionized water for multiple times, and then dialyzed with a dialysis bag, and the deionized water is replaced every 48 hours to obtain a graphene oxide dispersion, wherein the acid solution is a mixture of concentrated sulfuric acid and concentrated phosphoric acid in a volume ratio of 10:1; (2) 0.5 g of ammonium molybdate tetrahydrate, 0.55 g of thiourea, 0.25 g of polyvinyl pyrrolidone and 45 mL of deionized water were mixed and stirred for 3 h to obtain a second mixture, and then 15 mL of 10 mg / mL graphene oxide dispersion was added dropwise to the second mixture under stirring and continued stirring for 3 h. The mixture was then hydrothermally reacted at 190 °C for 22 h. After cooling to room temperature, deionized water and anhydrous ethanol were used alternately as washing solvents for multiple centrifugal washings, and freeze-dried for 48 h to obtain a MoS2 / RGO composite material. (3) The MoS2 / RGO composite material was mixed with lithium sulfide in a mass ratio of 1:4, and then ball-milled at 500 rpm for 7 h in a nitrogen atmosphere to obtain the Li2S / MoS2 / RGO composite material, wherein each ball-milling treatment was 0.5 h and rested for 10 min. Example 3 This embodiment provides a method for preparing a Li2S / MoS2 / RGO composite material, the steps comprising: (1) 3 g of graphite powder and 21 g of potassium permanganate were uniformly stirred to obtain a first mixture; 400 mL of acid solution was slowly added to the first mixture under ice water bath and continuous stirring, and then stirred and reacted at 42° C. for 10.5 h. After cooling to room temperature, 30% hydrogen peroxide and deionized water were added and continuously stirred. After standing, the supernatant was removed, and the mixture was washed with deionized water for multiple times, and then dialyzed with a dialysis bag, and the deionized water was replaced every 48 h to obtain a graphene oxide dispersion, wherein the acid solution was a mixture of concentrated sulfuric acid and concentrated phosphoric acid in a volume ratio of 8:1; (2) 0.5 g of ammonium molybdate tetrahydrate, 0.6 g of thiourea, 0.28 g of polyvinyl pyrrolidone and 55 mL of deionized water were mixed and stirred for 3 h to obtain a second mixture, and then 22 mL of 10 mg / mL graphene oxide dispersion was added dropwise to the second mixture under stirring and stirred for 3 h. The mixture was then hydrothermally reacted at 205 °C for 20 h. After cooling to room temperature, deionized water and anhydrous ethanol were used alternately as washing solvents for multiple centrifugal washings, and freeze-dried for 50 h to obtain a MoS2 / RGO composite material. (3) The MoS2 / RGO composite material was mixed with lithium sulfide in a mass ratio of 1:4, and then ball-milled at 600 rpm for 6 h in a nitrogen atmosphere to obtain the Li2S / MoS2 / RGO composite material, wherein each ball-milling treatment was 0.5 h and rested for 15 min. Example 4 Example 4 is substantially the same as Example 1, and the only difference between the two is that in Example 4, hexadecyltrimethylammonium bromide is used instead of polyvinylpyrrolidone in Example 1.

[0032] Comparative Example 1 Comparative Example 1 is substantially the same as Example 1, the only difference between the two being that polyvinyl pyrrolidone is removed in Comparative Example 1.

[0033] Comparative Example 2 Comparative Example 2 is substantially the same as Example 1, the only difference between the two being that in Comparative Example 2, thermal drying (drying at 60° C. for 12 h) is used instead of freeze drying for 48 h in Example 1.

[0034] Comparative Example 3 Comparative Example 3 is substantially the same as Example 1, the only difference between the two being that the ball milling speed in step (3) of Comparative Example 3 is 1000 rpm.

[0035] Comparative Example 4 Comparative Example 4 is substantially the same as Example 1, except that in step (3), the MoS2 / RGO composite material and lithium sulfide are mixed in a mass ratio of 1:4, and then the mixture is ball-milled at 400 rpm for 8 h under a nitrogen atmosphere to obtain the Li2S / MoS2 / RGO composite material.

[0036] Comparative Example 5 Comparative Example 5 is substantially the same as Example 1, and the only difference between the two is that in Comparative Example 5, cocamidopropyl betaine (CAPB) is used instead of polyvinyl pyrrolidone in Example 1.

[0037] The RGO in the MoS2 / RGO prepared in step (2) of Example 1 was observed using a transmission electron microscope. The results are as follows: Figure 1 As shown. Figure 1 It can be seen that RGO is in the form of wrinkled thin layers. The overlapping layers and wrinkled structure of RGO can form an interconnected conductive network, which provides a stable structure as a carrier and promotes the transport of electrons.

[0038] The Li2S / MoS2 / RGO composite material prepared in step (3) of Example 1 was observed using a transmission electron microscope. The results are as follows: Figure 2 As shown. Figure 2 It can be seen that RGO can provide abundant sites for the growth of layered MoS2, reducing the stacking and agglomeration of MoS2. Li2S is also uniformly coated by MoS2 and evenly distributed between RGO layers.

[0039] The MoS2 in the MoS2 / RGO prepared in step (3) of Comparative Example 1 was observed using a scanning electron microscope. The results are as follows: Figure 3 As shown. Figure 3 It can be seen that MoS2 has a two-dimensional layered structure. Figure 3 It can also be seen that in the absence of surfactant, MoS2 is prone to aggregate and form a spherical structure.

[0040] The Li2S / MoS2 / RGO composite materials prepared in Examples 1 to 4 and Comparative Examples 1 to 5 were applied to lithium-sulfur batteries according to the following method.

[0041] Li2S / MoS2 / RGO composite material, Super P, and PVDF were mixed in a ball mill at a mass ratio of 8:1:1, and an appropriate amount of N-methylpyrrolidone was added. The mixture was ball milled at 400 rpm for 5 h, with a rest period of 10 min for each ball milling for 0.5 h to obtain a uniformly dispersed slurry, which was coated on carbon paper and dried in a vacuum oven at 60 °C for 12 h to obtain a 16 mm uniformly coated disc positive electrode. The negative electrode was a lithium sheet, the diaphragm was a polypropylene diaphragm, the electrolyte was 1M LiTFSI (lithium bistrifluoromethanesulfonyl imide), 1wt% LiNO3 (lithium nitrate), and DOL (1,3-dioxolane) / DME (1,2-dimethoxyethane) (1:1 Vol%) mixed solution. The 2032 button cell was assembled in a glove box (H2O, O2 < 0.1 ppm) with a Li2S loading of 10 mg / cm 2 .

[0042] The above-mentioned 2032 button cells were placed on a battery tester and charged to 3.5V at a low current density of 0.05C at a constant temperature of 25±2°C for material activation. Then, a charge-discharge long cycle test was performed at a constant current density of 1.0C at 1.5-3.0V. The test results are shown in Table 1. The long cycle curve of the lithium-sulfur button cell assembled with the Li2S / MoS2 / RGO composite material prepared in Example 1 as the positive electrode material is shown in Table 1. Figure 4 As shown, Figure 4 Included are the charge and discharge curves and their corresponding coulombic efficiencies.

[0043] Table 1

[0044] By comparing Example 1 with Comparative Example 1, it can be seen that the lithium-sulfur battery of Example 1 has excellent cycle stability and high coulombic efficiency, which is due to the high conductivity and large number of active sites of the MoS2 / RGO composite material of Example 1. This shows that the introduction of a surfactant (at least one of polyvinyl pyrrolidone and hexadecyltrimethylammonium bromide) in the hydrothermal reaction can guide the uniform growth of MoS2 on the RGO surface and prevent agglomeration, thereby significantly improving the conductivity and number of active sites of the MoS2 / RGO composite material.

[0045] By comparing Example 1 with Comparative Example 2, it can be seen that the lithium-sulfur battery of Example 1 has excellent cycle stability and high coulombic efficiency. This is because freeze-drying technology is used after the hydrothermal reaction to avoid the presence of liquid solvents through direct sublimation of ice crystals (solid → gas), eliminate capillary forces, and use porous ice crystal skeletons to physically isolate graphene sheets, weaken the influence of van der Waals forces, maintain the dispersed state of graphene sheets, increase the specific surface area and form a uniform conductive network layer.

[0046] By comparing Example 1 with Comparative Examples 3-4, it can be seen that the lithium-sulfur battery of Example 1 has better cycle stability and high coulombic efficiency. This indicates that in step (3), the ball milling speed is controlled to 400-600 rpm to avoid the problem of excessive mechanical energy resulting from excessive rotation speed and causing agglomeration of lithium sulfide powder; and intermittent ball milling is used to avoid heat accumulation and the problem of agglomeration and agglomeration of lithium sulfide powder, thereby making Li2S uniformly dispersed and uniformly coated with MoS2, and finally significantly improving the cycle stability and coulombic efficiency of the battery.

[0047] Comparing Example 1 with Comparative Example 5, it can be seen that the lithium-sulfur battery of Example 1 has better cycle stability and high coulombic efficiency, which indicates that not all surfactants can achieve the effects of polyvinyl pyrrolidone and hexadecyltrimethylammonium bromide of the present application. Specifically, cocamidopropyl betaine (CAPB) is a zwitterionic surfactant, and its molecular structure contains both positively charged groups (quaternary ammonium groups) and negatively charged groups (carboxylic acid groups). The positively charged groups (quaternary ammonium groups) of CAPB can be adsorbed on the surface of negatively charged MoS2 nanosheets through electrostatic action, but the charge characteristics of CAPB are greatly affected by pH value, resulting in unstable adsorption on the MoS2 surface. Due to unstable adsorption behavior, CAPB may cause uneven coverage of MoS2 nanosheets on the RGO surface, thereby affecting dispersibility, and ultimately cannot significantly improve the cycle stability and coulombic efficiency of the battery.

[0048] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the essence and scope of the technical solution of the present invention.

Claims

1. A method for preparing a Li2S / MoS2 / RGO composite material, characterized in that the steps include: (1) stirring graphite powder and potassium permanganate to obtain a first mixture; adding an acid solution to the first mixture in an ice-water bath under continuous stirring, and then heating the mixture, cooling, adding hydrogen peroxide and water, washing, and dialyzing to obtain a graphene oxide dispersion, wherein the acid solution is a mixture of concentrated sulfuric acid and concentrated phosphoric acid; (2) stirring a molybdenum source, a sulfur source, a surfactant and water to obtain a second mixture, then dropping the graphene oxide dispersion into the second mixture and continuously stirring, and then sequentially performing hydrothermal reaction, washing and freeze drying to obtain a MoS2 / RGO composite material, wherein the surfactant is selected from at least one of polyvinyl pyrrolidone and hexadecyltrimethylammonium bromide; (3) Under an inert atmosphere, the MoS2 / RGO composite material is mixed with lithium sulfide by mechanical ball milling to obtain a Li2S / MoS2 / RGO composite material, wherein the rotation speed of the mechanical ball milling is 400-600 rpm, the time is 6-10 h, and each ball milling treatment is 0.5-1 h and rested for 5-15 min.

2. The method for preparing the Li2S / MoS2 / RGO composite material according to claim 1, characterized in that: The mass ratio of the graphite powder to the potassium permanganate is 1:6-7, and the volume ratio of the concentrated sulfuric acid to the concentrated phosphoric acid is 8-10:

1.

3. The method for preparing the Li2S / MoS2 / RGO composite material according to claim 1, characterized in that: The concentration of the graphene oxide dispersion obtained in step (1) is 4-10 mg / mL.

4. The method for preparing the Li2S / MoS2 / RGO composite material according to claim 1, characterized in that: The temperature-raising reaction in step (1) comprises: stirring the reaction at 40-50° C. for 10-12 hours.

5. The method for preparing the Li2S / MoS2 / RGO composite material according to claim 1, characterized in that: The mass ratio of the molybdenum source, the sulfur source, the graphene oxide dispersion, the surfactant and the water in step (2) is 1-1.4:0.9-1.3:0.4-0.6:0.3-0.5:80-120.

6. The method for preparing the Li2S / MoS2 / RGO composite material according to claim 1, characterized in that: The molybdenum source is ammonium molybdate tetrahydrate, and the sulfur source is thiourea.

7. The method for preparing the Li2S / MoS2 / RGO composite material according to claim 1, characterized in that: The temperature of the hydrothermal reaction is 180-200° C. and the time is 20-24 hours.

8. The method for preparing the Li2S / MoS2 / RGO composite material according to claim 1, characterized in that , the continuous stirring time in step (2) is 2.5~3.5h; the freeze-drying time is 40~60h.

9. A Li2S / MoS2 / RGO composite material, characterized in that: The Li2S / MoS2 / RGO composite material is prepared by the preparation method of any one of claims 1 to 8.

10. An application of Li2S / MoS2 / RGO composite material, characterized in that: The Li2S / MoS2 / RGO composite material prepared by the preparation method of the Li2S / MoS2 / RGO composite material according to any one of claims 1 to 8 is used as a positive electrode material for a lithium-sulfur battery.

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