Cobalt-doped molybdenum disulfide lithium-sulfur battery composite positive electrode material, preparation method and application

Abstract

According to the cobalt-doped molybdenum disulfide lithium-sulfur battery composite positive electrode material, the preparation method and the application, a cobalt-doped stable molybdenum disulfide defect structure composite nitrogen-doped graphene material is prepared by combining a solvothermal method and an annealing technology, and the process is mature; the method is simple, and the composite positive electrode material with high sulfur content is easy to obtain. According to the lithium-sulfur battery composite sulfur positive electrode material disclosed by the invention, the strong chemical adsorption effect and catalytic conversion effect of the cobalt-doped stable molybdenum disulfide defect structure composite nitrogen-doped graphene carrier on polar lithium polysulfide are utilized, so that the diffusion of the lithium polysulfide in an ether electrolyte is inhibited, and the electrochemical reaction kinetics is improved; therefore, the shuttling effect is slowed down, and the lithium-sulfur battery has the characteristics of high capacity and long service life.

Description

Technical Field

[0001] The present invention belongs to the field of new energy electrode materials, and particularly relates to a cobalt-doped molybdenum disulfide lithium-sulfur battery composite cathode material, a preparation method and an application thereof. Background Art

[0002] As a next-generation battery energy storage technology with commercialization prospects, due to its high energy density, low cost and environmental friendliness, lithium-sulfur batteries show great application potential in the fields of power batteries and large-scale energy storage devices. Compared with traditional lithium-ion batteries, the multi-electron transfer mechanism of lithium-sulfur batteries has a higher energy density and can effectively meet the dual requirements of future high-performance energy systems for high energy density and low cost. However, lithium-sulfur batteries face a series of technical challenges during use. Firstly, in ether-based electrolytes, the dissolution-deposition reaction mechanism leads to the dissolution and diffusion of soluble intermediate polysulfides, triggering a serious shuttle effect; the low conductivity of sulfur and the discharge product lithium sulfide results in slow reaction kinetics of the sulfur electrode; the side reaction between the diffused polysulfides and the lithium anode aggravates the consumption of the lithium anode, induces the evolution of dead lithium and dead sulfur products, and further aggravates the loss of active sulfur in the sulfur electrode; in addition, lithium dendrite growth occurs during the charging process of the lithium anode, inducing potential safety hazards in battery cycling. These problems greatly exacerbate the capacity decay and cycle stability of lithium-sulfur batteries. Moreover, lithium-sulfur batteries face the practical requirements of high sulfur load and small amount of electrolyte, and the dissolution and diffusion of polysulfides and the reaction kinetics of the sulfur electrode are even slower. Therefore, enhancing the reaction kinetics of the sulfur electrode and accelerating the reaction kinetics of the sulfur electrode are crucial.

[0003] Due to the complex solid-liquid-solid phase transformation and many consecutive reactions during the electrochemical reaction process of lithium-sulfur batteries, the kinetic transformation of polysulfide intermediates becomes a key factor determining the performance of lithium-sulfur batteries. Usually, regulating the catalytic ability of the sulfur cathode carrier material can effectively improve the redox kinetics of the sulfur electrode. Therefore, it is of great significance to find an efficient catalyst to achieve rapid redox of the sulfur cathode and inhibit the shuttle of polysulfides. Transition metal sulfides have been widely used in the field of energy catalysis due to their high conductivity, nano-size characteristics, high adsorption-catalytic activity for polysulfides and many other advantages.

[0004] In addition, the introduction of defect structures changes the local electronic structure of the material, generating more active sites for adsorbing polysulfide lithium and reducing the free energy of the polysulfide lithium conversion reaction. Defects such as oxygen vacancies and sulfur vacancies will break the crystal periodicity of the material, resulting in a redistribution of the electron density around the atoms. Defects not only enhance the polar interaction (chemical adsorption), but also may optimize the van der Waals force, and the defects can regulate the surface energy barrier to accelerate the conversion of polysulfide lithium, significantly improving the reaction kinetics. It can be said that defect engineering realizes the dual functions of polysulfide lithium anchoring and catalytic conversion by reconstructing the electronic structure, exposing active sites, reducing the reaction energy barrier and optimizing the transport path. At the same time, the breakthrough progress of the single-atom catalytic system reveals the unique catalytic mechanism of transition metals (such as cobalt and nickel) in lithium-sulfur batteries - significantly improving the redox kinetics of polysulfides by exposing highly active sites. Therefore, the regulation of defect structures by doping transition metal sulfides may promote the practical development of lithium-sulfur batteries. Summary of the Invention

[0005] To overcome the above deficiencies, the present invention provides a preparation method of a composite cathode material for a lithium-sulfur battery with a cobalt-doped and stabilized molybdenum disulfide defect structure and its application in a lithium-sulfur battery. By using the catalytic conversion and strong adsorption effects of the cobalt-doped and stabilized molybdenum disulfide defect structure on polysulfide lithium, the dissolution and shuttle of polysulfide lithium in an ether-based electrolyte are slowed down, thereby improving the cycling performance of the lithium-sulfur battery.

[0006] To achieve the above object, the technical solution adopted by the present invention is as follows: A composite cathode material for a lithium-sulfur battery with a cobalt-doped and stabilized molybdenum disulfide defect structure composite nitrogen-doped graphene support, characterized in that it uses a cobalt-doped and stabilized molybdenum disulfide defect structure composite nitrogen-doped graphene as a support, and uniformly loads sulfur.

[0007] A preparation method of a composite cathode material for a lithium-sulfur battery with a cobalt-doped and stabilized molybdenum disulfide defect structure, the specific steps are as follows: Step 1: Dissolve ammonium molybdate in an aqueous polymer solution, add nitrogen-doped graphene and ultrasonically mix, then add a sulfur source, add a cobalt source and stir until a clear mixed solution is obtained. Place the mixed solution in a high-pressure reaction kettle and continuously react in an oven at 200-240 °C for 18-36 hh. Wash the reaction kettle product multiple times, dry it and then anneal it in a tube furnace under a specific atmosphere to obtain a cobalt-doped and stabilized molybdenum disulfide defect structure / nitrogen-doped graphene composite material; Step 2: Grind and mix the composite material with sulfur, and then place the mixture in a reaction kettle under an argon atmosphere again, and obtain a composite cathode material for a lithium-sulfur battery through heat treatment.

[0008] Further optimization, the polymer is polyethylene glycol, its molecular weight is ~20000, and the solution concentration is 15-25 mM.

[0009] Further optimization: the sulfur source is one of L-cysteine, thioacetamide, and thiourea; the cobalt source is one of cobalt nitrate (Co(NO3)2·6H2O, Co(COOH)24H2O).

[0010] Further optimization: the multiple washing method is to perform centrifugal washing with deionized water and absolute ethanol 2 - 4 times respectively.

[0011] Further optimization: the tube furnace annealing treatment under a specific atmosphere is 5 - 10% H2 / Ar or Ar.

[0012] Further optimization: the temperature of the tube furnace annealing treatment under a specific atmosphere is 500 - 700 °C, and the time is 2 - 5 h.

[0013] Further optimization: the mass ratio of the cobalt-doped molybdenum disulfide defect structure / nitrogen-doped graphene and sulfur is 20 - 40:60 - 80.

[0014] Further optimization: the specific conditions of the heat treatment method are to keep warm at 150 - 165 °C for 10 - 20 h under an argon atmosphere.

[0015] Application of the cobalt-doped molybdenum disulfide lithium-sulfur battery composite cathode material in a lithium-sulfur battery.

[0016] The beneficial effects of the present invention are as follows: (1) Using the solvothermal technique to prepare MoS2 nanosheets uniformly distributed and grown on nitrogen-doped graphene and regulate whether Co is doped; combining the high-temperature annealing process (700 °C) under a specific atmosphere to regulate the sulfur defect structure of the material, and then compounding with sulfur by the heat treatment method to obtain the composite sulfur cathode material. The method is simple, the process is mature, and it is easy to regulate the sulfur content of the composite cathode material; (2) The cobalt-doped stable molybdenum disulfide defect structure / nitrogen-doped graphene composite material shows a synergistic improvement in material conductivity, polysulfide anchoring ability, and catalytic ability through Co doping regulation, thereby greatly inhibiting the diffusion of lithium polysulfide in the ether-based electrolyte, improving the reaction kinetics, slowing down the shuttle effect, and then obtaining a lithium-sulfur battery with high specific capacity, long cycle life, and excellent rate performance. Description of the Drawings

[0017] Figure 1 XRD patterns of the molybdenum disulfide-based composite nitrogen-doped graphene composites prepared in Examples 1 - 3; Figure 2 ESR patterns of the molybdenum disulfide-based composite nitrogen-doped graphene composites prepared in Examples 1 - 3; Figure 3 Initial charge-discharge curves of the lithium-sulfur battery composite cathode materials prepared in Examples 1 - 3 at a room temperature 0.2 C rate; Figure 4 Cycling performance graph of the battery assembled with the lithium-sulfur battery composite cathode material prepared in Examples 1 to 3 at room temperature and a 0.2 C rate. Detailed implementation mode

[0018] In order to more clearly understand the above-mentioned objects, features and advantages of the present invention, the present invention will be described in detail below in conjunction with specific embodiments. The following embodiments are implemented on the premise of the technical solution of the present invention, and the detailed implementation methods and specific operation processes are given. However, the present invention can also be implemented in other ways different from those described herein. Therefore, the protection scope of the present invention is not limited to the following embodiments.

[0019] Example 1 A lithium-sulfur battery composite cathode material based on a cobalt-doped stable molybdenum disulfide defect structure composite nitrogen-doped graphene carrier is specifically prepared according to the following steps: Step 1, preparation of cobalt-doped stable molybdenum disulfide defect structure composite nitrogen-doped graphene: First, dissolve 2.7 g of ammonium molybdate (NH₄)₂MoO₄·2H₂O in 60 ml of an aqueous solution containing PEG-20000 (20 mM), stir vigorously for 0.5 h, then ultrasonically disperse 50 mg of commercial nitrogen-doped graphene (N-G) into the above solution, add 1.5 g of L-cysteine, and continuously stir for 0.5 h. Immediately add 0.4 g of cobalt nitrate Co(NO₃)₂·6H₂O and stir until a clear solution is obtained. Finally, transfer the homogeneous solution to a 100 ml stainless steel autoclave and continuously react in an oven at 220 °C for 24 h. Cool to room temperature, collect the product in the autoclave, and centrifuge and wash it 3 times with deionized water and absolute ethanol respectively, and then dry it in an oven at 60 °C. Finally, calcine the dried product in a 10% H₂ / Ar atmosphere at 700 °C for 5 h to obtain the composite material.

[0020] Step 2, composite the cobalt-doped stable molybdenum disulfide defect structure / nitrogen-doped graphene obtained in Step 1 with sulfur: Mix the cobalt-doped stable molybdenum disulfide defect structure composite nitrogen-doped graphene with sulfur in a mass ratio of 28:72, and grind to obtain a mixture of cobalt-doped stable molybdenum disulfide defect structure composite nitrogen-doped graphene and sulfur; put the mixture containing cobalt-doped stable molybdenum disulfide defect structure composite nitrogen-doped graphene and sulfur into the autoclave under an argon atmosphere, and then heat it at 155 °C for 12 h in a high-pressure autoclave sealed with argon to obtain the lithium-sulfur battery composite sulfur cathode material.

[0021] Example 2 A lithium-sulfur battery composite cathode material based on a molybdenum disulfide defect structure composite nitrogen-doped graphene carrier is specifically prepared according to the following steps: Step 1, Preparation of molybdenum disulfide defective structure composite nitrogen-doped graphene support: First, dissolve 2.7 g of ammonium molybdate (NH4)2MoO4·2H2O in 60 ml of an aqueous solution containing PEG-20000 (20 mM), and vigorously stir for 0.5 h. Then, ultrasonically disperse 50 mg of commercial nitrogen-doped graphene (N-G) into the above solution. Immediately afterwards, add 1.5 g of L-cysteine and continuously stir for 0.5 h. Finally, transfer the homogeneous solution to a 100 ml stainless steel autoclave and continuously react in an oven at 220 °C for 24 h. Cool to room temperature, collect the autoclave product, and centrifuge and wash it 3 times with deionized water and anhydrous ethanol respectively, and then dry it in an oven at 60 °C. Finally, calcine the dried product in a 10% H2 / Ar atmosphere at 700 °C for 5 h to obtain the composite material.

[0022] Step 2, Composite sulfur with the molybdenum disulfide defective structure / nitrogen-doped graphene support obtained in Step 1: Mix the molybdenum disulfide defective structure composite nitrogen-doped graphene support and sulfur in a mass ratio of 28:72 respectively, and grind to obtain a mixture of the molybdenum disulfide defective structure composite nitrogen-doped graphene support and sulfur; put the mixture of the molybdenum disulfide defective structure composite nitrogen-doped graphene and sulfur into a reaction kettle under an argon atmosphere, and then heat it at 155 °C in a high-pressure autoclave sealed with argon for 12 h to obtain the composite sulfur cathode material for lithium-sulfur batteries.

[0023] Example 3 A composite cathode material for lithium-sulfur batteries based on a molybdenum disulfide composite nitrogen-doped graphene support is specifically prepared according to the following steps: Step 1, Preparation of molybdenum disulfide composite nitrogen-doped graphene: First, dissolve 2.7 g of ammonium molybdate (NH4)2MoO4·2H2O in 60 ml of an aqueous solution containing PEG-20000 (20 mM), and vigorously stir for 0.5 h. Then, ultrasonically disperse 50 mg of commercial nitrogen-doped graphene (N-G) into the above solution. Immediately afterwards, add 1.5 g of L-cysteine and continuously stir for 0.5 h. Finally, transfer the homogeneous solution to a 100 ml stainless steel autoclave and continuously react in an oven at 220 °C for 24 h. Cool to room temperature, collect the autoclave product, and centrifuge and wash it 3 times with deionized water and anhydrous ethanol respectively, and then dry it in an oven at 60 °C. Finally, calcine the dried product in an Ar atmosphere at 500 °C for 2 h to obtain the composite material.

[0024] Step 2, Composite sulfur with the molybdenum disulfide / nitrogen-doped graphene obtained in Step 1: Mix molybdenum disulfide composite nitrogen-doped graphene and sulfur in a mass ratio of 28:72, and grind to obtain a mixture of molybdenum disulfide composite nitrogen-doped graphene and sulfur; place the mixture containing molybdenum disulfide composite nitrogen-doped graphene and sulfur in a reaction kettle under an argon atmosphere, and then heat it at 155 °C for 12 h in a high-pressure kettle encapsulated with argon to obtain a composite sulfur cathode material for lithium-sulfur batteries.

[0025] Fabricate electrode sheets from the composite sulfur cathode materials prepared in the above Examples 1 to 3 according to the following method, assemble the batteries and test them: (1)Electrode sheet fabrication Respectively select the composite sulfur cathode materials, Super-P conductive agent and polyvinylidene fluoride (PVDF) prepared in Examples 1-3, grind them evenly at a ratio of 70 wt%, 20 wt%, and 10 wt% for 0.5 h, add N-methylpyrrolidone (NMP), and stir until a uniform slurry is obtained. Then coat it on carbon-coated aluminum foil and dry it at 60 °C for 12 h to make an electrode sheet with a diameter of 10 mm.

[0026] (2)Battery assembly Use Celgard 2325 as the separator and a lithium sheet as the negative electrode. Assemble a 2032-type coin cell in a glove box filled with argon. The assembly sequence is negative electrode shell – shrapnel – gasket – lithium sheet – electrolyte – separator – electrolyte – positive electrode sheet – positive electrode shell. The electrolyte is composed of 1.0 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and 0.2 M lithium nitrate (LiNO3) additive dissolved in 1,2-dimethoxyethane (1,2-Dimethoxyethane, DME) and 1,3-dioxolane (1,3-dioxolane, DOL) (V / V, 1:1), and the ratio of electrolyte to sulfur (E / S) is 20 μL mg -1 . Place the obtained 2032 coin cell on a battery test system, let it stand for 6 h, and then perform charge-discharge tests at a rate of 0.2 C. The voltage cut-off range is set to 1.7~2.8 V, and calculate the discharge specific capacity of the battery with the composite sulfur cathode material as the active substance.

[0027] From the above test results, it can be seen that the composite sulfur cathode materials for lithium-sulfur batteries with different ratios of carriers, conductive agents, and polyvinylidene fluoride provided by the present invention all exhibit excellent electrochemical performance. By comparison, the composite cathode material prepared in Example 1 has the highest specific capacity and the best cycle performance. Specifically, the composite cathode material obtained in Example 1 has an initial discharge specific capacity of 901.9 mAh g in the first week -1 , and the discharge specific capacity can be maintained at 740.8 mAh g after 50 cycles -1, its capacity retention rate is 82.1%, significantly higher than that of the other two composite sulfur cathode materials. The data of other embodiments are listed in Table 1. This shows that the composite of cobalt-doped stable molybdenum disulfide defect structure and nitrogen-doped graphene carrier loaded with sulfur has excellent catalytic conversion and strong chemisorption of polysulfide lithium, and a composite cathode material for lithium-sulfur batteries with both high capacity and stable cycling can be obtained.

[0028] Table 1 Comparison of battery cycling performance of each embodiment The above shows and describes the main features, usage methods, basic principles and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited by the above embodiments. What is described in the above embodiments and the specification only illustrates the principle of the present invention. Without departing from the spirit and scope of the present invention, the present invention will also have various changes and improvements according to actual situations, and these changes and improvements all fall within the scope of the present invention claimed. The scope of the present invention claimed is defined by the appended claims and their equivalents.

Claims

1. Lithium-sulfur battery composite cathode material doped with cobalt molybdenum disulfide, characterized in that, Using cobalt-doped stable molybdenum disulfide defect structure composite nitrogen-doped graphene as a carrier, sulfur is uniformly loaded thereon.

2. Preparation method of lithium-sulfur battery composite cathode material of cobalt-doped molybdenum disulfide, characterized in that, The specific steps are as follows: Step 1: Dissolve ammonium molybdate in an aqueous polymer solution, add nitrogen-doped graphene and mix by ultrasonic, then add a sulfur source, add a cobalt source and stir until a clear mixed solution is obtained. Place the mixed solution in a high-pressure reactor and continuously react in an oven at 200-240 °C for 18-36 h. Wash the reactor product multiple times, dry it, and then anneal it in a tube furnace under a specific atmosphere to obtain a cobalt-doped stable molybdenum disulfide defect structure / nitrogen-doped graphene composite material; Step 2: Grind and mix the composite material with sulfur, and then place the mixture in a reactor again under an argon atmosphere. After heat treatment, a composite cathode material for lithium-sulfur batteries is obtained.

3. The preparation method of the cobalt-doped molybdenum disulfide lithium-sulfur battery composite cathode material according to claim 2, characterized in that, The polymer is polyethylene glycol with a molecular weight of ~20000 and a solution concentration of 15-25 mM.

4. The preparation method of the cobalt-doped molybdenum disulfide lithium-sulfur battery composite cathode material according to claim 2, characterized in that, The sulfur source is one of L-cysteine, thioacetamide, and thiourea, and the cobalt source is one of cobalt nitrate (Co(NO3)2·6H2O, Co(COOH)2 4H2O).

5. The preparation method of the cobalt-doped lithium molybdenum disulfide lithium-sulfur battery composite cathode material according to claim 2, characterized in that, The method of multiple washing is to centrifuge and wash with deionized water and absolute ethanol 2-4 times respectively.

6. The preparation method of the cobalt-doped molybdenum disulfide lithium-sulfur battery composite cathode material according to claim 2, wherein The annealing treatment in the tube furnace under a specific atmosphere is 5-10% H2 / Ar or Ar.

7. The preparation method of the cobalt-doped lithium sulfur battery composite cathode material of molybdenum disulfide according to claim 2, wherein, The annealing treatment temperature in the tube furnace under a specific atmosphere is 500-700 °C, and the time is 2-5 h.

8. The preparation method of the cobalt-doped molybdenum disulfide lithium-sulfur battery composite cathode material according to claim 2, characterized in that, The mass ratio of the cobalt-doped molybdenum disulfide defect structure / nitrogen-doped graphene to sulfur is 20-40:60-80.

9. The preparation method of the cobalt-doped molybdenum disulfide lithium-sulfur battery composite cathode material according to claim 2, characterized in that, The specific conditions of the heat treatment method are to keep the temperature at 150-165 °C for 10-20 h under an argon atmosphere.

10. The application of the cobalt-doped molybdenum disulfide lithium-sulfur battery composite cathode material as described in claim 1 in a lithium-sulfur battery.

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