Preparation method for constructing hierarchical conductive network structure Ti3C2-coated N-CNTs composite material and application of hierarchical conductive network structure Ti3C2-coated N-CNTs composite material in lithium-sulfur battery

Abstract

The invention provides a preparation method for constructing a hierarchical conductive network structure Ti < 3 > C < 2 >-N-CNTs composite material and application of the hierarchical conductive network structure Ti < 3 > C < 2 >-N-CNTs composite material to a lithium-sulfur battery to solve the problems of shuttling of polysulfide and slow reaction kinetics. Ti < 3 > C < 2 >-N-CNTs are obtained by adopting a template method and a vapor deposition method. Spherical Ti3C2 prevents stacking among lamellas, and exposed active sites provide more possibilities for anchoring and conversion of polysulfide. Meanwhile, the nitrogen-doped carbon nanotubes grown in situ are connected with the Ti3C2 spheres, so that a three-dimensional conductive network is formed, and rapid electron transmission can be realized. The synergistic effect of the two enhances the adsorption and catalytic conversion of polysulfide, and promotes rapid ion diffusion and electron transfer at the same time, so that the constructed hierarchical conductive network structure Ti3C2 (at) N-CNTs composite material improves the electrochemical performance of a lithium-sulfur battery, and solves the problem of rapid reduction of the capacity of the lithium-sulfur battery.

Description

Technical Field

[0001] The present invention relates to the field of construction and design of positive electrode materials for lithium-sulfur batteries, and in particular to the technical field of improving the performance of positive electrode materials with three-dimensional network structures in lithium-sulfur batteries. Background Art

[0002] In recent years, facing the increasingly unbalanced supply and demand of traditional energy and the deteriorating global environment, countries around the world have stepped up their efforts to develop new energy and energy technologies. Energy storage devices, as key technologies for storing and redistributing energy, have been widely used and studied. Among energy storage devices, lithium-ion batteries have a relatively low energy density (200-300 Wh kg -1 ) and potential safety hazards have limited further development and application. Sulfur has abundant reserves, low price and high theoretical specific capacity. The theoretical energy density of the designed lithium-sulfur battery is as high as 2600 Wh kg -1 The theoretical specific capacity can reach 1675 mAh g -1 ,Therefore, it has greater application potential and research significance.

[0003] Although lithium-sulfur batteries have great advantages in the research process, there are also some problems that hinder their development. The main problems are: (1) The poor conductivity of sulfur and discharge products leads to low utilization of active materials and slow reaction kinetics, which seriously affects the high-rate performance of the battery; (2) Soluble polysulfides are produced during the charge and discharge process. They are relatively soluble in ether electrolytes. As the reaction proceeds, the continuously accumulated polysulfides will undergo a shuttle effect under the action of the concentration gradient, resulting in irreversible capacity loss and low coulombic efficiency; (3) The multi-phase transformation during the charge and discharge process is accompanied by a large volume change, which seriously affects the stability of the positive electrode structure. At the same time, the poor reaction kinetics between phase transitions also affect the electrochemical performance of the battery.

[0004] In response to the above problems, the earliest strategy in the design of the main body of the positive electrode material is to physically confine the active substance sulfur in a porous conductive carbon-based material (mesoporous carbon, carbon nanotubes, carbon nanofibers, etc.), which can achieve the effect of reducing the shuttling of polysulfides through spatial confinement. However, there is only a weak van der Waals force between the non-polar carbon material and the polysulfide, which cannot fundamentally solve the problem of battery performance degradation caused by the shuttle effect and slow reaction kinetics. As a result, the strategy of chemical adsorption and catalysis was developed, and methods such as doping, defects, and heterogeneous structures were introduced in the design and synthesis of catalysts to achieve the purpose of enhancing the anchoring of polysulfides and accelerating the redox process. It is very important to develop a cathode main material with high conductivity, adaptability to volume changes and inhibition of the shuttle effect. It can not only solve the problem of poor electrochemical performance of lithium-sulfur batteries, but also help promote the further development of lithium-sulfur batteries in the direction of practical application. This has also become the main focus of this patent research. Summary of the invention

[0005] In view of the above existing technical problems, the first object of the present invention is to provide a Ti3C2@N-CNTs composite material with a hierarchical conductive network structure. The key features of the synthesized material are: The spherical shape of Ti3C2 prevents stacking of sheets, and the exposed active sites are conducive to the anchoring and conversion of polysulfides; In-situ grown nitrogen-doped carbon nanotubes connect Ti3C2 spheres to build a three-dimensional conductive network that enables faster electron transport; Nitrogen-doped carbon nanotube-composite Ti3C2 cathode materials promote the catalytic conversion of polysulfides and accelerate the redox reaction kinetics of lithium-sulfur batteries, thereby improving the electrochemical performance of lithium-sulfur batteries.

[0006] The second object of the present invention is to provide a method for preparing a Ti3C2@N-CNTs composite material having a hierarchical conductive network structure, comprising the following steps: Preparation of MXene / PMMA: Ti3AlC2 powder, LiF and hydrochloric acid solution are etched in a mass ratio of 1:1.5:7 and the precipitate of the reaction is washed with water. The upper layer of the washed liquid is collected and mixed with the PMMA dispersion in a ratio of 1:10 to obtain MXene / PMMA; Preparation of Ti3C2@N-CNTs: Take an appropriate amount of MXene / PMMA and Co(NO3)2·6H2O solution and stir and mix them, collect the above precipitate and calcine it with dicyandiamide in a tube furnace at a certain ratio to obtain the precursor of Ti3C2@N-CNTs, and then oscillate the precursor in a hydrochloric acid solution to obtain the product Ti3C2@N-CNTs; Ti3C2@N-CNTs were mixed with sulfur powder and melted to be used as the positive electrode material.

[0007] In step (1), the etching temperature is 40-50°C, the reaction time is 12-36 h, and the washing result is PH>6.

[0008] In step (2), the mass ratio of MXene / PMMA to Co(NO3)2·6H2O is 1:2.5, wherein the concentration of the Co(NO3)2·6H2O solution is 2.5 wt%.

[0009] In step (2), the mass ratio of MXene / PMMA immersed in Co(NO3)2·6H2O to dicyandiamide is 1:6, the calcination temperature is 700-800°C, the calcination time is 3-4 h, and the calcination atmosphere is an inert atmosphere.

[0010] In step (3), the mixing ratio of Ti3C2@N-CNTs and sulfur powder is less than 1, the melting temperature is 110-200 °C, and the melting time is 9-15 h.

[0011] The third object of the present invention is to provide a hierarchical conductive network structure Ti3C2@N-CNTs with good conductivity and high redox activity as a positive electrode material for lithium-sulfur batteries. Compared with traditional sulfur positive electrode materials, the present invention has the following advantages: The hollow spherical structure of Ti3C2 can be used for sulfur storage and can also adapt to the volume change of sulfur during the reaction. The more exposed active sites are conducive to the capture of polysulfides and the catalytic reaction.

[0012] The nitrogen-doped carbon nanotubes grown in situ on the Ti3C2 surface construct a network structure, which enhances the rapid transmission of electrons. At the same time, the hierarchical three-dimensional network structure can enhance the penetration of the electrolyte and facilitate the diffusion of lithium ions.

[0013] The introduction of nitrogen doping can enhance the adsorption and catalytic conversion performance of cathode materials for polysulfides, and can effectively improve the electrochemical performance of lithium-sulfur batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 X-ray diffraction diagram of Ti3C2@N-CNTs with hierarchical network structure of the positive electrode material of the present invention; Figure 2 The cycle performance diagram of Ti3C2@N-CNTs with hierarchical network structure of the positive electrode material of the present invention; Implementation

[0015] The present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. The specific implementation cases described here are only applicable to explaining the present application and are not intended to limit the present application.

[0016] The specific preparation method of the present invention is as follows: (1) Preparation of MXene / PMMA: 2 g Ti3AlC2 powder is added to 3.2 g LiF and 20 mL 9.0 mol L -1 The precipitate was etched in a mixed solution of hydrochloric acid and washed with water, and the washed single-layer Ti3C2T X Solution is 2 mg mL -1 Then take 25 mL Ti3C2T X Solution with 50 mL 10 mg mL -1 The PMMA dispersion was stirred and mixed to obtain MXene / PMMA; (2) Preparation of Ti3C2@N-CNTs: 200 mg of MXene / PMMA was mixed with 20 mL of 2.5 wt% Co(NO3)2·6H2O solution, and the above precipitate was calcined with 1.2 g of dicyandiamide in a tube furnace to obtain the precursor of Ti3C2@N-CNTs. The precursor was then heated to 1 mol L -1 The product Ti3C2@N-CNTs was obtained by oscillating in a hydrochloric acid solution; (3) Ti3C2@N-CNTs was mixed with sulfur powder in a ratio of 3:7 and melted to prepare the positive electrode material.

[0017] In the embodiment of the present invention, carbon nanotubes are in-situ grown on Ti3C2 spheres to construct a hierarchical three-dimensional conductive network structure, which improves the conductivity of the material, shortens the electron transmission path, accelerates the conversion kinetics of polysulfide during the reaction, and effectively improves the electrochemical performance of lithium-sulfur batteries. While ensuring long-cycle stability, lithium-sulfur batteries have been more widely used.

[0018] Figure 1 As shown, the Ti3C2@N-CNTs in the present invention has good crystallinity, and the Ti3C2 structure after heat treatment is well preserved, showing obvious (002) plane characteristic peaks, and a strong peak of carbon nanotubes is observed at 26.5°.

[0019] Figure 2 As shown, the cycling performance of lithium-sulfur batteries assembled with positive electrode material S / Ti3C2@N-CNTs at 1 C current density.

[0020] The present invention is not limited to the above-mentioned embodiments. For those skilled in the art, several improvements and modifications can be made without departing from the present invention, and these improvements and modifications are also considered to be within the scope of protection of the present invention. The contents not described in detail in this specification belong to the prior art known to professionals in the field.

Claims

1. A method for preparing a Ti3C2@N-CNTs composite material with a hierarchical conductive network structure, comprising the following steps: (1) Preparation of MXene / PMMA: Ti3AlC2 powder, LiF and hydrochloric acid solution were etched in a mass ratio of 1:1.5:7 and the precipitate was washed with water. The upper layer of the washed liquid was collected and mixed with the PMMA dispersion in a ratio of 1:10 to obtain MXene / PMMA; (2) Preparation of Ti3C2@N-CNTs: Take an appropriate amount of MXene / PMMA and Co(NO3)2·6H2O solution and stir and mix them. Collect the above precipitate and calcine it with dicyandiamide in a tube furnace at a certain ratio to obtain the precursor of Ti3C2@N-CNTs. Then, oscillate the precursor in a hydrochloric acid solution to obtain the product Ti3C2@N-CNTs. (3) Ti3C2@N-CNTs are mixed with sulfur powder and melted to form the positive electrode material.

2. The preparation method according to claim 1, characterized in that Carbon nanotubes are in-situ grown on Ti3C2 spheres to construct a hierarchical three-dimensional conductive network structure, which improves the conductivity of the material, shortens the electron transmission path, accelerates the conversion kinetics of polysulfides during the reaction, and effectively improves the electrochemical performance of lithium-sulfur batteries; The preparation method according to claim 1, characterized in that In step (1), the etching temperature is 40-50 °C, the reaction time is 12-36 h, and the washing result is PH>6; in step (2), the mass ratio of MXene / PMMA to Co(NO3)2·6H2O is 1:2.5, wherein the concentration of the Co(NO3)2·6H2O solution is 2.5 wt%; in step (2), the mass ratio of MXene / PMMA immersed in Co(NO3)2·6H2O to dicyandiamide is 1:6, the calcination temperature is 700-800 °C, the calcination time is 3-4 h, and the calcination atmosphere is an inert atmosphere; in step (3), the mixing ratio of Ti3C2@N-CNTs and sulfur powder is less than 1, the melting temperature is 110-200 °C, and the melting time is 9-15 h; According to the preparation method described in claims 1-2, the Ti3C2@N-CNTs positive electrode material is obtained, and the electrochemical performance is excellent; the Ti3C2@N-CNTs preparation method and the positive electrode material are used in lithium-sulfur batteries.

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