CN113346054B - Preparation method and application of MXene-carbon nanocage-sulfur composite material - Google Patents

Preparation method and application of MXene-carbon nanocage-sulfur composite material Download PDF

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CN113346054B
CN113346054B CN202110439488.8A CN202110439488A CN113346054B CN 113346054 B CN113346054 B CN 113346054B CN 202110439488 A CN202110439488 A CN 202110439488A CN 113346054 B CN113346054 B CN 113346054B
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mxene
sulfur
carbon
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powder
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CN113346054A (en
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耿显葳
杨莉
赵策洲
易若玮
刘晨光
孙艺
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Xian Jiaotong Liverpool University
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
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    • H—ELECTRICITY
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    • H—ELECTRICITY
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    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
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    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/38—Selection of substances as active materials, active masses, active liquids of elements or alloys
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/58—Selection of substances as active materials, active masses, active liquids of inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy; of polyanionic structures, e.g. phosphates, silicates or borates
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
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    • Y02E60/10—Energy storage using batteries

Abstract

Preparation method and application of MXene-carbon nanocage-sulfur composite material with MAX phase Ti3AlC2The two-dimensional MXene material is used as a raw material and is obtained by a chemical etching method. The carbon nanocage material is obtained by taking sodium citrate powder as a raw material through carbonization and chemical etching. And then the MXene and the carbon nanocage composite material are obtained by ultrasonic dispersion and mixing. And finally, melting the elemental sulfur into MXene-carbon nanocage powder at high temperature to obtain the MXene-carbon nanocage-sulfur composite material serving as the anode of the lithium-sulfur battery. The electrode material has excellent conductivity, increases the interlayer spacing of MXene, exposes more active sites and functional groups, and is beneficial to capturing polysulfide. Therefore, the battery has high specific discharge capacity and good cycle performance.

Description

Preparation method and application of MXene-carbon nanocage-sulfur composite material
Technical Field
The invention belongs to the field of material chemistry, and particularly relates to a method for preparing an MXene-carbon nanocage composite material by ultrasound, and then carrying out a sulfur melting and mixing process to obtain an MXene-carbon nanocage-sulfur composite lithium-sulfur battery cathode material.
Background
In recent years, with the rapid development of electric vehicles and various electronic appliances in daily life, people have increasingly high requirements on energy storage devices with high energy density. Therefore, only conventional lithium ion batteries with lower theoretical capacities have been unable to meet the current demand. The lithium-sulfur battery is considered to be one of important development directions of next-generation secondary batteries because the lithium-sulfur battery has high theoretical specific capacity (1675mAh/g) and energy density (2600Wh/Kg), and meanwhile, sulfur is an environment-friendly material, widely exists on the earth, and has low toxicity and low preparation cost.
However, elemental sulfur has three major drawbacks that limit the commercial use of lithium sulfur batteries. First, sulfur itself has poor conductivity, so that electrons are difficult to transport to cause electrochemical reaction, and the reaction efficiency of the electrode is low, resulting in low utilization of sulfur as an active material. Secondly, unlike the conventional lithium ion battery, in the process of charging and discharging of the lithium sulfur battery, sulfur reacts with lithium, the charging and discharging process is a process in which soluble and insoluble polysulfide is formed by cyclic S8 molecules, and the polysulfide dissolves in the electrolyte and migrates, thereby generating a shuttle effect. This can lead to loss of sulfur as an active material and corrosion of the lithium negative electrode, resulting in short cycle life. Finally, sulfur forms lithium sulfide with a density less than that of sulfur during charging, causing expansion of the electrode volume, about 80%, and repeated volume scaling during charging and discharging causes cracking and peeling of the electrode material, which in the long run results in a reduction in capacity.
To solve these drawbacks, researchers have adopted many methods, among which MXene, a layered two-dimensional material, has emerged in recent years. The transition metal titanium carbide nano material with a graphene-like structure is obtained by etching IIIA/IVA elements in a MAX phase of a precursor through a chemical liquid phase method, and has excellent conductivity and capability of capturing polysulfide. However, due to the strong interaction of surface groups, MXene etched by strong acid still cannot be uniformly dispersed in the form of a single layer sheet and is easy to agglomerate, so that the specific surface area is reduced, and the application of the MXene in a lithium-sulfur battery is hindered.
Therefore, how to solve the above-mentioned deficiencies of the prior art is a problem to be solved by the present invention.
Disclosure of Invention
The invention aims to provide a preparation method and application of an MXene-carbon nanocage-sulfur composite material.
In order to achieve the purpose, the invention adopts the technical scheme that:
a preparation method of MXene-carbon nanocage-sulfur composite material comprises the steps of preparing sheet MXene and preparing carbon nanocages;
the preparation method of the flaky MXene comprises the following steps:
step one, adding lithium fluoride into 27-37 wt% of hydrochloric acid, wherein 1-3 g of lithium fluoride is added into every 40ml of hydrochloric acid; stirring at the rotating speed of 300-600 rpm for 20-50 min to ensure that lithium fluoride and hydrochloric acid fully react to generate hydrogen fluoride;
step two, adding MAX-Ti3AlC2Slowly adding into the reaction solution generated in the step one, wherein MAX-Ti is contained3AlC2And the mass ratio of the lithium fluoride in the step one is 1: 0.5 to 2; then adjusting the reaction temperature to 30-40 ℃, and continuously stirring for 12-48 hours to obtain MAX-Ti3AlC2Al in the Ti is fully etched to obtain Ti3C2；
Step three, centrifuging the reaction solution obtained in the step two at the rotating speed of 3500-8000 rpm, removing the supernatant after centrifugation, and adding deionized water into the precipitate, wherein 25-100 ml of deionized water is needed for 40ml of hydrochloric acid in each use step, and the mixture is sufficiently shaken up to uniformly mix the precipitate and the deionized water; then carrying out ultrasonic treatment for 15-25 minutes to ensure that the precipitate is uniformly dispersed in the solution again; finally, continuously centrifuging the solution, and repeating the step three 4-6 times to clean the hydrogen fluoride;
step four, adding ethanol into the precipitate, wherein 25-100 ml of ethanol is needed for 40ml of hydrochloric acid in each use step I, and fully shaking up; carrying out ultrasonic treatment for 1-2 hours to realize intercalation operation of MXene, and further obtaining a few-layer MXene dispersion liquid; centrifuging at the rotating speed of 8000-10000 rpm, fully collecting the lower-layer precipitate, and then putting the precipitate into a vacuum drying box to be dried for 6-12 hours at the temperature of 50-75 ℃ to obtain less-layer flaky MXene powder;
wherein, the preparation steps of the carbon nano cage are as follows:
step one, ball-milling sodium citrate powder for 6-12 hours;
step two, heating the ball-milled sodium citrate powder for 1-3 hours at the temperature of 600-900 ℃ under the protection of inert gas to carbonize the sodium citrate powder;
adding the carbonized sodium citrate powder into a 10-25 wt% hydrochloric acid solution, and reacting for 12-24 hours to etch away impurities except carbon;
step four, filtering, washing and drying to obtain porous carbon nanocage powder with a three-dimensional structure;
after the preparation of the flaky MXene and the carbon nano cage is finished, preparing the MXene-carbon nano cage composite material, wherein the preparation steps comprise:
step one, mixing the flaky MXene powder and the carbon nanocage powder in a mass ratio of 7: 1-5, and adding the mixture into deionized water, wherein 100-200 ml of deionized water is used for every 1g of mixed powder; ultrasonically dispersing for 20-50 min to obtain aqueous dispersion of MXene-carbon nanocages;
step two, placing the dispersion liquid into a vacuum drying oven, vacuumizing and heating to 50-75 ℃, and keeping for 48-72 hours to fully dry water to obtain MXene-carbon nanocage composite material powder;
after the MXene-carbon nanocage composite material is prepared, preparing the MXene-carbon nanocage-sulfur composite material, wherein the preparing steps comprise:
mixing the MXene-carbon nanocage composite material powder and sulfur elementary substance in a mass ratio of 1: 3-5 grinding and mixing; putting the mixed powder into a sealed vessel, heating to 140-160 ℃, melting sulfur into the MXene-carbon nanocage composite material powder, and keeping for 6-12 hours; then, cooling to room temperature to obtain MXene-carbon nanocage-sulfur composite material powder.
The relevant content in the above technical solution is explained as follows:
1. in the scheme, the ultrasonic frequency used in the ultrasonic treatment and the ultrasonic dispersion is 40-50 KHz, and the ultrasonic power is 600-800W.
2. In the scheme, in the preparation of the flaky MXene and the MXene-carbon nanocage composite material, the MXene-containing material is dried in vacuum, kept in a vacuum environment, naturally cooled to room temperature, and then removed in vacuum.
3. In the scheme, in the preparation of the carbon nano cage, in the fourth step, the washing times are 2-5 times, and the drying process is drying for 4-6 hours at 70-85 ℃ in a vacuum drying oven.
In order to achieve the purpose, the invention adopts another technical scheme that:
a lithium sulfur battery applying MXene-carbon nanocage-sulfur composite material applies the prepared MXene-carbon nanocage-sulfur composite material as a positive electrode material to a positive electrode, and the positive electrode manufacturing step comprises the following steps:
according to the positive electrode material: acetylene black: binder = 8: 1:1 to form a mixture, adding N-methyl pyrrolidone serving as a solvent, grinding into slurry, uniformly coating on an aluminum foil current collector to prepare an electrode, and drying for 8-24 hours in a vacuum environment at a temperature of 50-75 ℃ to prepare the anode;
taking a lithium sheet as a negative electrode, wherein electrolyte is 1M lithium bistrifluoromethanesulfonylimide which is dissolved in 1, 3-dioxolane and ethylene glycol dimethyl ether according to the volume ratio of 1:1, wherein the mass ratio of 1, 3-dioxolane to ethylene glycol dimethyl ether is 1: 0.4 to 2.
The relevant content in the above technical solution is explained as follows:
1. in the scheme, the sulfur carrying amount of the coating film is 1-2 mg/cm2。
2. In the above scheme, the components including the positive electrode and the negative electrode are assembled in a glove box having an inert gas environment, so as to obtain the lithium-sulfur battery.
The working principle and the advantages of the invention are as follows:
the MXene-nano carbon cage-sulfur composite material is used as the anode of the lithium-sulfur battery, so that the interlayer spacing of MXene can be effectively opened, more surface active sites and functional groups are exposed, ion transmission channels are increased, the effective de-intercalation of Li + is realized, and the problem that MXene sheets are easy to stack is solved. The lithium-sulfur ion battery anode material not only greatly improves the specific capacity, but also shows excellent cycling stability when being used as the lithium-sulfur ion battery anode material.
The invention adopts a simple chemical reaction method to prepare MXene and uses a carbonization method to prepare the carbon nanocages, and the experimental steps are easy to operate. Although pure MXene has good conductivity and polysulfide capturing capability, MXene is easy to stack and causes the obstruction to lithium ion transmission, and sulfur particles with the size of 1-5 microns are formed when the MXene is molten and mixed with sulfur simple substances, so that the MXene is not beneficial to the full utilization of active substance sulfur, and the battery performance is poor. And the carbon nanocages have better conductivity than MXene, are close to the conductivity of acetylene black, and have the conductivity of about 400m2Ultra-high specific surface area around/mg. Mixing MXene and the carbon nanocages can greatly increase the specific surface area of the composite material, is beneficial to forming sulfur particles with smaller or even nano-scale sizes in the sulfur melting process, and effectively increases the utilization rate of active substance sulfur. Meanwhile, the carbon nano cage is inserted into the middle of the flaky MXene, so that the stacking of the MXene is reduced, and the transmission of lithium ions is promoted. And the interlamellar spacing of MXene can be increased, more active sites and functional groups are exposed, and the conductivity and polysulfide capturing capability of MXene are fully exerted. The synergistic effect of the two can greatly improve the performances of the lithium-sulfur battery in all aspects.
Compared with the prior art, the invention has the characteristics that:
on the structural level: the MXene-carbon nanocage-sulfur composite material contains pure MXene substances, the carbon material is in a carbon nanocage form, and the MXene-carbon nanocage-sulfur composite material only contains three materials, so that the MXene-carbon nanocage composite material is relatively simple in components, and controllable in process realization difficulty and cost;
in the process level: in the invention, MAX phase Ti is used3AlC2MXene material is obtained by a chemical etching method as a raw material. Sodium citrate powder is used as a raw material, and a hollow porous carbon nanocage material is obtained by a carbonization and chemical etching method. After the two materials are respectively prepared, MXene and the carbon nano tube are mixed by a method of dispersing in ultrapure water. And finally, melting the elemental sulfur into the carbon nanocage-MXene powder at high temperature by using a melting method to obtain the carbon nanocage-MXene-sulfur composite material.
Compared with the prior art, the invention has the beneficial effects that:
1. the carbon nanocage not only has the performance of the carbon nano tube, but also has the capability of capturing polysulfide to a certain degree due to the porous structure, so that the shuttle effect is reduced.
2. Pure carbon nanocages have conductivity close to that of acetylene black, but have not yet good polysulfide trapping ability. Simple MXene has various functional groups and active sites on the surface, so that the MXene has strong capability of capturing polysulfide, but the conductivity is not good enough. The invention compounds MXene and carbon nanocages, which is beneficial to reducing the stacking of MXene, expanding the interlayer spacing, exposing more active sites, and further improving the conductivity of the material and the capturing capability of polysulfide, thereby improving the rate capability and the cycling stability.
3. The conductivity of the carbon nanocages is close to that of acetylene black, so that the conductivity can be further increased, and the mechanical strength of the material can be increased.
4. The invention highlights the synergistic effect between MXene and carbon nanocage, and the synergistic effect of MXene and carbon nanocage can be better exerted while the respective effect is realized.
Drawings
Fig. 1 is an SEM morphology characterization diagram of various materials in an embodiment of the present invention, where (a) is carbon nanocages, (b) is lamellar MXene, (c) is MXene-carbon nanocages, (d) is carbon nanocage-sulfur, (e) is MXene-sulfur, and (f) is MXene-carbon nanocage-sulfur;
FIG. 2 is a graph of cycle performance of different composite electrode batteries according to different embodiments of the present invention, where (a) is 0.1C low current density and (b) is rate capability.
Detailed Description
The invention is further described with reference to the following figures and examples:
the present disclosure will be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the disclosure may be shown and described, and which, when modified and varied by the techniques taught herein, can be made by those skilled in the art without departing from the spirit and scope of the disclosure.
As used herein, the terms "comprising," "including," "having," and the like are open-ended terms that mean including, but not limited to.
As used herein, the term (terms), unless otherwise indicated, shall generally have the ordinary meaning as commonly understood by one of ordinary skill in the art, in this written description and in the claims. Certain words used to describe the disclosure are discussed below or elsewhere in this specification to provide additional guidance to those skilled in the art in describing the disclosure.
The first embodiment is as follows: the invention relates to a preparation method of an MXene-carbon nanocage-sulfur composite material, which comprises the following steps of preparing sheet MXene and preparing carbon nanocages, and the preparation method comprises the following steps:
firstly, the preparation step of the flaky MXene comprises the following steps:
1-1, adding 2g of lithium fluoride into 40ml of 12M hydrochloric acid, and stirring for 30min at the rotating speed of 400rpm to ensure that the lithium fluoride and the hydrochloric acid fully react to generate HF;
1-2 g of MAX-Ti3AlC2Slowly adding into the reaction solution generated in the step one, wherein MAX-Ti is contained3AlC2The mass ratio of the lithium fluoride to the lithium fluoride in 1-1 is 1: 0.5 to 2; then the reaction temperature was adjusted to 35 ℃ and stirring was continued for 24 hours to obtain MAX-Ti3AlC2Al in the Ti is fully etched to obtain Ti3C2；
1-3, centrifuging the reaction solution obtained after the step 1-2 is finished at the rotating speed of 3500 rpm, removing a supernatant after centrifugation, and adding deionized water into a precipitate, wherein 25-100 ml of deionized water is needed for 40ml of hydrochloric acid in each use step I, and fully shaking up to uniformly mix the precipitate and the deionized water; then sonicated for 20 minutes to re-disperse the precipitate uniformly in the solution; finally, continuously centrifuging the solution, and repeating the step for 6 times to clean the hydrogen fluoride;
1-4, adding ethanol into the precipitate, wherein 25-100 ml of ethanol is needed for every 40ml of hydrochloric acid in 1-1, and fully shaking up; carrying out ultrasonic treatment for 1.5 hours to realize intercalation operation of MXene (namely, fully dispersing the MXene into a single-layer flaky structure), and further obtaining a few-layer MXene dispersion liquid; centrifuging at 10000rpm, sufficiently collecting lower-layer precipitate, and drying in a vacuum drying oven at 60 ℃ for 12 hours to obtain small-layer flaky MXene powder, wherein the step is used for collecting dry powdery MXene precipitate;
secondly, the preparation steps of the carbon nano cage comprise:
2-1, ball-milling 3g of sodium citrate powder for 12 hours to ensure that the particle size of the powder particles is smaller, so that the chemical reaction is more sufficient in the subsequent steps;
2-2, heating the ball-milled sodium citrate powder for 2 hours at the temperature of 800 ℃ in a tube furnace under the protection of argon (or other inert gases) to carbonize the sodium citrate powder;
2-3, adding the carbonized sodium citrate powder into a 15wt% hydrochloric acid solution, and reacting for 24 hours to etch away impurities except carbon;
2-4, filtering, washing and drying to obtain porous carbon nanocage powder with a three-dimensional structure;
and thirdly, after the flaky MXene and the carbon nano cage are prepared, preparing the MXene-carbon nano cage composite material, wherein the preparation steps comprise:
3-1, taking 500mg of the flaky MXene powder obtained from 1-4, taking 150mg of the carbon nanocage powder obtained from 2-4, and adding the carbon nanocage powder into 65ml of deionized water; performing ultrasonic dispersion for 30min to obtain aqueous dispersion of MXene-carbon nanocages;
3-2, putting the dispersion liquid into a vacuum drying box, vacuumizing and heating to 60 ℃, and keeping for 72 hours to fully dry water to obtain MXene-carbon nanocage composite powder;
fourthly, after the MXene-carbon nano cage composite material is prepared, preparing the MXene-carbon nano cage-sulfur composite material, wherein the preparation steps comprise:
mixing powder of the MXene-carbon nanocage composite material obtained in the step 3-2 with sulfur simple substance in a mass ratio of 1: 4, grinding and mixing; then putting the mixed powder into a sealed glassware, heating to 155 ℃, melting sulfur into MXene-carbon nanocage composite powder, and keeping for 12 hours; then, cooling to room temperature to obtain MXene-carbon nanocage-sulfur composite material powder.
Preferably, the ultrasonic frequency used in the ultrasonic treatment and the ultrasonic dispersion is 40-50 KHz, and the ultrasonic power is 600-800W.
Preferably, in the preparation of the flake MXene and the MXene-carbon nanocage composite material, the MXene-containing material is dried in vacuum, kept in a vacuum environment, naturally cooled to room temperature, and then removed in vacuum. By the design, MXene in the material can be prevented from being oxidized.
Preferably, in the preparation of the carbon nanocages, in 2-4 times of washing, the drying process is drying for 4-6 hours in a vacuum drying oven at 80 ℃. The process can fully clean impurities except the carbon nanocages, and the impurities are fully dried, so that accurate weighing of the impurities is facilitated.
The lithium-sulfur battery using the MXene-carbon nanocage-sulfur composite material is now described as follows:
the MXene-carbon nanocage-sulfur composite material obtained by preparation is used as a positive electrode material to be applied to a positive electrode, and the positive electrode manufacturing step comprises the following steps:
according to the positive electrode material: acetylene black (super-P): binder (PVDF) = 8: 1:1 to form a mixture, adding N-methyl pyrrolidone (NMP) as a solvent, wherein the mass ratio of the solvent to the mixture is 10-25: grinding into slurry, uniformly coating to prepare an electrode, and drying for 12 hours in a vacuum environment at the temperature of 60 ℃ to prepare the cathode.
Taking a lithium sheet as a negative electrode, wherein electrolyte is 1M lithium bistrifluoromethanesulfonylimide which is dissolved in 1, 3-dioxolane and ethylene glycol dimethyl ether according to the volume ratio of 1:1, wherein the mass ratio of 1, 3-dioxolane to ethylene glycol dimethyl ether is 1: 0.4 to 2.
Preferably, the sulfur carrying amount of the coating film is 1-2 mg/cm2。
Preferably, the respective components including the positive electrode and the negative electrode are assembled in a glove box having an inert gas (e.g., argon) atmosphere to obtain a lithium sulfur battery.
As shown in fig. 1, the SEM morphology of the materials of the first example is shown, wherein (a) is carbon nanocages, (b) is lamellar MXene, (c) is MXene-carbon nanocages, (d) is carbon nanocage-sulfur, (e) is MXene-sulfur, and (f) is MXene-carbon nanocage-sulfur.
As can be seen from fig. 1 (a) - (c), the pure carbon nanocages have a honeycomb-like polygonal structure, the pure MXene has a lamellar structure, and after the pure MXene and the pure MXene are ultrasonically mixed together, the carbon nanocages are sandwiched between the lamellar MXene, so that the respective dispersion of the pure MXene and the lamellar MXene is facilitated, the carbon nanocages can play a role in better transmitting electrons in the middle, thereby further increasing the conductivity, effectively increasing the interlayer spacing between the lamellar mxenes, and exposing more active sites and functional groups. Thus, the structure can increase the electrical conductivity of the composite material while enhancing the effect on polysulfide capture.
After the melting process, as shown in fig. 1 (d) to (f), the single MXene and sulfur melt and mix to generate large-particle sulfur, and the single carbon nanocage and the MXene-carbon nanocage melt and mix to generate smaller and even nano-sized sulfur particles. Therefore, the MXene-carbon nanocage-sulfur composite material is beneficial to increasing the specific surface area of the material, so that the utilization rate of the active substance sulfur is improved.
In order to illustrate the advantages and effects of the technical scheme of the invention, besides the first embodiment, the applicant makes second and third embodiments different from the first embodiment of the invention, thereby intuitively reflecting the effect difference between different schemes through different experimental data.
Example two: the MXene-sulfur composite was prepared as follows from example one:
the preparation method of the MXene-sulfur composite material after the preparation of the flaky MXene comprises the following steps: mixing flake MXene and sulfur elementary substance in a mass ratio of 1: 4 grinding and mixing, putting the mixed powder into a sealed glass ware, heating to 155 ℃ to melt sulfur into the flaky MXene, and keeping for 12 hours. And finally cooling to room temperature to obtain the MXene-sulfur composite material.
Example three: the carbon nanocage-sulfur composite material was prepared, which was different from the first example in the following point:
after the carbon nanocages are prepared, the carbon nanocages-sulfur composite material is prepared, and the specific preparation method comprises the following steps: mixing carbon nanocage powder and sulfur elementary substance in a mass ratio of 1: 4 grinding and mixing, putting the mixed powder into a sealed glass ware, heating to 155 ℃, melting sulfur into the carbon nanocage powder, and keeping for 12 hours. And finally, cooling to room temperature to obtain the carbon nanocage-sulfur composite material.
As shown in fig. 2, the cycle performance of different composite electrode batteries in the first to third examples is shown, wherein (a) is 0.1C low current density, and (b) is rate capability. Wherein, MXene/CNC/S is MXene-carbon nanocage-sulfur composite material, CNC/S is carbon nanocage-sulfur composite material, and MXene/S is MXene-sulfur composite material.
As can be seen from fig. 2 (a), when the batteries with three different electrode materials are subjected to cyclic charge and discharge tests at a low current density of 0.1C, the coulombic efficiency is higher, and is basically higher than 98%, but the specific capacity is greatly different. The MXene-carbon nanocage-sulfur composite material has the highest first discharge specific capacity of 1275.5mAh/g, and still maintains 823.8mAh/g after 100 cycles, so that the composite material has the highest utilization rate of active substance sulfur, stronger capability of capturing polysulfide and good cycle stability. As can be seen from fig. 2 (b), the MXene-carbon nanocage-sulfur composite also has the best performance for the rate capability of three different electrode materials due to the excellent conductivity. When the current density is increased to 4C, the specific discharge capacity of 592.1mAh/g is still maintained. The electrode material is favorable for the rapid transmission of electrons and is suitable for rapid charge and discharge.
In summary, the invention relates to a preparation method of MXene-carbon nanocage-sulfur composite material and a lithium-sulfur battery using the composite material, wherein MAX phase Ti is used3AlC2The two-dimensional MXene material is used as a raw material and is obtained by a chemical etching method. The carbon nanocage material is obtained by taking sodium citrate powder as a raw material through carbonization and chemical etching. And then the MXene and the carbon nanocage composite material are obtained by ultrasonic dispersion and mixing. Finally, melting the elementary sulfur into MXene-carbon nanocage powder at high temperature to obtain the MXene-carbon nanocage-sulfur complexThe composite material is used as the positive electrode of a lithium-sulfur battery. The electrode material has excellent conductivity, increases the interlayer spacing of MXene, exposes more active sites and functional groups, and is beneficial to capturing polysulfide. Therefore, the battery has high specific discharge capacity and good cycle performance.
The above embodiments are merely illustrative of the technical ideas and features of the present invention, and the purpose thereof is to enable those skilled in the art to understand the contents of the present invention and implement the present invention, and not to limit the protection scope of the present invention. All equivalent changes and modifications made according to the spirit of the present invention should be covered within the protection scope of the present invention.

Claims (7)

1. A preparation method of MXene-carbon nanocage-sulfur composite material is characterized by comprising the following steps: the preparation method comprises the steps of preparing sheet MXene and preparing a carbon nanocage;
the preparation method of the flaky MXene comprises the following steps:
step one, adding lithium fluoride into 27-37 wt% of hydrochloric acid, wherein 1-3 g of lithium fluoride is added into every 40ml of hydrochloric acid; stirring at the rotating speed of 300-600 rpm for 20-50 min to ensure that lithium fluoride and hydrochloric acid fully react to generate hydrogen fluoride;
step two, adding MAX-Ti3AlC2Slowly adding into the reaction solution generated in the step one, wherein MAX-Ti is contained3AlC2And the mass ratio of the lithium fluoride in the step one is 1: 0.5 to 2; then adjusting the reaction temperature to 30-40 ℃, and continuously stirring for 12-48 hours to obtain MAX-Ti3AlC2Al in the Ti is fully etched to obtain Ti3C2；
Step three, centrifuging the reaction solution obtained in the step two at the rotating speed of 3500-8000 rpm, removing the supernatant after centrifugation, and adding deionized water into the precipitate, wherein 25-100 ml of deionized water is needed for 40ml of hydrochloric acid in each use step, and the mixture is sufficiently shaken up to uniformly mix the precipitate and the deionized water; then carrying out ultrasonic treatment for 15-25 minutes to ensure that the precipitate is uniformly dispersed in the solution again; finally, continuously centrifuging the solution, and repeating the step three 4-6 times to clean the hydrogen fluoride;
step four, adding ethanol into the precipitate, wherein 25-100 ml of ethanol is needed for 40ml of hydrochloric acid in each use step I, and fully shaking up; carrying out ultrasonic treatment for 1-2 hours to realize intercalation operation of MXene, and further obtaining a few-layer MXene dispersion liquid; centrifuging at the rotating speed of 8000-10000 rpm, fully collecting the lower-layer precipitate, and then putting the precipitate into a vacuum drying box to be dried for 6-12 hours at the temperature of 50-75 ℃ to obtain less-layer flaky MXene powder;
wherein, the preparation steps of the carbon nano cage are as follows:
step one, ball-milling sodium citrate powder for 6-12 hours;
step two, heating the ball-milled sodium citrate powder for 1-3 hours at the temperature of 600-900 ℃ under the protection of inert gas to carbonize the sodium citrate powder;
adding the carbonized sodium citrate powder into a 10-25 wt% hydrochloric acid solution, and reacting for 12-24 hours to etch away impurities except carbon;
step four, filtering, washing and drying to obtain porous carbon nanocage powder with a three-dimensional structure;
after the preparation of the flaky MXene and the carbon nano cage is finished, preparing the MXene-carbon nano cage composite material, wherein the preparation steps comprise:
step one, mixing the flaky MXene powder and the carbon nanocage powder in a mass ratio of 7: 1-5, and adding the mixture into deionized water, wherein 100-200 ml of deionized water is used for every 1g of mixed powder; ultrasonically dispersing for 20-50 min to obtain aqueous dispersion of MXene-carbon nanocages;
step two, placing the dispersion liquid into a vacuum drying oven, vacuumizing and heating to 50-75 ℃, and keeping for 48-72 hours to fully dry water to obtain MXene-carbon nanocage composite material powder;
after the MXene-carbon nanocage composite material is prepared, preparing the MXene-carbon nanocage-sulfur composite material, wherein the preparing steps comprise:
mixing the MXene-carbon nanocage composite material powder and sulfur elementary substance in a mass ratio of 1: 3-5 grinding and mixing; putting the mixed powder into a sealed vessel, heating to 140-160 ℃, melting sulfur into the MXene-carbon nanocage composite material powder, and keeping for 6-12 hours; then, cooling to room temperature to obtain MXene-carbon nanocage-sulfur composite material powder.
2. The method of claim 1, wherein: the ultrasonic frequency used in the ultrasonic treatment and the ultrasonic dispersion is 40-50 KHz, and the ultrasonic power is 600-800W.
3. The method of claim 1, wherein: in the preparation of the flaky MXene and the MXene-carbon nanocage composite material, the MXene-containing material is dried in vacuum, kept in a vacuum environment and naturally cooled to room temperature, and then removed in vacuum.
4. The method of claim 1, wherein: in the preparation of the carbon nano cage, in the fourth step, the washing times are 2-5 times, and the drying process is drying for 4-6 hours at 70-85 ℃ in a vacuum drying oven.
5. A lithium-sulfur battery applying MXene-carbon nanocage-sulfur composite material is characterized in that:
the MXene-carbon nanocage-sulfur composite material prepared by the preparation method of claim 1 is applied to a positive electrode as a positive electrode material, and the positive electrode is prepared by the steps of:
according to the positive electrode material: acetylene black: binder = 8: 1:1 to form a mixture, adding N-methyl pyrrolidone serving as a solvent, grinding into slurry, uniformly coating on an aluminum foil current collector to prepare an electrode, and drying for 8-24 hours in a vacuum environment at a temperature of 50-75 ℃ to prepare the anode;
taking a lithium sheet as a negative electrode, wherein electrolyte is 1M lithium bistrifluoromethanesulfonylimide which is dissolved in 1, 3-dioxolane and ethylene glycol dimethyl ether according to the volume ratio of 1:1, wherein the mass ratio of 1, 3-dioxolane to ethylene glycol dimethyl ether is 1: 0.4 to 2.
6. The lithium sulfur battery of claim 5, wherein: the sulfur carrying amount of the coating film is 1-2 mg/cm2。
7. The lithium sulfur battery of claim 5, wherein: and assembling the components including the positive electrode and the negative electrode in a glove box with an inert gas environment to obtain the lithium-sulfur battery.
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