CN111952568A - Sandwich structure composite material and preparation method and application thereof - Google Patents

Sandwich structure composite material and preparation method and application thereof Download PDF

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CN111952568A
CN111952568A CN202010836661.3A CN202010836661A CN111952568A CN 111952568 A CN111952568 A CN 111952568A CN 202010836661 A CN202010836661 A CN 202010836661A CN 111952568 A CN111952568 A CN 111952568A
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composite material
sodium
potassium
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张建新
张文扬
金辉鑫
张友健
杜逸群
王子晗
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Shandong University
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Abstract

The invention relates to a sandwich structure composite material and a preparation method and application thereof. Comprising Nb2C and N-doped carbon nanotubes (N-CNT), Nb2C is two-dimensional organ shape, and N-CNT is clamped in Nb2And C, between the C sheets. Improve Nb2The conductive capability of C, the insertion of N-CNT, avoid Nb2Stacking between C-layer structures, N-CNT improves Nb2The ion transmission capability of C improves the electrochemical reaction area. The preparation method comprises the following steps: nb2Etching AlC to obtain Nb2C；Nb2Dispersing C in water to obtain Nb2C, colloidal solution; N-CNT solution and Nb2And C, mixing the colloidal solution to perform self-assembly reaction to obtain the sandwich structure composite material. Has excellent electrical performance.

Description

Sandwich structure composite material and preparation method and application thereof
Technical Field
The invention belongs to the technical field of electrode materials, and particularly relates to a sandwich structure composite material and a preparation method and application thereof.
Background
The information in this background section is only for enhancement of understanding of the general background of the invention and is not necessarily to be construed as an admission or any form of suggestion that this information forms the prior art that is already known to a person of ordinary skill in the art.
Energy storage devices such as batteries play an important role in mobile devices. Alkali metal ion batteries, including lithium, sodium, and potassium ion batteries, are widely studied as rechargeable batteries. Their energy storage mechanism is similar to that of rocking chair batteries, in which lithium, sodium and potassium ions are continuously inserted and extracted at the two ends of positive and negative electrodes. The lithium ion battery has high open-circuit voltage, large energy density and power, but the lithium metal is expensive and has less reserve in the earth crust, which restricts the development of the lithium ion battery. The sodium and potassium metal reserves are abundant, the price is low, and the cathode material has potential to be used as the cathode material of the next generation battery. However, they have a low open circuit voltage and a large ionic radius, so that it is difficult to find a suitable electrode material. Moreover, all alkali metal batteries are subject to dendrite growth, which can cause severe performance degradation and safety concerns.
Successful preparation of two-dimensional materials such as graphene opens up a new idea for finding suitable electrode materials of rechargeable batteries. Compared with one-dimensional and three-dimensional materials, the two-dimensional material can have more ion channels and huge specific surface area, and provides a large amount of active sites and ion storage space for reaction, so that the two-dimensional material has excellent electrochemical performance as an ion battery material. Recently, a new two-dimensional material MXene has attracted much attention. It is aThe carbide or nitride of transition metal is prepared by selectively etching away Al layer from ceramic MAX phase. Mn+1XnTxThe compound has a chemical formula, wherein M represents transition metal elements such as Ti, Nb, V, Cr and Mo, N is 1,2 or 3.X is C or N, and T represents terminal groups such as O, OH, F, Cl and S. MXene materials have properties similar to graphene and good electrical conductivity. Because of the abundance of terminal groups, the fusion between the terminal groups and different solvents is better, but the existence of the terminal groups can also damage the conductivity of the conductive material to some extent. MXene has weak electric conduction capability, and the sheets of the material are easy to be stacked again under the van der Waals force, so that the ion transmission is hindered, and the electrochemical reaction area is reduced.
Disclosure of Invention
In view of the above problems in the prior art, the present invention is directed to a sandwich structure composite material, and a method for preparing the same and applications thereof. The defect of poor MXene conductive capability is overcome by compounding MXene and the carbon nano tube. The carbon nanotubes are filled between the MXene sheets and used as a spacer substance to provide more ion transmission and storage positions for the material, so that ions with larger radius, such as potassium ions, can be easily inserted into the material. The nitrogen doping process of the carbon nanotube can add external defects on the tube wall of the carbon nanotube to promote alkali metal ions to enter the tube and increase storage space for the alkali metal ions.
In order to solve the technical problems, the technical scheme of the invention is as follows:
in a first aspect, a sandwich structured composite material (N-CNT @ MXene) comprises Nb2C and N-doped carbon nanotubes (N-CNT), Nb2C is two-dimensional organ shape, and N-CNT is clamped in Nb2And C, between the C sheets.
Nb2C is Nb2And etching the Al layer by using AlC to obtain a two-dimensional organ shape, wherein the two-dimensional organ shape is a shape with a plurality of lamellar intervals which can be seen through a scanning electron microscope, and then inserting N-CNT into the lamellar intervals to form the sandwich structure composite material. N-CNT increasing Nb2The conductive capability of C, the insertion of N-CNT, avoid Nb2Stacking between C-sheet structures, N-CNT enhancement of Nb2The ion transmission capability of C improves the electrochemical reaction area。
In some embodiments of the invention, Nb2The mass ratio of C to N-CNT is 1: 0.08-0.12; preferably 1: 0.1. The reason for choosing this mass ratio range is: the density of N-CNT is small, Nb if N-CNT with excessive mass is added2The role of C in it is very insignificant. This ratio is preferably chosen to form N-CNTs sandwiched between Nb2C sandwich structure in the middle.
In a second aspect, the preparation method of the sandwich structure composite material comprises the following specific steps:
Nb2etching AlC to obtain Nb2C；
Nb2Dispersing C in water to obtain Nb2C, colloidal solution;
N-CNT solution and Nb2And C, mixing the colloidal solution to perform self-assembly reaction to obtain the sandwich structure composite material (N-CNT @ MXene).
In some embodiments of the invention, Nb2The AlC etching method comprises the step of etching Nb2Selectively etching AlC with HF solution to remove Al layer and obtain Nb2C。
Preferably, Nb2Crushing is carried out before AlC etching. Preferably, the mass fraction of HF is 35-45%. Preferably, Nb2The etching time for mixing AlC and HF solution is 20-30 h; further preferably, the etching time is 24-25 h. Preferably, the mixture is separated to obtain Nb2The method C comprises the following steps: centrifuging, collecting the precipitate, cleaning and drying. Further preferably, the centrifugal rotation speed is 3000-5000 rpm.
In some embodiments of the invention, Nb2The preparation method of AlC comprises the following steps: mixing Nb, Al and NbC, ball-milling, and reacting in inert atmosphere to obtain Nb2AlCrax phase.
Preferably, the mass ratio of Nb to Al to NbC is 2:1-1.5: 1; preferably 2:1.2: 1. Preferably, the time of ball milling is 20-40 min.
Preferably, the reaction temperature is 1300-1500 ℃, and the reaction time is 1.5-3 h; preferably, the reaction temperature is 1350-1450 ℃ and the reaction time is 2-3 h.
In some embodiments of the invention, Nb2The preparation method of the C colloid solution comprises the following steps: at normal temperature, Nb is added2Dispersing C into deionized water, ultrasonic dispersing, centrifuging, and collecting upper liquid to obtain Nb2C, colloidal solution. Nb2Dispersing C in deionized water, ultrasonic dispersing to improve dispersing effect, and centrifuging to remove Nb with more layers2C and Nb with a small number of layers2C is separated, and some Nb with a small number of layers is remained in the upper liquid2C. Since Nb2C is a multi-layered structure, and if the number of layers is more, the mass is larger, and the mass sinks to the bottom through the centrifugal part. Nb with less number of reserved layers2The purpose of C is: conveniently clamp N-CNT at every two Nb layers as much as possible2And C, the advantages of the sandwich structure are exerted to the maximum.
Preferably, Nb2The concentration of the C colloidal solution is 4-6 mg/mL. Preferably, Nb2And C is carried out in an inert atmosphere in the process of dispersing into the deionized water, wherein the inert atmosphere is argon or nitrogen.
In some embodiments of the invention, the solvent of the N-CNT solution is a surfactant, which may be cetyl trimethylammonium bromide (CTAB) or a polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymer (P123). Preferably, the mass ratio of the N-CNT in the N-CNT solution to the surfactant is 5-8; preferably 6 to 7. Preferably, the mass concentration of the N-CNT in the N-CNT solution is 0.04-0.06 g/mL.
In some embodiments of the invention, the N-CNT solution and Nb2The volume ratio of the colloidal solution C is 9-11: 1.
In some embodiments of the present invention, the temperature of the self-assembly reaction is normal temperature, the self-assembly reaction is performed with ultrasonic dispersion and then is performed with standing, the time of ultrasonic dispersion is 0.5-1.5h, and the time of standing is 4-6 h. Firstly, the N-CNT solution and Nb are dispersed by ultrasound2And fully mixing the C colloidal solution, and carrying out a self-assembly reaction process to obtain the N-CNT @ MXene.
In a third aspect, the sandwich structure composite material is applied to the field of electrode materials.
In a fourth aspect, an electrode material includes the above sandwich structure composite material.
In some embodiments of the invention, the electrode material further comprises polyvinylidene fluoride and acetylene black, and the mass ratio of the composite material with the sandwich structure to the polyvinylidene fluoride to the acetylene black is 7-9:1: 1; preferably 8:1: 1.
In a fifth aspect, a battery comprises a negative electrode and a positive electrode, wherein the positive electrode comprises the electrode material, and the negative electrode is lithium, sodium, potassium or liquid sodium-potassium alloy.
In some embodiments of the present invention, the liquid sodium-potassium alloy is prepared by the following steps: and (3) carrying out physical contact on solid sodium and potassium, and applying an external force, so that the solid sodium and the solid potassium are mutually dissolved to form the sodium-potassium alloy.
In some embodiments of the invention, the sodium-potassium alloy negative electrode is prepared by: and heating the sodium-potassium alloy, and automatically adsorbing the alloy on the foamed nickel to obtain the sodium-potassium alloy cathode. Preferably, the heating temperature is 400-450 ℃.
In a sixth aspect, the battery is applied to the field of energy storage devices.
One or more technical schemes of the invention have the following beneficial effects:
N-CNT@Nb2c is N-CNT and Nb2C recombination, N-CNT insertion into Nb2C in a layered structure. Improve Nb2Conductivity of C, avoiding Nb2The laminated structure of C is stacked again under the action of Van der Waals force, so that the ion transmission capability is improved, the electrochemical reaction area is increased, and the stability of the structure is improved. More ion transport and storage sites are provided for the material so that larger radius ions, such as potassium ions, can be easily inserted therein. The nitrogen doping process of the carbon nanotube can add external defects on the tube wall of the carbon nanotube to promote alkali metal ions to enter the tube and increase storage space for the alkali metal ions.
N-CNT@Nb2The C composite material has excellent electrochemical properties in lithium, sodium, liquid sodium-potassium alloy and potassium ion batteries, and has good effects on cycle performance, rate performance and electrochemical impedance performance. N-CNT @ Nb2C composite material will be alkaliMetal-ion batteries offer better electrode material selection and promote their commercial development.
Drawings
The accompanying drawings, which are incorporated in and constitute a part of this specification, are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description, serve to explain the invention and not to limit the invention.
FIG. 1 shows N-CNT @ Nb2C, a preparation flow chart and a scanning electron microscope picture corresponding to the product at each stage, wherein a and e are MAX phases (Nb) of the block2AlC), b and f are Nb2C, C and g are Nb2C, single-layer colloid solution, d and h are after self-assembly;
FIG. 2 is Nb2AlC MAX phase, Nb2C MXene,N-CNT,N-CNT@Nb2An XRD spectrum of C;
FIG. 3 is N-CNT @ Nb2A transmission electron microscope photo of C MXene, wherein C is an electron diffraction photo selected by b;
FIG. 4 is N-CNT @ Nb2C, taking lithium, sodium, liquid sodium-potassium alloy and potassium as a cycle curve and a charge-discharge voltage curve of the negative electrode, taking lithium as the negative electrode, taking sodium as the negative electrode, taking liquid sodium-potassium alloy as the negative electrode, taking potassium as the negative electrode, taking s as 1st, taking j as 50th, taking k as 100th and taking z as 500 th;
FIG. 5 shows N-CNT @ Nb with lithium, sodium, liquid sodium potassium alloy and potassium as negative electrodes2C, a cyclic voltammetry curve, wherein a is the negative electrode of lithium, b is the negative electrode of sodium, C is the negative electrode of liquid sodium-potassium alloy, d is the negative electrode of potassium, and s is 1 st;
FIG. 6 shows N-CNT @ Nb with lithium, sodium, liquid sodium potassium alloy and potassium as negative electrodes2A multiplying power performance curve of C, wherein a is the negative electrode of lithium, b is the negative electrode of sodium, C is the negative electrode of liquid sodium-potassium alloy, and d is the negative electrode of potassium;
FIG. 7 shows N-CNT @ Nb with lithium, sodium, liquid sodium potassium alloy and potassium as negative electrodes2And C, taking lithium as a negative electrode, taking sodium as a negative electrode, taking a liquid sodium-potassium alloy as a negative electrode, and taking potassium as a negative electrode.
Detailed Description
It is to be understood that the following detailed description is exemplary and is intended to provide further explanation of the invention as claimed. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.
It is noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments according to the present application. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, and it should be understood that when the terms "comprises" and/or "comprising" are used in this specification, they specify the presence of stated features, steps, operations, devices, components, and/or combinations thereof, unless the context clearly indicates otherwise.
Nb in the following examples2The preparation method of AlC MAX comprises the following steps:
MXene Nb2c is by selective etching of MAX phase Nb2Al layer in AlC. Nb is obtained by mixing Nb, Al and NbC at a ratio of 2:1.2:1, ball milling for 30 minutes, and heating at 1400 ℃ for two hours under argon atmosphere2AlC MAX phase.
The invention will be further illustrated by the following examples
Example 1
2g of Nb2The AlC was ball milled for 30 minutes to smaller particles and then selectively etched with 40% HF, and the mixture was stirred for 24 hours and centrifuged at 4000 rpm. Collecting the precipitate and repeatedly cleaning until the pH value of the supernatant is 6-7. After drying, Nb is obtained2C. Then Nb is added2Dispersing the C into a large amount of deionized water, continuously introducing argon gas for ultrasonic dispersion, and keeping the temperature at 25 ℃. The solution was then centrifuged at 3500rpm for 1 hour. Collecting the upper liquid, i.e. MXene Nb with a smaller number of layers2And C, colloidal solution. 1.55mg of nitrogen-doped carbon nanotubes (N-CNT) prepared by a chemical vapor deposition method and 0.25g of cetyltrimethylammonium bromide (CTAB) were mixed into 30ml of deionized water and stirred and ultrasonically dispersed for 30 min. Followed by 3mL of 5mg/mL Nb2C colloidal solution was added dropwise to the mixtureAnd sonicated for 1 hour with argon gas sparge. Let stand for 5 hours to allow it to complete self-assembly. And centrifuging and drying to obtain the sandwich structure N-CNT @ MXene.
The preparation process and the scanning electron micrograph are shown in figure 1, wherein a and e in figure 1 are MAX phases of the bulk, the surface area of which is small, and Nb is2The C and Al layers are stacked on top of each other leaving no room for ion transport. After the Al layer is selectively etched away, the MAX phase of the bulk is changed into two-dimensional organ-shaped Nb2C Mxene. The material has larger specific surface area and a large number of storage sites, and can be used as a battery material with excellent performance, such as b and f in figure 1. But sheets of pure MXene material are easily stacked together again so some sheet spacer is needed to fix the structure. In order to further enlarge the inter-lamellar spacing and storage space, the accordion Nb2The C Mxene was ultrasonically dispersed into individual pieces as shown in fig. 1C and g. With the aid of CTAB, Nb2The C sheet and N-CNT self-assembly form a sandwich structure, wherein the N-CNT is clamped in the Nb2C, e.g., d and h in fig. 1. With the N-CNTs as lamellar spacing, the overall structure stability is enhanced, as are the conductivity and lamellar spacing. The nitrogen doping sites on the N-CNTs not only improve the conductivity of the carbon nanotubes, but also serve as an external defect through which ions can be transported and stored inside the carbon nanotubes, increasing the storage space.
FIG. 2 is Nb2AlC MAX phase, Nb2C MXene,N-CNT,N-CNT@Nb2XRD pattern of C, and Nb2AlC,Nb2The major peaks of C, and N-CNT are marked in the figure with stars, squares and triangles. As can be seen in FIG. 2, Nb2The AlC crystal phase was matched to PDF #30-0033, with four major peak locations at 13 °,26 °,39 ° and 52 °, corresponding to (002), (004), (103) and (106) crystal planes, respectively. No bulge in the spectrum indicates good crystallinity. Nb2The crystal phase of C is matched with PDF #75-2169, the three main peaks correspond to crystal planes (201), (211) and (002), and the three main peaks are positioned at 33 degrees, 38 degrees and 60 degrees. The pattern has bumps because the etching process has an effect on its structure. N-CNT has only one main peak at 26 deg., corresponding to crystal plane (002) (PDF # 75-1621). End product N-CNT @ Nb2Main peak of C and Nb2C MXene and N-CNT are matched and no other crystalline phases are present. In addition, pure Nb2C and N-CNT @ Nb2Nb in C2Slightly modified (211) crystal plane position of C, N-CNT @ Nb2Nb in C2The peak position of C is shifted by 1 ° to the left as shown in fig. 2. This indicates that insertion of N-CNT expands Nb2The spacing between the C sheets provides space and channels for more ions to be inserted.
FIG. 3 b is a transmission electron micrograph showing Nb2The sheet Stacking structure of C with N-CNT sandwiched between sheets demonstrates N-CNT @ Nb2And C, successfully preparing. The lattice phase of N-CNT is also shown, with a interplanar spacing of 0.34nm, corresponding to the (002) crystal plane of N-CNT. The selected area electron diffraction pattern of the c diagram in FIG. 2 shows Nb2Diffraction rings for C and N-CNT as indicated by the indices.
Example 2
2g of Nb2The AlC was ball milled for 30 minutes to smaller particles and then selectively etched with 40% HF, and the mixture was stirred for 24 hours and centrifuged at 4000 rpm. Collecting the precipitate and repeatedly cleaning until the pH value of the supernatant is 6-7. After drying, Nb is obtained2C. Then Nb is added2Dispersing the C into a large amount of deionized water, continuously introducing argon gas for ultrasonic dispersion, and keeping the temperature at 25 ℃. The solution was then centrifuged at 3500rpm for 1 hour. Collecting the upper liquid, i.e. MXene Nb with a smaller number of layers2And C, colloidal solution. 1.55mg of nitrogen-doped carbon nanotubes (N-CNT) prepared by a chemical vapor deposition method and 0.25g of cetyltrimethylammonium bromide (CTAB) were mixed into 27ml of deionized water and stirred and ultrasonically dispersed for 30 min. Followed by 3mL of 5mg/mL Nb2The colloidal solution C was added dropwise to the mixture and sonicated under argon for 1 hour. Let stand for 5 hours to allow it to complete self-assembly. And centrifuging and drying to obtain the sandwich structure N-CNT @ MXene.
Example 3
2g of Nb2The AlC was ball milled for 30 minutes to smaller particles and then selectively etched with 40% HF, and the mixture was stirred for 24 hours and centrifuged at 4000 rpm. Collecting the precipitate and repeatedly cleaningWashing until the pH value of the supernatant is 6-7. After drying, Nb is obtained2C. Then Nb is added2Dispersing the C into a large amount of deionized water, continuously introducing argon gas for ultrasonic dispersion, and keeping the temperature at 25 ℃. The solution was then centrifuged at 3500rpm for 1 hour. Collecting the upper liquid, i.e. MXene Nb with a smaller number of layers2And C, colloidal solution. 1.8mg of nitrogen-doped carbon nanotubes (N-CNT) prepared by a chemical vapor deposition method and 0.25g of cetyltrimethylammonium bromide (CTAB) were mixed into 30ml of deionized water and stirred and ultrasonically dispersed for 30 min. Followed by 3mL of 5mg/mL Nb2The colloidal solution C was added dropwise to the mixture and sonicated under argon for 1 hour. Let stand for 5 hours to allow it to complete self-assembly. And centrifuging and drying to obtain the sandwich structure N-CNT @ MXene.
Comparative example 1
Compared with example 1, the nitrogen-doped carbon nanotube (N-CNT) is changed into a Carbon Nanotube (CNT), and the obtained sandwich structure is CNT @ MXene.
The biological conductivity of the N-CNT @ MXene is better than that of the CNT @ MXene. The N-CNT serving as the interlayer of the MXene material can improve the conductivity of the MXene material, further enlarge the interlayer spacing of the MXene material and provide space for ion transmission and storage.
Example 4
A battery, the negative electrode is a liquid sodium-potassium alloy negative electrode, and the preparation method of the liquid sodium-potassium alloy is that solid sodium and potassium (the mass ratio of sodium is 9.2-58.2%) are in physical contact and are mutually dissolved to form the sodium-potassium alloy after external force is applied. After the alloy is heated to 420 ℃, the alloy is automatically adsorbed on the foamed nickel to obtain the sodium-potassium alloy cathode which can be used for a battery.
The positive electrode is N-CNT @ MXene with a sandwich structure, and the preparation method of the positive electrode comprises the following steps: mixing N-CNT @ MXene, polyvinylidene fluoride and acetylene black in a sandwich structure in a ratio of 8:1:1, adding N-methylpyrrolidone, and stirring for 24 hours. The mixed slurry was then coated on a copper foil and vacuum dried at 60 ℃ for 10 hours. Thus, a positive electrode was obtained.
Example 5
A battery with a negative electrode of metallic sodium and a positive electrode of the sandwich structure of N-CNT @ MXene prepared in example 1.
The preparation method of the anode comprises the following steps: mixing N-CNT @ MXene, polyvinylidene fluoride and acetylene black in a sandwich structure in a ratio of 8:1:1, adding N-methylpyrrolidone, and stirring for 24 hours. The mixed slurry was then coated on a copper foil and vacuum dried at 60 ℃ for 10 hours. Thus, a positive electrode was obtained.
Example 6
A battery with potassium metal as the negative electrode and N-CNT @ MXene with a sandwich structure prepared in example 1 as the positive electrode.
The preparation method of the anode comprises the following steps: mixing N-CNT @ MXene, polyvinylidene fluoride and acetylene black in a sandwich structure in a ratio of 8:1:1, adding N-methylpyrrolidone, and stirring for 24 hours. The mixed slurry was then coated on a copper foil and vacuum dried at 60 ℃ for 10 hours. Thus, a positive electrode was obtained.
Example 7
A battery with a negative electrode of lithium metal and a positive electrode of the sandwich structure of N-CNT @ MXene prepared in example 1.
The preparation method of the anode comprises the following steps: mixing N-CNT @ MXene, polyvinylidene fluoride and acetylene black in a sandwich structure in a ratio of 8:1:1, adding N-methylpyrrolidone, and stirring for 24 hours. The mixed slurry was then coated on a copper foil and vacuum dried at 60 ℃ for 10 hours. Thus, a positive electrode was obtained.
Examples of the experiments
Electrochemical performance tests were performed using the cells of example 4, example 5, example 6, and example 7, wherein the positive electrode was made into a small disk and the cell was packed in a glove box. The glass fiber filter membrane is used as a battery diaphragm. The battery assembly is completed in a glove box.
The battery voltage interval is set to be 0.01-3V, the current density used in the cycle performance test is 50mA/g, the current density used in the multiplying power performance test is 50,100,200,500,200,100 and 50mA/g, the sweeping speed of a CV curve is 0.5mV/s, and the testing frequency range of an Electrochemical Impedance Spectroscopy (EIS) is 1MHz-0.01 Hz.
(1) Cycle performance
The cycle performance results are shown in fig. 4, and it can be seen from the graphs a-d in fig. 4 that the cycle can be performed for 500 cycles (500th) or more in all the battery systems. When the metallic lithium is used as the negative electrode, the cycle is very stableThe initial specific capacity was 1587.2mAh/g (including irreversible capacity of SEI film formation). N-CNT @ Nb in lithium ion batteries after brief drop and adjustment2The capacity of C will rise again. At 500 weeks, the capacity is as high as 650mAh/g, and the capacity retention rate is 90%. This capacity increase is due to the internal structure adjustment, further activation of the active material and partial amorphization. And with the further insertion of lithium ions, the inter-lamellar spacing of the material is enlarged and strengthened, thereby promoting circulation and improving capacity. Good cycling Performance N-CNT @ Nb2The C cycle and the capability of storing lithium ions are stronger. N-CNT @ Nb when sodium is used as the negative electrode2C fluctuates briefly at 160 weeks due to the reaction mechanism and structural changes characteristic in sodium ion batteries. After conditioning, N-CNT @ Nb2C also exhibits good stability in sodium ion batteries. N-CNT @ Nb when liquid sodium-potassium alloy is used as the negative electrode2C shows better cycle performance due to the dendrite suppression effect of the liquid negative electrode.
As can be seen from the e-h diagram in fig. 4, in the upward trend curve of f, 50 weeks (50th), 100 weeks (100th) and 500 weeks (500th) substantially coincide, and in the downward trend curve, 50 weeks, 100 weeks and 500 weeks substantially coincide; g, the 100 weeks and the 500 weeks in the graph are basically coincided, and in the downward trend curve, the 50 weeks, the 100 weeks and the 500 weeks are basically coincided; in the graph h, the downward trend curves are substantially coincident with 50 weeks, 100 weeks, and 500 weeks.
The overlap of the charge-discharge curves of all the cells except the first week (1st) in the number of the latter weeks was good, indicating that the reversibility of the cells was good. In fig. e, there is a small plateau at the 1.6V position and a slightly longer plateau at the 0.75V position, which are due to decomposition of the electrolyte and formation of the SEI film, respectively. The same plateaus can be seen in fig. f-h, but the plateaus of electrolyte decomposition are sometimes less pronounced. The cell response that contributes most to capacity is N-CNT @ Nb2The insertion and extraction of ions in C occurs at a position where the voltage is low. In addition, N-CNT @ Nb with a liquid sodium-potassium alloy as a negative electrode can be noted2The C-curve has a higher overlap, since it suppresses dendrite growthThe high reversibility due to long performance.
(2) Cyclic voltammetric properties
Cyclic voltammetry behavior is shown in fig. 5 a-d, where the curves at 2 and 3 weeks are substantially coincident, and the two irreversible peaks in graph a appear at the 1.6 and 0.75V positions, corresponding to the electrolyte decomposition and SEI film formation reactions, consistent with the charge and discharge plateau positions described above. Similar two irreversible peaks can be observed in other battery systems, but when the metal potassium is used as a negative electrode, the two peaks are close to each other, so that the two peaks are combined into one. Ionic intercalation of N-CNT @ Nb in all cell systems2The reversible process between the lamellae and inside the carbon nanotubes in C occurs at the 0.01V position. When lithium, sodium, liquid sodium-potassium alloy and potassium are used as negative electrodes, the corresponding ion extraction processes are 0.25V,0.1V,0.3V and 0.3V. The cyclic voltammetry curves of the liquid sodium-potassium alloy and the metal potassium are similar, which is also because the liquid sodium-potassium alloy can be used as the negative electrode of the potassium-ion battery. In a potassium ion battery system with potassium and liquid sodium-potassium alloy as a negative electrode, potassium ions are separated from N-CNT @ Nb2The peak of C elution is split into two, the first anodic peak is due to the particles escaping from between the MXene sheets and initially separating from the carbon nanotubes, and the second anodic peak is due to the particles separating from the nitrogen doping sites of the carbon nanotubes.
(3) Rate capability
Rate performance as shown in a-d of fig. 6, it can be seen from fig. 6 that all cells exhibit good rate performance. When the current density is increased, the capacity decays slowly; when the current density is gradually reduced to the initial value, the capacity can be largely restored to the initial level. This illustrates N-CNT @ Nb2C has the ability to withstand large current surges while maintaining structural functionality. N-CNT @ Nb with lithium, sodium, liquid sodium-potassium alloy and potassium as negative electrodes2The rate capacity retention of C was 100%, 75%, 67%, 65%. This illustrates N-CNT @ Nb2The C composite material has stable structure and strong conductivity, and can be used in high-current occasions. We can also observe that the change of the capacity value is very small when the liquid sodium-potassium alloy is used as the negative electrode, which shows that the negative electrode can bear large current rushAnd the surface is kept smooth and free of dendrites, so that the electrochemical performance of the battery is better.
(4) Electrochemical impedance performance
Electrochemical impedance properties are shown in a-d of FIG. 7. from FIG. 7, it can be seen that the intercept at the Z' axis of the high frequency portion, the size of the semicircle, and the slope of the slope at the low frequency correspond to the ohmic resistance, the interfacial resistance, and the Warburg impedance, respectively. Ohmic resistance, i.e., the internal resistance of the electrolyte and electrodes, interface resistance and Warburg resistance, are caused by charge transport and mass transport at the interface. It can be seen that the semicircle of the impedance spectrum is split into two when lithium is used as the negative electrode, because the capacitance of the SEI film is decreased and the pseudocapacitance of the electric double layer is increased during the process of forming the SEI film (i.e., the first intercalation and first charging process of lithium ions), so that the semicircle of the SEI film and the semicircle of the charge transport are moved to high and low frequencies, respectively. So N-CNT @ Nb2The interfacial resistance of C with lithium as the negative electrode can be obtained by the sum of the two circle sizes. It can be seen that the ohmic resistance values of all battery systems are similar, but the interface resistances differ significantly. N-CNT @ Nb with lithium, sodium and liquid sodium-potassium alloy as negative electrode2The interface resistance of C is smaller, but the interface resistance of solid potassium as a cathode is relatively larger, which also proves that the electrochemical performance of the liquid sodium-potassium alloy is better than that of the solid potassium.
As can be seen from the above, N-CNT @ Nb2The electrochemical performance of C in lithium, sodium and potassium ion batteries is excellent. When the liquid sodium-potassium alloy is used as the negative electrode of the potassium ion battery instead of solid potassium, the composite material has better electrochemical performance due to the property that the liquid surface of the composite material does not generate dendrite. N-CNT @ Nb2The C composite material will provide a better electrode material choice for alkali metal ion batteries and promote their commercial development.
The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims (10)

1. A sandwich structure composite material is characterized in that: comprising Nb2C and N-doped carbon nanotubes (N-CNT), Nb2C is two-dimensional organ shape, and N-CNT is clamped in Nb2And C, between the C sheets.
2. The sandwich structured composite material of claim 1 wherein: nb2The mass ratio of C to N-CNT is 1: 0.08-0.12; preferably 1: 0.1.
3. The method for preparing a composite material of sandwich structure according to claim 1 or 2, characterized in that: the method comprises the following specific steps:
Nb2etching AlC to obtain Nb2C；
Nb2Dispersing C in water to obtain Nb2C, colloidal solution;
N-CNT solution and Nb2And C, mixing the colloidal solution to perform self-assembly reaction to obtain the sandwich structure composite material.
4. The method of preparing a sandwich structured composite material according to claim 3, wherein: nb2The AlC etching method comprises the step of etching Nb2Selectively etching AlC with HF solution to remove Al layer and obtain Nb2C；
Preferably, Nb2Crushing before AlC etching;
preferably, the mass fraction of HF is 35-45%;
preferably, Nb2The etching time for mixing AlC and HF solution is 20-30 h; further preferably, the etching time is 24-25 h;
preferably, the mixture is separated to obtain Nb2The method C comprises the following steps: centrifuging, collecting the precipitate, cleaning and drying.
5. The method of preparing a sandwich structured composite material according to claim 3, wherein: nb2The preparation method of AlC comprises the following steps: mixing Nb, Al and NbC, ball-milling, and performing inert millingReacting in an atmosphere to obtain Nb2AlCMAX phase;
preferably, the mass ratio of Nb to Al to NbC is 2:1-1.5: 1; preferably 2:1.2: 1;
preferably, the ball milling time is 20-40 min;
preferably, the reaction temperature is 1300-1500 ℃, and the reaction time is 1.5-3 h; preferably, the reaction temperature is 1350-1450 ℃ and the reaction time is 2-3 h.
6. The method of preparing a sandwich structured composite material according to claim 3, wherein: nb2The preparation method of the C colloid solution comprises the following steps: at normal temperature, Nb is added2Dispersing C into deionized water, ultrasonic dispersing, centrifuging, and collecting upper liquid to obtain Nb2C, colloidal solution;
preferably, Nb2The concentration of the C colloidal solution is 4-6 mg/mL;
preferably, Nb2C is carried out in an inert atmosphere in the process of dispersing into deionized water;
or, the solvent of the N-CNT solution is a surfactant, and the surfactant can be cetyl trimethyl ammonium bromide or polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymer;
preferably, the mass ratio of the N-CNT in the N-CNT solution to the surfactant is 5-8; preferably 6 to 7;
preferably, the mass concentration of the N-CNT in the N-CNT solution is 0.04-0.06 g/mL;
or, N-CNT solution and Nb2The volume ratio of the colloidal solution C is 9-11: 1;
or, the temperature of the self-assembly reaction is normal temperature, the self-assembly reaction is firstly carried out ultrasonic dispersion and then is carried out standing, the time of the ultrasonic dispersion is 0.5-1.5h, and the time of the standing is 4-6 h.
7. Use of a sandwich structured composite material according to claim 1 or 2 in the field of electrode materials.
8. An electrode material, characterized in that: comprising the sandwich structured composite material of claim 1 or 2;
the electrode material also comprises polyvinylidene fluoride and acetylene black, wherein the sandwich structure composite material and the mass ratio of the polyvinylidene fluoride to the acetylene black are 7-9:1: 1; preferably 8:1: 1.
9. A battery, characterized by: the composite material comprises a negative electrode and a positive electrode, wherein the positive electrode comprises the sandwich structure composite material, and the negative electrode is lithium, sodium, potassium or liquid sodium-potassium alloy;
preferably, the preparation method of the liquid sodium-potassium alloy comprises the following steps: carrying out physical contact on solid sodium and potassium, and applying external force, and mutually dissolving the solid sodium and the solid potassium to form a sodium-potassium alloy;
preferably, the preparation method of the sodium-potassium alloy negative electrode comprises the following steps: heating the sodium-potassium alloy, and automatically adsorbing the alloy on foamed nickel to obtain a sodium-potassium alloy cathode; preferably, the heating temperature is 400-450 ℃.
10. Use of the cell of claim 9 in the field of energy storage devices.
CN202010836661.3A 2020-08-19 2020-08-19 Sandwich structure composite material and preparation method and application thereof Pending CN111952568A (en)

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Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN106025200A (en) * 2016-05-24 2016-10-12 浙江大学 Preparation method and application of nitrogen-doped MXene battery anode material
US20180108910A1 (en) * 2015-04-20 2018-04-19 Drexel University Two-dimensional, ordered, double transition metals carbides having a nominal unit cell composition m'2m"nxn+1
CN109167066A (en) * 2018-09-03 2019-01-08 济南大学 A kind of preparation method of few layer titanium carbide growth in situ nitrogen-doped carbon nanometer pipe three-dimensional composite material
CN109671576A (en) * 2018-12-12 2019-04-23 福建翔丰华新能源材料有限公司 Carbon nano tube-MXene composite three-dimensional porous carbon material and preparation method thereof

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20180108910A1 (en) * 2015-04-20 2018-04-19 Drexel University Two-dimensional, ordered, double transition metals carbides having a nominal unit cell composition m'2m"nxn+1
CN106025200A (en) * 2016-05-24 2016-10-12 浙江大学 Preparation method and application of nitrogen-doped MXene battery anode material
CN109167066A (en) * 2018-09-03 2019-01-08 济南大学 A kind of preparation method of few layer titanium carbide growth in situ nitrogen-doped carbon nanometer pipe three-dimensional composite material
CN109671576A (en) * 2018-12-12 2019-04-23 福建翔丰华新能源材料有限公司 Carbon nano tube-MXene composite three-dimensional porous carbon material and preparation method thereof

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
AYEONG BYEON等: "Lithium-ion capacitors with 2D Nb2CTx (MXene) e carbon nanotube electrodes", 《JOURNAL OF POWER SOURCES》 *

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