CN110850301A - MoS2/graphene/MoS2Sandwich structure and Na-ion battery capacity prediction method - Google Patents

MoS2/graphene/MoS2Sandwich structure and Na-ion battery capacity prediction method Download PDF

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CN110850301A
CN110850301A CN201911138168.8A CN201911138168A CN110850301A CN 110850301 A CN110850301 A CN 110850301A CN 201911138168 A CN201911138168 A CN 201911138168A CN 110850301 A CN110850301 A CN 110850301A
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温新竹
秦少平
刘明真
杨雪婷
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Abstract

The invention discloses a MoS2/graphene/MoS2A sandwich structure and a Na-ion battery capacity prediction method belong to the field of physical chemistry. Aiming at solving the problems that the prior art has the precision of atomic scale which cannot be achieved by experimental means and the experimental environment is difficult to realize MoS2/graphene/MoS2Material Synthesis, MoS2The theoretical capacity of Na ions of the two-dimensional nano-layered material is not high enough, and the Na ions are in the materialThe optimal diffusion problem of (2). The invention constructs a brand new three-layer MoS through a first principle2/graphene/MoS2The sandwich structure material firstly calculates the stable structure of the material under corresponding pressure, obtains the optimal interlayer spacing when in stable structure, calculates the relevant characteristics of possible electron migration, and predicts the Na ion embedding/extracting performance of the material according to the result, and the result shows that the material can be used as a very potential Na Ion Battery (NIB) electrode (cathode) material.

Description

MoS2/graphene/MoS2Sandwich structure and Na ionMethod for predicting capacity of sub-battery
Technical Field
The invention relates to a MoS2/graphene/MoS2A sandwich structure and a Na-ion battery capacity prediction method belong to the field of physical chemistry.
Background
Two-dimensional (2D) atomic crystals generally have different properties from their three-dimensional (3D) bulk crystals, and graphene was the most studied 2D material in the past years, grown by mechanical cutting and chemical lift-off of 2D monoliths, or by chemical vapor deposition, however, only the simple chemistry of C — C bonds may limit its practical application.
With the continuous and intensive research on graphene and its derived two-dimensional layered structure, complex layered materials composed of more than one element may offer new opportunities because their structural components are varied and can be tailored to specific properties and applications, especially within two-dimensional layered transition metal-sulfur compounds (with structures very similar to graphite) whose physical properties are induced by the motion of d-electrons, and thus are of interest to many researchers. MoS in transition metal-sulfur compounds2Is unique in condensed state, MoS2Instead of a layered planar structure in graphite, it is composed of three atomic layers, where the Mo atoms in a triangular prism coordination are sandwiched between two layers of pyramidal S atoms. Like graphite, these MoS' s2The layers are held together by weak Van Der Waals (VDW) interactions. The unique structure endows MoS2 with a plurality of excellent performances such as catalysis, photovoltaics and lubricants, and simultaneously provides enough interlayer space for the insertion of foreign molecules and atoms due to the unique layered structure, reversible Li + insertion/extraction of lithium ions is highly supported, the unique layered structure is long considered as an ideal electrode material of a high-grade Lithium Ion Battery (LIB), and a large number of experiments prove the effectiveness of the unique layered structure. Research on Na ion battery electrode material through first-nature principle calculation and establishment of Na ionThe theoretical basis for selecting the electrode material of the sub-battery can not only reduce the high experimental cost, but also can more accurately develop some electrode materials with better performance. Currently, few studies are being made on the theoretical simulation of NIB electrode materials.
However, in the prior art, the accuracy of atomic scale which cannot be achieved by experimental means exists, and the MoS is difficult to realize in experimental environment2/graphene/MoS2Material Synthesis, MoS2The theoretical capacity of Na ions of the two-dimensional nano-layered material is not high enough, and the Na ions are optimally diffused in the material.
Disclosure of Invention
Aiming at the defects in the prior art, the invention aims to provide a MoS2/graphene/MoS2A sandwich structure and a Na-ion battery capacity prediction method are used for solving the problems that in the prior art, the accuracy of atomic scale cannot be achieved by experimental means, and the MoS is difficult to realize in experimental environment2/graphene/MoS2Material Synthesis, MoS2The two-dimensional nano-layered material has the problems that the theoretical capacity of Na ions is not high enough and the Na ions are optimally diffused in the material.
In order to achieve the purpose, the invention is realized by the following technical scheme: MoS2/graphene/MoS2Sandwich structure, Na-ion battery capacity prediction method and MoS2(space group P63/mmc) and graphene (space group P63/mmc) are similar in structure but are greatly different in properties, and MoS2For indirect bandgap semiconductors, the bandgap is 1.29eV, and it is formed by S-Mo-S sheets with weak van der waals forces, and how to form a stable structural model is first determined by calculation, which includes the steps of:
1) preliminarily determining the atomic component ratio of the crystal: by graphene lattice parameter:MoS2lattice parameters:the interlayer forces are weak Van Der Waals (VDW) interactions, if consideredTo the lattice matching problem, a structure consisting of 5 × 5 graphene unit layers and 4 × 4 MoS is constructed2A unit layer reducing the lattice mismatch rate to below 2%;
2) respectively introducing graphene (graphene) unit cells and MoS in a Masterial Studio software package2Model, keeping original unit cell lattice constant unchanged, establishing 5 × 5 and 4 × 4 super unit cells;
3) respectively processing the surfaces of the just-built super-cell in the (001) direction, and only keeping the thickness of one atomic layer in the z-axis direction;
4) for the graphene (graphene) surface and MoS obtained in the third step2Surface synthesis creates a new layer structure named MoS2/graphene/MoS2(abbreviated as mgm structure); the lattice parameter of the layer structure is 1.25nm, b is 1.25nm, c is 1.26nm, the variation range of the initial unit cell parameter is within 2 percent, and the primarily constructed MoS2/graphene/MoS2The sandwich structure model is shown in figure 1, and the MoS is built through build layers at the moment2-MoS2The interlayer spacing is
5) The structure of the constructed model is not optimized and is not a stable structure, and MoS is found after preliminary optimization for finding the stable structure of the material2-MoS2Is stabilized in
Figure BDA0002280119820000032
In the vicinity, and thus in the interlayer spacingThe lowest point of energy of the structure was found and the result is shown in FIG. 2, which shows that the lowest point of total energy of the system is 5476.61664kcal/mol, corresponding to the layer spacing
Figure BDA0002280119820000034
Comparing MoS when no graphene (graphene) layer is inserted2-MoS2Inter-layer distance of
Figure BDA0002280119820000035
Insertion of the visible graphene layer significantly increased MoS2-MoS2The interlayer spacing of (a);
6) structure optimization using a Forcite tool to obtain MoS taking into account weak Van Der Waals (VDW) interactions between layers2/graphene/MoS2Sandwich stable structure, MoS at this time2-MoS2Has a layer spacing of
Figure BDA0002280119820000036
7) Considering that graphene (graphene) is easily partially oxidized by contacting air in the experimental process, a small amount of hydroxyl or carboxyl is inserted into the graphene layer to better meet the practical situation of the experiment; the final determined model is shown in FIG. 3;
8) the experimental determination of the interlayer spacing can be characterized by xrd diffraction data, and here, it can also be simulated that ideal xrd diffraction data before and after the intercalation of graphene is ready for future experimental verification, as shown in fig. 4; visible double layer MoS2The main peak of the compound is shifted to a small angle, two symmetrical auxiliary main peaks appear, the slightly different intensity is the influence of the added hydroxyl and carboxyl, and the change of the interlayer distance can be judged to be correspondingly compared with a theoretical model through the shifting condition of the peaks in the experimental process;
9) adding 1 Na atom in the final model, calculating the diffusion coefficient of the Na atom, performing molecular dynamics simulation calculation by using an optimized structure under a Forcite module, in the case, the implementation step is to expand the unit cell to 5 multiplied by 5 again, take the step length of 1fs, firstly perform 2000-step simulation of NPT system, perform 2000000-step simulation of NVT system, analyze MSD (mean square displacement) from the calculated xtd file after the calculation is finished, select the slope of MSD of the selected object fitted in the middle part when the energy is stable, and divide the slope by 6 to obtain the simple diffusion coefficient of the Na/Li atom in the material;
10) the invention calculates the relation between the diffusion coefficient of Na atoms and the interlayer spacing and finds that the interlayer spacing is
Figure BDA0002280119820000041
(best system stability) the self-diffusion coefficient of Na atoms is the greatest and diffusivity is the best, and as shown in FIG. 5, this data also shows that the self-diffusion coefficient of Na atoms is greatly affected when the interlayer spacing becomes smaller than when the interlayer spacing becomes larger;
11) for comparison of MoS2After the graphene (graphene) intercalation is added, the function of the expanded interlayer spacing in increasing the embedding amount of Na atoms is realized, and the formation energy of different numbers of Na atoms when the Na atoms are respectively embedded into two super-cells is calculated;
12) as shown in FIG. 6, for the original MoS2(interlayer spacing 6.2 ℃ A), 1, 2, 4, 8, 12, 16, 20, and 24 Na atoms in the intercalate were calculated, corresponding to NaxMoS2The Na content is respectively 0.06, 0.125, 0.25, 0.5, 0.75, 1, 1.125 and 1.5; for MoS2/graphene/MoS2(interlayer spacing 9.923 ° a), the Na atom insertion numbers were calculated to be 1, 2, 4, 8, 12, 16, 20, 24, 28, 32, and 48, giving Na holding ratios of 0.06, 0.125, 0.25, 0.5, 0.75, 1, 1.125 and 1.5, 1.75, 2 and 2.5, respectively;
13) FIG. 6 shows the original MoS2The formation energy of medium Na starts to be negative, indicating a more favorable exothermic reaction between the MoS2 layer and Na, and changes to positive at an insertion number of 20, indicating a less favorable Na insertion, corresponding to the original MoS2The open-circuit voltage (OCV) of the experimental value in the layer changes from x being 1-1.11 to negative, and MoS2/graphene/MoS2The positive value of the formed Na-containing layer is changed to 50 along with the embedding number of Na atoms, and the Na-containing rate is 3 times, which shows that the insertion of the graphene layer increases MoS2Interlayer spacing, significant improvement of theoretical MoS2Na capacity ratio between layers.
The invention has the beneficial effects that: the invention constructs a brand new three-layer MoS through a first principle2/graphene/MoS2The stable structure of the material under corresponding pressure is firstly calculated to obtain the optimal interlayer spacing in the stable structure, the possible electron migration related characteristics are calculated, and the Na ion embedding/deinserting performance of the material is predicted according to the resultThe results show that the material can be used as a very potential Na Ion Battery (NIB) electrode (negative electrode) material. The problem of the accuracy of atomic scale that the present experimental means can not reach is solved. Solves the problem that the prior experimental environment is difficult to realize MoS2/graphene/MoS2The material synthesis problem. Solution to MoS2The theoretical capacity of Na ions of the two-dimensional nano-layered material is not high enough. The problem of optimal diffusion of Na ions in the material is solved.
Drawings
Other features, objects and advantages of the invention will become more apparent upon reading of the detailed description of non-limiting embodiments with reference to the following drawings:
FIG. 1 shows a MoS of the present invention2/graphene/MoS2A schematic diagram of a sandwich structure.
FIG. 2 shows a MoS of the present invention2-MoS2The layer spacing is plotted against the total energy of the system.
FIG. 3 shows a MoS of the present invention2/graphene/MoS2And (5) optimizing the material of the sandwich structure.
Fig. 4 is a comparison graph of xrd diffraction before and after intercalation of graphene in accordance with the present invention.
FIG. 5 is a graph of the relationship between the interlayer spacing and the diffusion coefficient of Na atoms according to the present invention.
Fig. 6 is a graph showing the effect of graphene layers of the present invention on Na atom intercalation formation energy.
Detailed Description
In order to make the technical means, the creation characteristics, the achievement purposes and the effects of the invention easy to understand, the invention is further described with the specific embodiments.
Referring to FIGS. 1-6, the present invention provides a MoS2/graphene/MoS2The scheme of the sandwich structure and Na-ion battery capacity prediction method comprises the following steps: MoS2(space group P63/mmc) and graphene (space group P63/mmc) are similar in structure but are greatly different in properties, and MoS2For indirect bandgap semiconductors, the bandgap is 1.29eV, and it is formed by S-Mo-S sheets with weak van der waals forces, and how to form a stable structural model is first determined by calculation, which includes the steps of:
1) preliminarily determining the atomic component ratio of the crystal: by graphene lattice parameter:
Figure BDA0002280119820000051
MoS2lattice parameters:
Figure BDA0002280119820000052
the interlayer acting force is weak Van Der Waals (VDW) interaction, and if the lattice matching problem is considered, the graphene structure is formed by 5 multiplied by 5 graphene unit layers and 4 multiplied by 4 MoS2A unit layer reducing the lattice mismatch rate to below 2%;
2) respectively introducing graphene (graphene) unit cells and MoS in a Masterial Studio software package2Model, keeping original unit cell lattice constant unchanged, establishing 5 × 5 and 4 × 4 super unit cells;
3) respectively processing the surfaces of the just-built super-cell in the (001) direction, and only keeping the thickness of one atomic layer in the z-axis direction;
4) for the graphene (graphene) surface and MoS obtained in the third step2Surface synthesis creates a new layer structure named MoS2/graphene/MoS2(abbreviated as mgm structure); the lattice parameter of the layer structure is 1.25nm, b is 1.25nm, c is 1.26nm, the variation range of the initial unit cell parameter is within 2 percent, and the primarily constructed MoS2/graphene/MoS2The sandwich structure model is shown in figure 1, and the MoS is built through build layers at the moment2-MoS2The interlayer spacing is
Figure BDA0002280119820000061
5) The structure of the constructed model is not optimized and is not a stable structure, and MoS is found after preliminary optimization for finding the stable structure of the material2-MoS2Is stabilized in
Figure BDA0002280119820000062
In the vicinity, and thus in the interlayer spacing
Figure BDA0002280119820000063
The lowest point of energy of the structure was found and the result is shown in FIG. 2, which shows that the lowest point of total energy of the system is 5476.61664kcal/mol, corresponding to the layer spacing
Figure BDA0002280119820000064
Comparing MoS when no graphene (graphene) layer is inserted2-MoS2Inter-layer distance of
Figure BDA0002280119820000065
Insertion of the visible graphene layer significantly increased MoS2-MoS2The interlayer spacing of (a);
6) structure optimization using a Forcite tool to obtain MoS taking into account weak Van Der Waals (VDW) interactions between layers2/graphene/MoS2Sandwich stable structure, MoS at this time2-MoS2Has a layer spacing of
Figure BDA0002280119820000066
7) Considering that graphene (graphene) is easily partially oxidized by contacting air in the experimental process, a small amount of hydroxyl or carboxyl is inserted into the graphene layer to better meet the practical situation of the experiment; the final determined model is shown in FIG. 3;
8) the experimental determination of the interlayer spacing can be characterized by xrd diffraction data, and here, it can also be simulated that ideal xrd diffraction data before and after the intercalation of graphene is ready for future experimental verification, as shown in fig. 4; visible double layer MoS2The main peak of the compound is shifted to a small angle, two symmetrical auxiliary main peaks appear, the slightly different intensity is the influence of the added hydroxyl and carboxyl, and the change of the interlayer distance can be judged to be correspondingly compared with a theoretical model through the shifting condition of the peaks in the experimental process;
9) adding 1 Na atom in the final model, calculating the diffusion coefficient of the Na atom, performing molecular dynamics simulation calculation by using an optimized structure under a Forcite module, in the case, the implementation step is to expand the unit cell to 5 multiplied by 5 again, take the step length of 1fs, firstly perform 2000-step simulation of NPT system, perform 2000000-step simulation of NVT system, analyze MSD (mean square displacement) from the calculated xtd file after the calculation is finished, select the slope of MSD of the selected object fitted in the middle part when the energy is stable, and divide the slope by 6 to obtain the simple diffusion coefficient of the Na/Li atom in the material;
10) the invention calculates the relation between the diffusion coefficient of Na atoms and the interlayer spacing and finds that the interlayer spacing is
Figure BDA0002280119820000071
(best system stability) the self-diffusion coefficient of Na atoms is the greatest and diffusivity is the best, and as shown in FIG. 5, this data also shows that the self-diffusion coefficient of Na atoms is greatly affected when the interlayer spacing becomes smaller than when the interlayer spacing becomes larger;
11) for comparison of MoS2After the graphene (graphene) intercalation is added, the function of the expanded interlayer spacing in increasing the embedding amount of Na atoms is realized, and the formation energy of different numbers of Na atoms when the Na atoms are respectively embedded into two super-cells is calculated;
12) as shown in FIG. 6, for the original MoS2(interlayer spacing 6.2 ℃ A), 1, 2, 4, 8, 12, 16, 20, and 24 Na atoms in the intercalate were calculated, corresponding to NaxMoS2The Na content is respectively 0.06, 0.125, 0.25, 0.5, 0.75, 1, 1.125 and 1.5; for MoS2/graphene/MoS2(interlayer spacing 9.923 ° a), the Na atom insertion numbers were calculated to be 1, 2, 4, 8, 12, 16, 20, 24, 28, 32, and 48, giving Na holding ratios of 0.06, 0.125, 0.25, 0.5, 0.75, 1, 1.125 and 1.5, 1.75, 2 and 2.5, respectively;
13) FIG. 6 shows the original MoS2The formation energy of medium Na starts to be negative, indicating a more favorable exothermic reaction between the MoS2 layer and Na, and changes to positive at an insertion number of 20, indicating a less favorable Na insertion, corresponding to the original MoS2The open-circuit voltage (OCV) of the experimental value in the layer changes from x being 1-1.11 to negative, and MoS2/graphene/MoS2The positive value of the formed Na-containing layer is changed to 50 along with the embedding number of Na atoms, and the Na-containing rate is 3 times, which shows that the insertion of the graphene layer increases MoS2Interlayer spacing, remarkable improvementTheoretical MoS2Na capacity ratio between layers.
While there have been shown and described what are at present considered the fundamental principles and essential features of the invention and its advantages, it will be apparent to those skilled in the art that the invention is not limited to the details of the foregoing exemplary embodiments, but is capable of other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. Any reference sign in a claim should not be construed as limiting the claim concerned.
Furthermore, it should be understood that although the present description refers to embodiments, not every embodiment may contain only a single embodiment, and such description is for clarity only, and those skilled in the art should integrate the description, and the embodiments may be combined as appropriate to form other embodiments understood by those skilled in the art.

Claims (1)

1. MoS2/graphene/MoS2The sandwich structure and the Na-ion battery capacity prediction method are characterized in that the MoS2(space group P63/mmc) and graphene (space group P63/mmc) are similar in structure but are greatly different in properties, and MoS2For indirect bandgap semiconductors, the bandgap is 1.29eV, and it is formed by S-Mo-S sheets with weak van der waals forces, and how to form a stable structural model is first determined by calculation, which includes the steps of:
1) preliminarily determining the atomic component ratio of the crystal: by graphene lattice parameter:
Figure FDA0002280119810000011
MoS2lattice parameters:the interlayer acting force is weak Van Der Waals (VDW) interaction, and if the lattice matching problem is considered, the graphene structure is formed by 5 multiplied by 5 graphene unit layers and 4 multiplied by 4 MoS2A unit layer reducing the lattice mismatch rate to below 2%;
2) respectively introducing graphene (graphene) unit cells and MoS in a Masterial Studio software package2Model, keeping original unit cell lattice constant unchanged, establishing 5 × 5 and 4 × 4 super unit cells;
3) respectively processing the surfaces of the just-built super-cell in the (001) direction, and only keeping the thickness of one atomic layer in the z-axis direction;
4) for the graphene (graphene) surface and MoS obtained in the third step2Surface synthesis creates a new layer structure named MoS2/graphene/MoS2(abbreviated as mgm structure); the lattice parameter of the layer structure is 1.25nm, b is 1.25nm, c is 1.26nm, the variation range of the initial unit cell parameter is within 2 percent, and the primarily constructed MoS2/graphene/MoS2The sandwich structure model is shown in figure 1, and the MoS is built through build layers at the moment2-MoS2The interlayer spacing is
Figure FDA0002280119810000013
5) The structure of the constructed model is not optimized and is not a stable structure, and MoS is found after preliminary optimization for finding the stable structure of the material2-MoS2Is stabilized in
Figure FDA0002280119810000014
In the vicinity, and thus in the interlayer spacing
Figure FDA0002280119810000015
The lowest point of energy of the structure was found and the result is shown in FIG. 2, which shows that the lowest point of total energy of the system is 5476.61664kcal/mol, corresponding to the layer spacing
Figure FDA0002280119810000016
CompareMoS without insertion of graphene (graphene) layer2-MoS2Inter-layer distance of
Figure FDA0002280119810000021
Insertion of the visible graphene layer significantly increased MoS2-MoS2The interlayer spacing of (a);
6) structure optimization using a Forcite tool to obtain MoS taking into account weak Van Der Waals (VDW) interactions between layers2/graphene/MoS2Sandwich stable structure, MoS at this time2-MoS2Has a layer spacing of
Figure FDA0002280119810000022
7) Considering that graphene (graphene) is easily partially oxidized by contacting air in the experimental process, a small amount of hydroxyl or carboxyl is inserted into the graphene layer to better meet the practical situation of the experiment; the final determined model is shown in FIG. 3;
8) the experimental determination of the interlayer spacing can be characterized by xrd diffraction data, and here, it can also be simulated that ideal xrd diffraction data before and after the intercalation of graphene is ready for future experimental verification, as shown in fig. 4; visible double layer MoS2The main peak of the compound is shifted to a small angle, two symmetrical auxiliary main peaks appear, the slightly different intensity is the influence of the added hydroxyl and carboxyl, and the change of the interlayer distance can be judged to be correspondingly compared with a theoretical model through the shifting condition of the peaks in the experimental process;
9) adding 1 Na atom in the final model, calculating the diffusion coefficient of the Na atom, performing molecular dynamics simulation calculation by using an optimized structure under a Forcite module, in the case, the implementation step is to expand the unit cell to 5 multiplied by 5 again, take the step length of 1fs, firstly perform 2000-step simulation of NPT system, perform 2000000-step simulation of NVT system, analyze MSD (mean square displacement) from the calculated xtd file after the calculation is finished, select the slope of MSD of the selected object fitted in the middle part when the energy is stable, and divide the slope by 6 to obtain the simple diffusion coefficient of the Na/Li atom in the material;
10) hair brushThe relation between the diffusion coefficient of Na atoms and the interlayer spacing is clearly calculated and found to be
Figure FDA0002280119810000023
(best system stability) the self-diffusion coefficient of Na atoms is the greatest and diffusivity is the best, and as shown in FIG. 5, this data also shows that the self-diffusion coefficient of Na atoms is greatly affected when the interlayer spacing becomes smaller than when the interlayer spacing becomes larger;
11) for comparison of MoS2After the graphene (graphene) intercalation is added, the function of the expanded interlayer spacing in increasing the embedding amount of Na atoms is realized, and the formation energy of different numbers of Na atoms when the Na atoms are respectively embedded into two super-cells is calculated;
12) as shown in FIG. 6, for the original MoS2(interlayer spacing 6.2 ℃ A), 1, 2, 4, 8, 12, 16, 20, and 24 Na atoms in the intercalate were calculated, corresponding to NaxMoS2The Na content is respectively 0.06, 0.125, 0.25, 0.5, 0.75, 1, 1.125 and 1.5; for MoS2/graphene/MoS2(interlayer spacing 9.923 ° a), the Na atom insertion numbers were calculated to be 1, 2, 4, 8, 12, 16, 20, 24, 28, 32, and 48, giving Na holding ratios of 0.06, 0.125, 0.25, 0.5, 0.75, 1, 1.125 and 1.5, 1.75, 2 and 2.5, respectively;
13) FIG. 6 shows the original MoS2The formation energy of medium Na starts to be negative, indicating a more favorable exothermic reaction between the MoS2 layer and Na, and changes to positive at an insertion number of 20, indicating a less favorable Na insertion, corresponding to the original MoS2The open-circuit voltage (OCV) of the experimental value in the layer changes from x being 1-1.11 to negative, and MoS2/graphene/MoS2The positive value of the formed Na-containing layer is changed to 50 along with the embedding number of Na atoms, and the Na-containing rate is 3 times, which shows that the insertion of the graphene layer increases MoS2Interlayer spacing, significant improvement of theoretical MoS2Na capacity ratio between layers.
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