A method for enhancing the fluoride ion diffusion performance of monolayer Mo2C through vacancy induction

Abstract

This invention discloses a method for enhancing the fluoride ion diffusion performance of monolayer Mo2C by vacancy-induced defects. It primarily utilizes vacancy defects to atomically control the structure of monolayer Mo2C, reducing the diffusion energy barrier for fluoride ions and thus inducing rapid diffusion of fluoride ions within the vacancy system of monolayer Mo2C. This significantly improves the rate performance of monolayer Mo2C as an electrode material for fluoride-ion batteries. Firstly, the vacancy defect formation energy of monolayer Mo2C is calculated using first-principles methods. The vacancy defect system of monolayer Mo2C is then screened and optimized to obtain its most stable structure, Mo2C. 1.78 C 0.89 Based on this, the most stable fluoride ion adsorption sites were explored, and the structure corresponding to the most stable fluoride ion adsorption system was obtained; and the fluoride ion adsorption sites in Mo were discovered. 1.78 C 0.89 The optimal diffusion path in the fluoride ion transport pathway corresponds to a fluoride ion diffusion barrier of 0.07 eV, which is much smaller than the diffusion barrier of fluoride ions in monolayer Mo₂C. This indicates that introducing vacancy defects can significantly improve the transport performance of fluoride ions in monolayer Mo₂C. Furthermore, Mo… 1.78 C 0.89 It exhibits good thermal stability at both 300K and 500K. This invention provides a concept and method for designing fluoride-ion battery electrode materials with high diffusion performance.

Description

Technical Field

[0001] This invention belongs to the field of computational materials science, specifically relating to a method for vacancy-induced improvement of fluoride ion diffusion performance in monolayer Mo2C. Background Technology

[0002] In recent years, with the rapid development of electronic devices and electric vehicles, people have placed higher demands on the safety, high capacity, and low cost of the energy storage products they support. Developing high-performance rechargeable batteries has become a research hotspot. Fluorine-ion batteries, as a novel energy storage device, have the following unique advantages: First, fluorine ions are relatively stable and less likely to cause safety hazards; second, fluorine-ion batteries have a high energy density, typically 6 to 10 times that of lithium-ion batteries; and third, the Earth's crust contains abundant fluorine resources, giving fluorine-ion batteries a low-cost advantage. However, their poor rate performance severely restricts the further development of fluorine-ion batteries. Inducing rapid diffusion of fluorine ions in electrode materials is an effective method to solve this problem.

[0003] Transition metal carbides, as a class of two-dimensional materials, possess the advantage of high electrical conductivity; moreover, their unique layered structure and relatively stable adsorption sites enable them to have high capacity. Among the many transition metal carbides, monolayer Mo₂C exhibits stable chemical properties and a large specific surface area. Its sandwich structure, composed of alternating (Mo-C-Mo) atomic layers, is conducive to the storage of fluoride ions, especially as monolayer Mo₂C possesses a high energy density; however, the diffusion performance of fluoride ions within it still needs further improvement. Based on the intrinsic relationship between vacancy defects and ion diffusion, this invention introduces vacancy defects into monolayer Mo₂C to improve fluoride ion diffusion performance, achieving a fluoride ion diffusion barrier of only 0.07 eV. The aim is to design a monolayer Mo₂C-based fluoride ion battery electrode material with excellent rate performance. Summary of the Invention

[0004] The purpose of this invention is to provide a method for enhancing the fluoride ion diffusion performance of monolayer Mo2C through vacancy induction, thereby making it a fluoride ion battery electrode material with excellent fluoride ion diffusion performance. The specific implementation method is carried out according to the following steps:

[0005] Step 1: Screening for the most stable structure (Mo) corresponding to monolayer Mo2C containing vacancy defects. 1.78 C 0.89 ), and calculate its vacancy formation energy;

[0006] Step 2: Obtain Mo 1.78 C 0.89 The band gap was determined, and its conductivity was confirmed.

[0007] Step 3: The concentration of fluoride ions in Mo was calculated. 1.78 C 0.89 Adsorption energies at different sites on the surface were analyzed, and the most stable Mo was selected. 1.78 C 0.89 F 0.125 ;

[0008] Step 4: In Mo 1.78 C 0.89 F 0.125 In this study, the optimal fluoride ion diffusion path and its corresponding fluoride ion diffusion energy barrier were obtained.

[0009] Step 5: Proved Mo 1.78 C 0.89 It exhibits good thermal stability at temperatures of 300K and 500K;

[0010] This invention addresses the poor fluoride ion diffusion performance of monolayer Mo2C by proposing a vacancy-induced method to enhance the fluoride ion diffusion performance of monolayer Mo2C, thereby obtaining a design scheme and technical route for achieving high fluoride diffusion performance within it. Firstly, monolayer Mo2C with different vacancy structures is screened to obtain the most stable vacancy defect structure (Mo). 1.78 C 0.89 Then for Mo 1.78 C 0.89 Band gap calculations were performed on the structure, proving its excellent electronic conductivity; then, the band gap of fluorine in Mo was calculated and obtained. 1.78 C 0.89 The adsorption energies corresponding to different sites were used to screen for the most stable Mo. 1.78 C 0.89 Fluorine adsorption system (Mo) 1.78 C 0.89 F 0.125 Based on this, the climbing elastic band (CI-NEB) method was used to search for fluoride ions in Mo. 1.78 C 0.89 The lowest diffusion barrier in Mo and its corresponding fluoride ion diffusion pathway were investigated; finally, ab initio molecular dynamics (AIMD) was used to study the diffusion path of Mo. 1.78 C 0.89 Thermal stability at temperatures of 300 K and 500 K, respectively. The fluoride ion in Mo provided by this invention... 1.78 C 0.89 The lowest diffusion barrier in it is 0.07 eV, and it also has good electronic conductivity, and Mo 1.78 C 0.89 It exhibits good thermal stability at both 300K and 500K. Attached Figure Description

[0011] The embodiments illustrated in the accompanying drawings will be described in detail below. The accompanying drawings are merely some embodiments of the present invention. In the accompanying drawings:

[0012] Figure 1 This is an overall flowchart of a method for improving the fluoride ion diffusion performance of monolayer Mo2C through vacancy induction.

[0013] Figure 2 Top view (a) and side view (b) of a single-layer Mo2C structure;

[0014] Figure 3 The structural diagrams are for monolayer Mo2C with different vacancy defect types.

[0015] Figure 4 for Mo 1.78 C 0.89 Band diagram;

[0016] Figure 5 (a) is Mo 1.78 C 0.89 Various fluoride ion adsorption sites, Figure 5 (b) lists the corresponding fluoride ion adsorption energies;

[0017] Figure 6 (a) shows fluoride ions in Mo 1.78 C 0.89 Two different diffusion paths; Figure 6 (b) is its corresponding fluoride ion diffusion barrier curve;

[0018] Figure 7 (a) shows fluoride ions in Mo2C and Mo 1.78 C 0.89 The optimal diffusion path in; Figure 7 (b) shows the corresponding fluoride ion diffusion barrier curve;

[0019] Figure 8 for Mo 1.78 C 0.89 The structures after treatment at temperatures of 300K (a) and 500K (b), respectively, and the curves showing the change of their total energy over time. Detailed Implementation

[0020] This embodiment provides a method for improving the fluoride ion diffusion performance of monolayer Mo2C through vacancy induction, comprising the following steps:

[0021] First, single-layer Mo2C structures with different vacancy types were screened to obtain Mo 1.78 C 0.89 The most stable structure and its vacancy formation energy; then for Mo 1.78 C 0.89The band gap of the structure was calculated to analyze its electronic conductivity; then the band gap of fluoride ions in Mo was calculated. 1.78 C 0.89 Adsorption energies at different sites were used to screen for the most stable Mo. 1.78 C 0.89 Adsorption fluoride ion system (Mo 1.78 C 0.89 F 0.125 Based on this, the climbing elastic band (CI-NEB) method was used to find the lowest diffusion barrier of fluoride ions in the system and its corresponding diffusion path. Figure 1 This is an overall flowchart of a method for improving the fluoride ion diffusion performance of monolayer Mo2C through vacancy induction.

[0022] Optimization and screening of single-layer Mo2C structures with vacancy types were carried out. Figure 2 and Figure 3 The images show monolayer Mo₂C and its structures with different vacancy defects. Specific calculation parameters are as follows: the Perdew-Burke-Ernzerh (PBE) method with the generalized gradient approximation (GGA) is used to handle the exchange correlation energy between electrons; the projected plane wave (PAW) method is used to describe the interaction between electrons and ions; and the valence electrons of W, C, and F are considered to be 4f... 14 5d 4 6s 2 1s 2 2s 2 2p 2 and 1s 2 2s 2 2p 5 The plane wave cutoff energy was set to 500 eV. The Monkhorst-Pack (MP) method was used to set up the k-point grid; 2×2×1 and 4×4×1 k-point grids were used for the monolayer Mo₂C vacancy system and the Mo₂C vacancy system, respectively. 1.78 C 0.89 F 0.125 Structural optimization and electronic structure calculation were performed; the van der Waals forces between monolayer Mo2C and fluoride ions were considered using the DFT-D2 method; the force convergence accuracy and energy convergence criterion between atoms were set to... and 10 -5 eV / atom. Furthermore, the vacuum layer thickness in the Z direction is set to... This avoids interlayer interactions. The vacancy formation energy (E) of the monolayer Mo₂C vacancy system is calculated according to equation (2-1). f ).

[0023] E f =E def +E a -E tot(2-1)

[0024] Among them, E def E represents the total energy of a monolayer Mo2C system with vacancy defects; a The energy of a single atom or molecule; E tot This represents the total energy of a monolayer Mo2C. According to equation (2-1), the smaller the vacancy formation energy of a monolayer Mo2C, the more stable its corresponding vacancy defect structure. Therefore, this method can be used to screen for the most stable vacancy defect structure of Mo2C. The study investigated Mo2C (atomic composition Mo...) 18 Five different vacancy defect structures of C9 (e.g.) Figure 3 (As shown in (AE)): (A) Single Mo atom vacancy defect; (B) Single C atom vacancy defect; (C) Single Mo2C molecule vacancy defect (adjacent Mo atom defects appear in the upper and lower Mo layers); (D) Single Mo2C molecule vacancy defect (Mo atom defects appear in the same Mo layer); and (E) Single Mo2C molecule vacancy defect (non-adjacent Mo atom defects appear in the upper and lower Mo layers). By calculating and comparing the vacancy formation energies of these five monolayer Mo2C vacancy defect structures, it can be seen that the structure of type D has the lowest vacancy formation energy, meaning that Mo... 1.78 C 0.89 It is the most stable vacancy defect structure in a single layer of Mo2C.

[0025] To Mo 1.78 C 0.89 Perform band gap calculations and analyze the electronic conductivity of the material. Figure 4 for Mo 1.78 C 0.89 The band structure diagram, from Figure 4 It can be seen that Mo 1.78 C 0.89 The energy band intersects with its Fermi level, indicating that it has metallic properties and excellent electronic conductivity, which is conducive to the rapid transport of electrons within it.

[0026] Next, Mo with three fluorine adsorption sites was constructed using Materials Studio software. 1.78 C 0.89 Adsorption fluoride ion system (e.g.) Figure 5 (a) shows that the three fluorine adsorption sites are as follows: the position directly above Mo (T Mo ), directly above C (T) C The position directly above the center of the triangle formed by the three Mo atoms (H). At this point, based on the different positions occupied by Mo and C, five possible positions directly above Mo (T) are considered. Mo (each marked as T) Mo1 T Mo2 TMo3 T Mo4 T Mo5 ); Four possible positions directly above C were considered (T) C (each marked as T) C1 T C2 T C3 T C4 The positions directly above the center of the triangle formed by the three Mo atoms in the five types (H) are labeled as H1, H2, H3, H4, and H5, respectively.

[0027] Then calculate Mo according to equation (2-2). 1.78 C 0.89 F 0.125 Fluoride ion adsorption energy (E) ad ), thereby obtaining the most stable Mo 1.78 C 0.89 Adsorption fluoride ion system (Mo 1.78 C 0.89 F 0.125 ).

[0028]

[0029] In equation (2-2), and Mo 1.78 C 0.89 F 0.125 and Mo 1.78 C 0.89 Total energy; E F That is the energy of a single fluorine atom.

[0030] Mo was obtained 1.78 C 0.89 The fluoride ion adsorption energies at different adsorption sites in the fluoride ion adsorption system are calculated as follows: Figure 5 As shown in (b). From Figure 5 It can be seen that: T Mo1 site and T Mo2 The site (directly above the Mo atom) is where Mo is most stable. 1.78 C 0.89 The site of fluoride ion adsorption.

[0031] A low diffusion barrier is a crucial indicator of excellent rate performance in electrode materials and is also the core innovation of this invention. Therefore, based on the most stable Mo... 1.78 C 0.89 For the fluoride ion adsorption system, we further used the climbing elastic band (CI-NEB) method to search for its optimal fluoride ion diffusion path.

[0032] Select Mo 1.78 C 0.89 China TMo1 site and T Mo2 The sites (directly above the Mo atom) represent the initial and final states of fluoride ions during diffusion, and the distance (d) between them is... The relationship between the interatomic spacing and the average distance between transition states can be obtained, as shown in equation (2-3):

[0033] N=d / Δx (2-3)

[0034] Where d is the interatomic distance; Δx is the average distance of the transition states; here, the value of Δx is taken as... The number of transition states is 4.

[0035] Four transition state structures were inserted between the initial and final states, forming a complete fluoride ion diffusion path together with the initial and final states. Materials Studio software was used to adjust these four inserted transition state structures. Based on this, the fluoride ion diffusion path was optimized, and the corresponding fluoride ion diffusion energy barriers were compared to obtain the Mo... 1.78 C 0.89 The optimal fluoride ion diffusion path.

[0036] After the transition state optimization calculation is completed, the output file of the diffusion barrier is processed. Based on the diffusion barrier in the output file corresponding to different paths, Mo is obtained. 1.78 C 0.89 The diffusion pathway of fluoride ions and its corresponding diffusion energy barrier. From Figure 6 (a) It can be seen that when fluoride ions are in Mo 1.78 C 0.89 During diffusion, two main diffusion paths were considered (labeled Path1 and Path2, respectively). Path1 represents the diffusion of fluoride ions from a stable T... Mo1 The bit passes through H to reach the adjacent equivalent T. Mo2 bit (T) Mo1 →H→T Mo2 ), Path2 represents the fluoride ion from a stable T Mo1 The position passed by T C The bit reaches the adjacent equivalent T Mo2 bit (T) Mo1 →T C →T Mo2 ). Calculations of the fluoride ion diffusion barrier revealed that the fluoride ion diffusion barriers for Path1 and Path2 are 0.17 eV and 0.07 eV, respectively. Figure 6 (b)). The smaller the energy barrier for fluoride ion diffusion, the easier it is for fluoride ions to diffuse; therefore, Path2 is Mo. 1.78 C 0.89The optimal fluoride ion diffusion path.

[0037] Further comparison of Mo 1.78 C 0.89 The diffusion pathway and diffusion barrier of fluoride ions in monolayer Mo2C are as follows: Figure 7 (a) and Figure 7 As shown in (b). From Figure 7 (b) It can be seen that: in Mo 1.78 C 0.89 The fluoride ion diffusion barrier of Mo₂C is much smaller than that of monolayer Mo₂C (0.07 eV vs. 0.16 eV). Therefore, compared with monolayer Mo₂C, Mo₂C has a lower fluoride ion diffusion barrier. 1.78 C 0.89 It exhibits superior fluoride ion diffusion performance. In summary, this invention verifies that vacancy-induced diffusion is an effective method for improving the fluoride ion diffusion performance of monolayer Mo₂C.

[0038] Furthermore, excellent thermal stability is also an important indicator for evaluating the performance of electrode materials. Therefore, we used ab initio molecular dynamics (AIMD) to study the properties of Mo. 1.78 C 0.89 The thermal stability at 300K and 500K was determined, and the structures after treatment at 300K and 500K were obtained. The variation of the total energy over time was also presented to simulate its thermal stability at room temperature and above room temperature. The calculation results are as follows: Figure 8 As shown in (a) and 8(b). From Figure 8 (a) and 8(b) show that Mo 1.78 C 0.89 After treatment with 300K and 500K respectively, its structure did not undergo significant distortion, and it always maintained a stable distribution over time, indicating that Mo 1.78 C 0.89 It has good thermal stability.

[0039] Finally, it is necessary to state that the specific embodiments described above are merely illustrative of the principles of the present invention and do not constitute a limitation thereof. Any non-essential improvements and adjustments made by those skilled in the art based on the above description of the present invention are within the scope of protection of the present invention.

Claims

1. A method for enhancing the fluoride ion diffusion performance of monolayer Mo2C through vacancy induction, characterized in that, The method includes the following steps: Step 1: Calculate the vacancy defect formation energy of monolayer Mo2C, and screen the vacancy types and stable structures of monolayer Mo2C. The molecular formula of this structure is Mo. 1.78 C 0.89 ; Step 2: For Mo 1.78 C 0.89 Band structure calculations were performed to analyze its electronic conductivity. Step 3: Based on structural symmetry, construct the fluoride ion in Mo 1.78 C 0.89 By comparing the total energies of the adsorption structures corresponding to different sites and calculating the corresponding adsorption energies, the Mo can be obtained. 1.78 C 0.89 F 0.125 The most stable structure; Step 4: Use the climbing elastic band method to search for fluoride ions in Mo 1.78 C 0.89 The optimal diffusion path; Step 5: Study Mo using ab initio molecular dynamics. 1.78 C 0.89 Thermal stability at temperatures of 300K and 500K; In step 1, the formation energy of different vacancy defect systems in monolayer Mo2C is calculated according to equation (1-1). Thus, a stable Mo2C vacancy structure is obtained; （1-1） In equation (1-1), This represents the energy of a monolayer Mo2C vacancy defect system. For the energy of a single atom or molecule, This is the total energy of a single layer of Mo2C; In step 3, according to Mo 1.78 C 0.89 The structures of fluoride ions adsorbed at different sites are determined, and the corresponding fluoride ion adsorption energies are calculated according to equation (1-2). ; （1-2） In equation (1-2), , Mo 1.78 C 0.89 F 0.125 and Mo 1.78 C 0.89 The total energy, That is the energy of a single fluorine atom.

2. The method for improving the fluoride ion diffusion performance of monolayer Mo2C according to claim 1, characterized in that, In step 2, Mo 1.78 C 0.89 Band structure calculations were performed to analyze the electronic conductivity of the vacancy defect structure; the specific steps are as follows: Step 2.1: Process the Mo obtained in Step 1 1.78 C 0.89 The structures were optimized and statically self-consistently calculated using 2×2×1 and 4×4×1 Brillouin zone k-point grids of Monkhorst-Pack type, respectively. Step 2.2: Use the output file obtained from the static self-consistent calculation in Step 2.1 for Mo 1.78 C 0.89 Band structure calculation.

3. The method for improving the fluoride ion diffusion performance of monolayer Mo2C according to claim 1, characterized in that, In step 4, the climbing elastic band method is used to search for the optimal diffusion path of fluoride ions in monolayer Mo2C and its vacancy defect system; the specific steps are as follows: Step 4.1: Obtain Mo in step 3 1.78 C 0.89 The most stable fluoride ion adsorption site is Mo 1.78 C 0.89 F 0.125 Based on the existing system, we will continue to search for adjacent equivalent fluoride ion adsorption sites; Step 4.2: Two adjacent equivalent fluoride ion adsorption sites are designated as the initial and final states of the fluoride ion diffusion path, respectively, and the distance between them is measured. Then, based on the relationship between this distance and the transition states, the number of inserted transition states is obtained, thereby designing the fluoride ion diffusion path in Mo. 1.78 C 0.89 The diffusion path in; Step 4.3: Optimize the fluoride ion concentration in Mo using the climbing elastic band method. 1.78 C 0.89 The diffusion path in the fluoride ion diffusion barrier was determined and its corresponding diffusion barrier was obtained.

4. The method for improving the fluoride ion diffusion performance of monolayer Mo2C according to claim 1, characterized in that, In step 5, the ab initio molecular dynamics method is used to prove Mo 1.78 C 0.89 It exhibits good thermal stability at temperatures of 300K and 500K.

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