- Email: [email protected]
Adsorption mechanisms of Mo2CrC2 MXenes as potential anode materials for metal-ion batteries: A first-principles investigation
Journal Pre-proofs Full Length Article Adsorption Mechanisms of Mo2CrC2 MXenes as Potential Anode Materials for Metal-ion Batteries: A First-Principles Investigation Yi Xiao, Weibin Zhang PII: DOI: Reference:
S0169-4332(20)30639-5 https://doi.org/10.1016/j.apsusc.2020.145883 APSUSC 145883
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
12 August 2019 11 February 2020 23 February 2020
Please cite this article as: Y. Xiao, W. Zhang, Adsorption Mechanisms of Mo2CrC2 MXenes as Potential Anode Materials for Metal-ion Batteries: A First-Principles Investigation, Applied Surface Science (2020), doi: https:// doi.org/10.1016/j.apsusc.2020.145883
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2020 Published by Elsevier B.V.
Adsorption Mechanisms of Mo2CrC2 MXenes as Potential Anode Materials for Metal-ion Batteries: A First-Principles Investigation Yi Xiao a*, Weibin Zhang b* a Institute b
of Materials Science, TU Darmstadt, 64287 Darmstadt, Germany
School of Physics and Optoelectronic Engineering, Yangtze University, Jingzhou, 434023, P. R. China
E-mail addresses: [email protected] (Y. Xiao), [email protected] (W. Zhang).
ABSTRACT: Two dimensional MXene are attracting increasing interests as electrode materials for metal ion batteries (MIBs) because metal ions can diffuse in a 2D lattice surface. In this work, first-principles calculations were carried out to investigate the adsorption performance of monolayer Mo2CrC2 for Li, Na, and K. The results show strong storage ability for various metal cations. There are three layers of Li, two layers of Na, one layer of K, and four layers of Mg on both sides of the monolayer. The electrochemical and thermodynamics of the intercalation of metal ions on the Mo2CrC2 Mxene have been intensively investigated. There is no band gap at the Fermi level of any configuration indicating that it has excellent electronic conductivity leading to an asymmetric electron localization distribution. The binding energy of metal ions decreases as the ion concentration increases. The adsorption of metal ions on a mono-layer of Mo2CrC2 has been studied, and the results indicate that the metal ions obviously improved the diffusion of metal ions. The CI-NEB method was used to study the diffusion of a cation on a Mo2CrC2 surface and search for its lowest energy along the diffusion pathway. Their structural transformations were studied via ab initio molecular dynamic (AIMD) simulations at a series of temperatures. The metal ions gradually evolved via thermal motion with increasing temperature. These results suggest that Mo2CrC2 could be a promising electrode material for Li- and Na-ion batteries in terms of Mo2CrC2 specific capacity, diffusion dynamics, and structural stability.
Keywords: Mo2CrC2 MXene; Metal ion batteries; Dynamics simulation; Ion diffusion; Conversion reaction.
1. Introduction The 2D MXene materials have shown promise as new energy storage devices. [1-2] There are many electrode materials with MXene-layered nanostructured materials; MXene-layered materials have excellent environmental protection, low cost, and availability. [3-4] This work studied their compatibility with the specific capacity characteristics of rechargeable batteries. Their safety issues and widespread use as cathode materials for lithium-ion batteries deserve further study. 2D MXene materials have a high specific capacity, high safety alternatives, and
high-volume energy storage performance versus other common cathodes. [5] MXenes are widely used in electronic devices, catalysts, energy storage, or conversion materials.[6] In addition, the electrochemical performance of lithium-ion batteries is competitive with other electrode materials, and it is well known that the structure can be considered a monolayer with the general formula Mn+1Xn with M = Sc, V, Hf, Nb, Zr, Ta, Cr, and Mo; here, X is C or N, and n is set to 1, 2, or 3. [7-8] MXene usually have some surface-terminated functional groups such as H, F, O, or OH, and these terminated MXene species are defined by the general formula Mn+1XnTx. MXenes have been synthesized via experimental methods including Ti3C2, Ti2C, (Cr0.5Nb0.5)2C, Ta4C3, (V0.5Cr0.5)3C2, Ti3CN, V2C, and Nb2C. [9-10] These MXenes have attracted great attention and demonstrated excellent electrochemical properties. [11] A new promising MXene material called Mo2CrC2 has been developed for anode materials for metal-ion batteries. It exhibits promising electrochemical performances with large capacities, excellent kinetic properties, and good stabilities.[11] However, there are still many challenges before practical use. It is difficult to synthesize and suffers from thermodynamic instability with frequent charging and discharging. The results show that the Mo2CrC2 MXene monolayer has high performance stability and storage capacity: Layered Mo2CrC2 can improve Li+ ion mobility due to its high electronic conductivity and lithium-ion diffusion coefficient. Recently, there have been many efforts to replace the terminated group on the layered surface in Mo2CrC2. These have been theoretically investigated to explain the synthesis of other terminated MXenes as well as their physical and chemical properties.[12] To better understand the electrochemical properties of MXene as an electrode, it is essential to investigate its electronic structure, capacity, stability, and dynamics via simulation. This work combines first-principles density functional theory (DFT) calculations with ab initio molecular dynamics (AIMD) simulations. [13] We used molecular dynamics simulations (MD) to study the thermal stability of these materials and improve their first-principles calculations. The MD simulations explain the effect of structural disorder of Mo2CrC2A2 (A=Li, Na, K, and Mg). We then used the CI-NEB method [14] with bond valence force fields to study structural and lithium-ion transport properties. The calculations indicate that Mo2CrC2A2 increases the adsorption energy of the cations due to charge transfer and bonding. The basic structure of Mo2CrC2 in the monolayer is not obviously influenced by the concentration of adsorbed metal ions. The adsorption energy of cations is subjected to adsorption concentrations. We further show that metal ions tend to adsorb on both sides of Mo2CrC2 layer rather than only one side in two-dimensional materials. The stability of the lithiated structures will be studied via ab initio molecular dynamics. The calculated energy barriers of one Li-ion diffusion on the MXene surface are very small.
2. Simulation Method All calculations were performed with density functional theory (DFT) within the Generalized Gradient Approximation and Perdew Burke Ernzerh (GGA-PBE) method as implemented in the Vienna ab initio simulation package (VASP) code. [15-16] To consider the action of the interlayer, the van der Waals (VdW) density functional (vdWDF) of optB86b was calculated for all simulations. [17] A plane-wave cutoff energy at 500 eV confirmed this convergence (10-5 eV in energy and 0.01 eV in force) of the structural relaxation within the conjugate gradient (CG) method. The Brillouin zone was integrated using an 11×11×1 MonkhorstPack mesh k-point grid during the relaxation for the band structure and density of states (DOS) calculations. [18] A vacuum separation of the interlayer was set to 20 Å to avoid any interactions during the use of periodic boundary conditions. Metal ion adsorption was studied on a 2×2×1 MXene nanosheet with a Brillouin zone sampling of 4×4×1 k-mesh. The adsorption energies of the first layer (equation 1) and additional layers (equation 2) were calculated via the following equation:
Ead(ave) (EMXeneMm EMXene mEM ) / m (M Li, Na, K, Mg)
Ead(ave） ( EMXene Mn EMXene M ( n 1) mEM ) / m ( M Li, Na, K , Mg ) . The open-circuit voltage (OCV) was expressed by the following equation:
(1) （2）
V (EMXeneMm EMXene mEM ) / mze (M Li, Na, K, Mg) .
(3)
Here, EMXene+Mn is the total energy of a metal-treated nanosheet, EMXene is the total energy of a pristine MXene nanosheet, and EM and m are the total energy of bulk metal ions and the number of adsorbed ions in each layer (for a 2×2 supercell is 8 on both sides), respectively. Terms n and c represent the number of adsorption layers and the valence state of fully ionized cations, respectively; m is the number of adsorbed metal atoms. For example, z=1 for Li+, Na+, and K+; z=2 for Mg2+. The theoretical capacities [19] of metal ion batteries Q were calculated by
Q czF / 3.6MMo2CrC2Mx .
(4)
Here, F is the Faraday constant at 96,485.3383 C/mol, and M stands for the molar mass of the Mo2CrC2Mx in g/mol; c and z represent the number of metal ion and the valence state of fully ionized cations, respectively. To investigate the diffusion pathway and energy barriers, the minimum energy paths of metal diffusion were considered. We used the climbing image-nudged elastic band method (CI-NEB) to search the transition state. This was done via VASP code, and this method was within eight images as simulated between the initial and final states. The molecular dynamic simulations were performed at various temperatures (300, 900, and 1500 K) with the NVT ensemble and the Nose-Hooveŕ method via ab initio molecular dynamics (AIMD). [20] Trajectory analysis of metal ion migration was based on Einstein’s diffusion equations as follows:
MSD t (t ) r (0) D
2
1 1 MSD t (t ) r (0) 4t 4t
2
.
(5) The radial distribution function (RDF) was used to estimate the lengths of different types of bonds and distances between atoms; this is defined as the following equation:
G (r )
1
(r r ) i0
i
V
N 1 ( r ri ) N
4 r ( r ) (0) 4 r 2 dr .
(6) Here, ρ, V, and N are the density, the volume, and the number of particles of the system, respectively. Term r is the atomic coordinate. [21]
3. Results and Discussion 3.1 Adsorption and storage capacity The intercalating ions from Figure 1 show the structures of Mo2CrC2 and three high-symmetry sites in the Brillouin zone. We conducted binding energy calculations as a function of the metal ions located on the top of a Mo atom, a C atom, and a Cr atom on both sides of the MXene. Thus, a total of three potential configurations have been considered. Taking dispersive interactions into account, [22] the following ions were considered: Li+, Na+, K+, and Mg2+. To examine the energetic stability of metal ions on the monolayer Mo2CrC2 MXene, we calculated the average adsorption energy of Li+, Na+, K+, and Mg2+ at the various states; these metals ions have similar adsorption sites (Figure 1e). The binding energies of the metal ions top-on-M (Mo), top-on-C, and top-on-Cr sites were calculated via PBE functionals. As the metal ions adsorbed at the top-on-Mo site, they spontaneously moved to the top-on-Cr site. Thus, the top-on-Cr sites were discussed in this work. All adsorption energies are negative after being fully adsorbed on Li+, Na+, K+, and Mg2+. This suggests that these ions can be adsorbed by the Mo2CrC2 MXenes nanosheets. K, in particular, has higher adsorption energies than the other ions. This may be related to the strong interaction between them and the MXene surface.
Figure 1. The structures of Mo2CrC2 supercell (2×2×1) modeling. (a) Top view of Mo2CrC2, (b) side view of Mo2CrC2, (c) the considered adsorption sites on the surface of Mo2CrC2 monolayer (top view). (d) A schematic diagram of the top-view Mo2CrC2 with three high-symmetry sites. (e) Adsorption energy (Eav) of metal ions in the first layer is located at Mo, Cr, and C atom sites on both sides for Mo2CrC2 monolayer surface.
Nonetheless, the Li and Mg adsorption energies of the previous work show an average adsorption energy of -0.76 eV vs. Li/Li+ on Mo2CrC2 electrode and -0.51 eV/atom for Mg. These species may be physisorbed by the MXenes surface. Some adsorption energies calculations about other MXenes were listed for comparison (in Table S3, see Supporting Information) [23-24]. The adsorption behavior of the Mo-based Mo2CrC2 MXenes resembles that of the Cr3C2 materials. This similar structure nicely explains why the adsorption profiles of Mo2CrC2 are similar to Cr3C2 within the variation of metal ion concentrations. The theoretical capacity of Mo2CrC2M2 increases from 193.06 mAh g-1 for Li, 173.05 mAh g-1 for Na, and 343.19 mAh g-1 for Mg (Figure 2b). Some other MXenes are listed for comparison in Table S4 (see Supporting Information). According to the additional adsorption layers calculation, a double layer was adsorbed on the Na ion. A double layer of Na ions formed after being full adsorbed on the monolayer Mo2CrC2 MXene in Figure 2d. Similar to other alkali metal ions, adsorption capacity is limited by the concentration of ions. Metal ion adsorption on MXene nanosheets with these metals ions is at similar adsorption sites regardless of the cation. The total open-circuit voltage (OCV) was calculated via the adsorption energy of the Mo2CrC2Mx (M=Li, Na, K, and Mg) structure (7.16 eV for Mg) in Figure 2a. This value is still higher than those of other metal ions (0.59–5.13 eV) with monolayer adsorption. The adsorption energy first decreases from 1.83 to 2.10 eV and then increases slightly from 2.10 to 1.96 eV. This finding offers an intrinsic advantage for ion conductivity at high ionic concentration. This may facilitate the fundamental science and technological development of 2D materials in energy storage applications. The average open-circuit voltages of alkali metal ions were observed at ≈0.76 V for Li+/Li, ≈0.89 V for Na+/Na, ≈1.06 V for K+/K, and ≈0.51 V for Mg2+/Mg in Table S1. This suggests that lithium and magnesium ion batteries should be encouraged for compounds with average open-circuit voltage close to or below zero.
Figure 2. Variation of Eav as a function of metal ions contents in Mo2CrC2. Here, metal content x is defined as the number of adsorbed metal ions per Mo2CrC2Mx (M=Li, Na, K, and Mg) unit cell. (a) Total of OCV for monolayer adsorption, (b) Theoretical capacity for monolayer adsorption, (c) E adsorption for multilayer adsorption, and (d) Theoretical capacity for multilayer adsorption on a metal-coated Mo2CrC2 nanosheet.
Here, a 2×2×1 supercell of the Mo2CrC2 monolayer was adopted in the calculations. We focus on Mo2CrC2M2n compositions here (M=Li, Na, K, and Mg; n is the number of adsorption layers). Importantly, Li, Na, and Mg can form stable multilayers on bare Mo2CrC2 MXenes via our proposed criteria. As expected of multilayer adsorption, Mg and Li can form stable adsorption layers four or three layers thick on all bare MXenes. The Mg ion is as high as 927.50 mAh g-1 on Mo2CrC2 with four metal adsorption layers. These are the highest capacities predicted for the Mg-ion battery. The Li ions also show a high capacity of 519.49 mAhg-1 on Mo2CrC2. This is higher than the theoretical capacity of K and Na ions; the theoretical storage capacity of Na ion can reach as high as 297.91 mAh g-1. The charge-discharge process of the Mo2CrC2 monolayer is the common cell reaction A+/A: Mo2CrC2+xM++xe- Mo2CrC2Mx. Here, we evaluate the average adsorption energy (Eav) of Li/Na/K/Mg of Mo2CrC2 monolayer. This is multilayer adsorption in the interlayer expanded bare Mo2CrC2 monolayer MXenes. Furthermore, the theoretical capacity profiles of Li- and Na-adsorbed of Mo2CrC2 were similar to the profiles of VS2 and Ti2B2 reported in the literature [25] indicating the reliability of our DFT calculation results.
3.2 Electronic properties Periodic calculations in the solid phase (Figure 3) showed that Cr-C has a similar bond length range from 2.095 to 2.101 Å, and Mo-C has a similar bond length range from 2.191 to 2.195 Å. In Mo2CrC2Li2, the length of Cr–C and Mo–C bonds (2.095 and 2.193 Å) is a little shorter than the corresponding bonds (2.196 and 2.097 Å) in the Mo2CrC2 monolayer, while the Cr-C and Mo-C bond lengths in Mo2CrC2Mg2 are 2.191 and 2.101 Å, respectively. They are slightly longer than the corresponding bonds in the Mo2CrC2 monolayer. The vacuum of Mo2CrC2M2n (M=Li, Na, K, and Mg) monolayer is 20 Å with ion adsorption from atomic layer distances of 4.686, 5.168, 5.512, and 4.760 Å, respectively. We studied the electrical properties of the Mo2CrC2M2n (M=Li, Na, K, and Mg) monolayer via DFT calculations. Figures 4 and S3 (Supporting Information) illustrate the total density of state
(TDOS) and partial density of state (PDOS) of these systems.[26-27] The lowest total energy of the structure is achieved when the metal atoms in the top atomic layer of the MXene are located directly above the Cr atoms. The three compounds under investigation have similar electronic properties (Figure 4) because of their structural similarity.
Figure 3. Geometric structures for single layer adsorption on both sides of Mo2CrC2Mx (M=Li, Na, K and Mg) MXenes: (a) Li, (b) Na, (c) K, and (d) Mg ion adsorption on a metal-coated Mo2CrC2 nanosheet.
The partial density of states (PDOS) of all models was studied and shows full monolayer adsorption. Figure S3 presents the PDOS for metal ions, Li-s, and s,p orbitals of Na, K, and Mg for the first layer of metal atoms. These results clearly demonstrate that the PDOS of the adsorption layer does not change significantly by adding the metal atoms; thus, we showed only one Na layer. The strong hybridization between Mo-4d and Na-3s states in the valence region shows the bonding states between C-2p and Mo-4d in the first layer. Hybridization between Cr-3d and Na-3p in the conduction region shows the anti-bonding state between Mo and Na. In the K adsorption layer, the hybridization between Mo-4d and K-4s remains similar in the valence band region, but this is reduced in the conduction state region. The Mo2CrC2 monolayer has intriguing properties and promising applications in metal ion batteries. The previous results can be used for further analysis to determine the reproduction in the electronic properties of the density of states as illustrated in Figure 4. The highest occupied states mainly have Mo4d features. This explains the relatively fast recovery of the electronic structure of insulators or semiconductors, and more insights into the electronic properties and chemical bonding can be gleaned from their partial density of state (PDOS).
Figure 4. For clarity, the total DOS and PDOS of metal ions atom on a metal-coated Mo2CrC2 nanosheet: (a) Li, (b) Na, (c) K, and (d) Mg. MXenes with geometry structures. Dashed lines represent the Fermi level at 0/eV.
The DOS and PDOS of metal ions on a metal-treated Mo2CrC2 nanosheet (Figures 4 and S3) show that the valence band is mainly composed of Li from -1 to 6 eV, and the conduction band is formed mainly by Mo4d and Li-s orbitals. The valence electrons fill the C2p band that is mainly built by the electronic states with Mo4d localized at the metal atoms. The electronic charge is distributed mainly on Mo and C atoms; these show a strong spd hybridization between M4d and Cr3d with C2p bands. The ionic character contribution is the weakest between Mo and metal cations in Mo2CrC2M2. Various interactions of Mo-A form the major types of interactions and determine the metal cation migration capabilities. However, battery charge and discharge are determined by the metal ion migration capabilities.
3.3 Multilayer adsorption In general, the theoretical capacity and voltage are two important indicators that determine battery performance. The charge and discharge process of Mo2CrC2 is described via the following half-cell reaction versus Mn+/M0. [28] Our results show that the first layer was located at the most stable adsorption site (top Cr site), and the second adsorption layer had a stable adsorption site on the top of the C site. In real-world applications, it is valuable to explore the storage capacity of the electrode materials for the batteries. In our work, the average adsorption energies are used to study the storage capacity of metal ions on the Mo2CrC2 monolayer on both sides. We proposed a practical adsorption model in which 2×2 supercells are investigated and can be labeled as Mo2CrC2M2n (here M is Li, Na, and Mg) with n representing different adsorption layers (n=1, 2, 3, and 4). For the first-layer metal ion adsorption as shown in Figure 5, all of the different metal atoms are absorbed at the top of Cr sites; there are eight metal atoms adsorbed on both sides of the Mo2CrC2 MXene monolayer. The average adsorption energy of metal atoms for the first layer is -0.64, -0.28, -0.20, and -0.89 eV for Li, Na, K, and Mg: This indicates that these metal atoms can adsorb full coverage and remain stable. After adsorption of the first layer, it will form the second adsorbed layer without K atoms, and these metal atoms can be adsorbed at the top of C atom sites with Mo2CrC2Li4, Mo2CrC2Na4, and Mo2CrC2Mg4; there are 16 metal atoms for full coverage
adsorption. The corresponding average adsorption energies are -0.31, -0.04, and -0.42 eV for full adsorption of Li, Na, K, and Mg, respectively. However, according to Eq. (2), the adsorption energies for the second adsorption layers of metal atoms are 0.02 for Li, 0.20 for Na, and 0.025 for Mg ion. However, according to this theory, we deduced that the positive adsorption energies indicate that the two layers of these metal ions are not stable indicating weak repulsive interactions between Mo2CrC2 and Li, Na, and Mg; this decreases the adsorption energy. Meanwhile, the electrons of the metal ions are spread out in the metal layers and form a negative electron cloud with clustering—these can be adsorbed for full coverage layer-by-layer. According to our DFT calculations, the average adsorption energies are negative for full adsorption for Li, Na, K, and Mg. Metal adatoms can be adsorbed stably with the clustering effect. For example, the third layer for Li atoms is adsorbed at the top of Mo sites, and the adsorption energy is -0.02 eV, which means that the adsorption of two layers of Li is stable. Furthermore, the -0.27 eV value is seen with clustering. The second layer for K cations had a positive adsorption energy of 0.32 eV, which indicates that the K cations can only form one layer on the monolayer. This suggests that these metal cations could form stable layer-by-layer adsorption. Further calculations indicated that Mg atoms can accumulate up to the fourth layer, and Mg cations could be adsorbed on Mo2CrC2Mg8 with negative average adsorption energies of -0.18 eV for the outermost Mg cation absorbed layer. In summary, the Li cations of the three adsorption layers preferred the top-of-Mo site. The Na cations in the two adsorption layers preferred the top-of-C site, and Mg cations can achieve four adsorption layers on the top of the first Mg layer site. Based on these results, a fully adsorbed Mo2CrC2 monolayer for the different metal cations was Mo2CrC2Li6, Mo2CrC2Na4, and Mo2CrC2Mg8.
Figure 5. Geometric structures for multilayer adsorption on both sides of fully adsorbed Mo2CrC2Mx (M=Li, Na, and Mg) MXenes monolayers with three Li layers, two Na layers, and four Mg layers. (a) Mo2CrC2Li6, (b) Mo2CrC2Na4, and (c) Mo2CrC2Mg8. Top
(first row) and side (second row) views. The different colors of the number represent the average adsorption energy of each layer.
We also estimated the vertical distance of the metal cations and Mo2CrC2 monolayer after the adsorption. The vertical distance is 4.786 Å for the first layer, 6.018 Å for the second layer, and 7.264 Å for the third layer of Li adsorption. In Mo2CrC2Na4, 5.190 Å was seen in the first layer, and 7.174 Å was seen in the second layer. The vertical distance of the Mo2CrC2Na8 monolayer was for the single layer (Mo2CrC2Na2), two layers (Mo2CrC2Na4), and three layers (Mo2CrC2Na6) adsorption values of 4.772, 6.244, and 7.792 Å, respectively, for Mg cations arranged layer-by-layer up to four layers deep (8.690 Å). These results suggest that the Mo2CrC2 monolayer is a robust anode material in both lithium ion battery (LIBs) and magnesium ion battery (MIBs). These capacities were attributed to many factors such as stacking layers and side reactions under experimental conditions, [29] but these factors were not considered in our DFT calculations. The open-circuit voltage (OCV) of Mo2CrC2 monolayers at different metal cation adsorption layers was calculated with equation (3). The theoretical discharge (Mn+/M0) profile voltage was obtained via the average adsorption energy. Figure 6(b) shows that all of the specific capacities were obtained in a zero-voltage window indicating that the Mo2CrC2 monolayers were suitable anode materials for Li-ion and Mg-ion batteries. According to Figure 6, the corresponding theoretical capacity and OCV of the Mo2CrC2Li6 are 519.49 mAhg-1 and -0.21 eV, respectively. Meanwhile, the theoretical capacity and open-circuit voltage (OCV) of the Mo2CrC2Mg8 are 927.50 mAhg-1 and 0.18 V, respectively. This increases the adsorbed Mg atoms from 6 to 24 in 2×2 supercell of Mo2CrC2 monolayer. Therefore, the calculated theoretical capacity of the Mo2CrC2Na2 of sodium-ion battery (NIBs) is 297.91 mAhg-1, and the average open-circuit voltage is 0.04 V. The differences in electron density obviously change for the four systems: The electron density evolved between metal cations and the Mo2CrC2 surface with a simultaneous decrease for layer-by-layer assembly of Mo2CrC2Li6, Mo2CrC2Na4, and Mo2CrC2Mg8. The electron localization function (ELF) [30] maps showed a distribution of electronic localization in the two-dimensional nanomaterial surface between the cation ions and the surface in the nanosheets. This indicates strong bonding between them suggesting excellent chemical stability for the Mo2CrC2 monolayers as electrodes for Li and Mg ion batteries in practical applications.
Figure 6. Calculated voltage profiles and capacity of Mo2CrC2 monolayers at different cation adsorption for (a) voltage and (b) theoretical capacity.
Next, we studied the adsorption behavior of different metal cations on Mo2CrC2 monolayers by electron localization function (ELF) (Figure S4). This is similar to a homogeneous electron gas; the value of the isosurface level is set to 0.0–0.8 a.u. Values of 0.8 and 0.0 correspond to fully localized and fully delocalized electrons, respectively. Figure S4 maps the electron localization functions (ELF) on the (110) planes for all of the configurations of the Mo2CrC2M2 (M=Li, Na, K, and Mg) monolayer with one adsorption layer. This showed that the valence electrons of the adatoms were fully ionized and spread out in the metal cations layer forming a negative electron cloud. Therefore, the K cations can form only one layer of adsorption on the Mo2CrC2 monolayer. This
indicates that the resulting ionic bonds are stronger than the metallic bonds. The local regions of electron density are usually the main contributors to the atomic and electron transfer between the metal cations. The electron density covers an entire localized region. Molybdenum and lithium atoms are usually regarded as electron donors and do not bind to either ionic or covalent processes regardless of electron gain or loss. They are left with an overall positive or negative charge. There are substantial concentrations of electrons around the outermost metal cations layer like the free electron gas localized on the Mo2CrC2 MXene surface. The results show an ionic bond character. However, there is almost no free electron delocalization [32, 44] in the internal layers of Mo2CrC2 MXene. The electron density of Mo2CrC2Li6, Mo2CrC2Na4, and Mo2CrC2Mg8 is analyzed based on Figures 5 and S4. The results show the electron density (red region) around the metal cations illustrating that the electron density around the metal cations increases. This implies partial electron transfer to the cations (red region) from adjacent Mo2CrC2 surfaces (blue region). Therefore, the charge transfer can significantly improve the electronic conductivity and theoretical specific capacity of the materials.
3.4 Ion diffusion The ion diffusion properties are a critical metric of metal ion batteries. They determine the charge/discharge rate of rechargeable batteries. Based on the previous analysis, the top of the Cr atoms was the most favorable adsorption site for all adatoms. There are three possible diffusion pathways between the two neighboring favorable adsorption sites (Figure 7). In pathway 1, a cation moves from the top of one Cr site (A) to an adjacent Cr site (A`) across the top of a C site (B). This was defined as A→B→A`. In pathway 2, the cations migrate from one A site and move directly to the nearest neighboring A` site. Pathway 3 is from the A site to the A` site across the top of a Mo site (C) or A→C→A`. The diffusion barrier profiles (Figure 8) of Mo2CrC2 monolayers for the three pathways and all cations show that pathway 3 has the largest diffusion barrier: There is only one saddle point in pathway 1. This illustrates that the diffusion of activated metal cations diffusing in the Mo2CrC2 layer occurs via one transition state (TS) via pathway 3. In comparison, the energy barrier along A→C→A` was significantly smaller. According to the Arrhenius equation, the diffusion coefficient (DLi) of the cations can be estimated by the following equation: DLi=d2ν0exp-Ea/kBT. (8). Here, Ea is the energy barrier (activation energy), kB is the Boltzmann constant, T is room temperature, and ν0 is the attempt frequency—a constant for each chemical reaction that relates to collision theory in the correct orientation. The calculated diffusion coefficient in Mo2CrC2A2 (A=Li, Na, K, and Mg) at room temperature (300 K) ranges from 10-9 up to 10-7 cm2/s. The Mo2CrC2 can provide better kinetic properties than the other 2D materials. [33]
Figure 7. A schematic diagram of the top and side views for possible pathway of a sodium ions diffusion on a 2×2×1 supercell of (a) Li, (b) Na (c) K, or (d) Mg ion adsorption on a metal-coated Mo2CrC2 nanosheet Mxenes.
Figure 8. Energy profiles of metal ion diffusion on a 2×2×1 supercell of (a) Li, (b) Na (c) K, and (d) Mg ion diffusion on Mo2CrC2 nanosheet Mxenes.
For all adsorbed cations, a local energy minimum was observed at the intermediate C site. There are two saddle points in the middle of the A → C → A` and A → A` bridges for all metal cations without K cation. The calculated diffusion barriers along pathway 1 were 0.026, 0.027, and 0.021 eV for Li+, Na+, and K+, respectively. The obtained diffusion barrier for Mg2+ was consistent with our previous adsorption energies of metal cations results. For Mg2+, path 2 (A→A` bridges) has the smallest diffusion energy barrier versus other pathways (0.083 eV). There is a much smaller diffusion barrier value than in the other 2D energy storage materials: TMD (MoS2: 0.49 eV for Li)[34] and (MoN2: 0.78 eV for Li, 0.56eV for Na and 0.49eV for Mg) [35] and graphene (0.32 eV for Li) [36]. In addition, the diffusion barriers for Mg2+ were larger than those for Li+, Na+, and K+. Therefore, the diffusion barrier followed the ordered sequence from lowest to highest: Mg2+ < Li+ < Na+ < K+ along the most favorable diffusion pathways in the three pathways. The diffusion barrier was similar to the diffusion path of Li+, Na+, and K+. In addition, the adsorption distance of the Cr–Li bonds (4.69 Å at A site and 3.45 Å at B site) in Mo2CrC2Li2 is shorter than that in Mo2CrC2Na2 (5.17 Å for A site and 4.24 Å for B site); these values were also calculated for Mo2CrC2K2 (5.51 Å for A site and 4.18 Å for B site) from Figure 7. Therefore, after layer-by-layer adsorption, cations migrate on one layer of the blocks, and the metal cation diffusion makes this more difficult. Our work determined the kinetic pathways for metal cations. The vacancy migration is consistent with the most-favored pathway calculated via the CI-NEB method; this work also presents the barrier energy involved. The atomic arrows indicate the motion of neighboring favored adsorption site atoms (Figure 8). The TS state has a barrier energy of 0.0210.083 eV relative to that of the most stable cations vacancy state. The lower energy barrier usually leads to faster metal-ion transport in rechargeable batteries. Path 1 has a lower energy barrier indicating that spatial effects play an important role in accelerating the metal-ion migration—especially that of K (0.021 eV). To the best of our knowledge, alkaline earth metals (Mg2+) have a high diffusion barrier versus other metal adatoms. Indeed, we found that the diffusion of Mg2+ across the top of C or Mo atoms must overcome a barrier energy of about 0.376 eV in the A→C→A` direction or 0.085 eV in the A→B→A` direction.
3.5 MD simulation
Metal ion diffusion occurs in the cation layer via vacancy jumping because of the layered structure of 2D Mo2CrC2 MXene materials. This work primarily studied the thermal motion of metal ions within a supercell (2x2x1) with four formula units via MD simulations at 300, 900, and 1500 K within 10 ps simulation times. Figure 10 shows the mean square displacements (MSD) [37-40] of Mo2CrC2M2 (M= Li, Na, K, and Mg) at various temperatures (300, 900, and 1500 K). The metal cation MSD values are given preference in Figure 9 suggesting that MSD values should be fairly constant. These were calculated via the Einstein diffusion equation for four complexes along with MD calculated displacements. We used linear fitting of the MSD values and favored cationic displacements for the metal ions to obtain the self-diffusion coefficient of the cations. This provides insight into how the potential can be improved during the 10-ps simulation. This contributes to the self-diffusion coefficient caused by the relative motion of metal ions calculated by equation 5, i.e., the computer simulation of the MSD of a cation considered as implemented in the NVT ensemble. The value of the self-diffusion coefficient was calculated from the linear relation of the MSD curve with simulation time. In Mo2CrC2A2, the self-diffusion coefficients are 5.12×10-7, 1.32×10-7, 1.37×10-7, and 5.71×10-7 m2/s for Li+, Na+, K+, and Mg2+ ions at room temperature, respectively.
Figure 9 Mean square displacements between the metal ion sites of (a) Li, (b) Na, (c) K, and (d) Mg ion diffusion on Mo2CrC2 nanosheet Mxenes at 300, 900, and 1500 K. (a) Li, (b) Na (c) K, and (d) Mg ion diffusion on Mo2CrC2 nanosheet Mxenes.
In general, the radial distribution function (RDF) g(r) can give the probability of finding one particle in the interatomic separation (r) from another nanoparticle. [40-43] Significant results have been obtained for cations (Li, Na, K, and Mg).[44-47] Here, we studied cations with potentially significant interactions between metals ions and the Mo2CrC2 surface according to RDF calculations. The plot of RDF demonstrates a series of peaks corresponding to the evolution of particle separation within nearest neighbor distances. As temperature increases, the peak widths gradually become broader, and the peak height gradually reduced due to the thermal motion of the metal ions. The RDFs of Mo-C and Cr-C are all shown in Figure 10. To further analyze the intensity of the first peak positions, we found that the contribution order of cations is K+ > Li+> Na+> Mg2+ indicating the order of the binding strength between transition metals (TMs) and carbon. This study shows that the RDFs of the Mo2CrC2 surface with metal ion layer had a prominent difference for the interatomic separation. The Mo2CrC2Li2 results on RDF of the nearest atomic distance of Mo-C (around the position of 2.1 Å) and Cr-C (around the position of 2.2 Å) pair functions are consistent with prior experimental studies. As the temperature increases, the difference in the RDF of the Li-ions gradually becomes less clear; the peaks of RDFs of the Mo-C pair finally overlap [see Figure 10(d)]. The differences between calculated values of the self-diffusion coefficient are obvious. There is an obvious first peak in all configurations. The characteristic of the RDF is determined by the interatomic separation (Mo-C and Cr-C) (from 2.1 to 2.3 Å).
Figure 10. The radial pair distribution function g(r) at 300, 900, and 1500 K. (a) Li, (b) Na, (c) K, and (d) Mg ion diffusion on Mo2CrC2 nanosheet Mxenes.
Figure 11. The motion of ions at 300 K: (a) Li, (b) Na, (c) K, and (d) Mg on Mo2CrC2 nanosheet Mxenes.
Figure 11 shows that by using the widely varying simulation level with NVT ensemble conditions at room temperature, all of these composites found common features of metal ion motion with long (3−5 ps) dynamics. This was seen in the solid phase at room temperature by exploring the motion displacement of ions during the atomic level dynamics. [48-50] Our calculations highlight that Li ions are most likely to migrate with obvious localization in supercells in the Z directions; see Figure 11a for a random snapshot extracted from the simulated time evolution. This is in line with the thermally activated carrier recombination indicating a cation-induced shifting of the supercell surface in different spatial regions. This might underlie possible ionic transport mechanisms and could be explored via cation dynamics.[51-54] The snapshot of MD simulations results in Figure S6 shows that the most energetically stable configuration is used as the initial structure for AIMD. The simulation temperature is set to 300, 900, and 1500 K. At 300 K, the structural transformation of Mo2CrC2Li2 and Mo2CrC2Mg2 did not differ significantly during the simulation time. However, a structural transformation was seen with Mo2CrC2Na2 and Mo2CrC2K2 at 300 K. This leads to stronger kinetic properties than Li and Mg ions on Mo2CrC2. The diffusion of cations is quite dependent on the diffusion path. The simulation temperature is then set to 900 and 1500 K with significantly high temperature conditions. There are obvious structural transformations at all configurations. Our simulation model is not sufficiently large, and the simulation time is limited. [55-56] The simulation results show that both Mo2CrC2Na2 and Mo2CrC2K2 begin
to transform into the corresponding disordered structures at 900 and 1500 K (see Figure S5). We selected Mo2CrC2Mg2 as a representative species for MD simulations at 300 K. The structure retains its original state, and the structural transformation is not observed by Mg ion monolayer adsorption. The MD simulations reveal that the Mo2CrC2 monolayer has good stability during the Li and Mg ion insertion processes. [57]
4. Conclusion In summary, we designed a new 2D electrode material with a Mo2CrC2 monolayer for energy storage. The promising properties of the Mo2CrC2 monolayers as the electrode for Li+, Na+, K+, and Mg2+ ions batteries were investigated using first-principles calculations. Li ions have three adsorption layers, and Na cations have two adsorption layers with a preference for the top of the Mo2CrC2 surface. Importantly, Mg ions can achieve four adsorption layers—this leads to high theoretical capacities of fully adsorption system, i.e., 519.49 mAh g-1 for Li, 297.91 mAh g-1 for Na, 154.88 mAh g-1 for K, and 927.51 mAh g-1 for Mg during the multilayer adsorption. The promising conductivity and low diffusion barriers of Mo2CrC2 monolayers ensured excellent kinetic properties for electron transportation and ion diffusion for rechargeable batteries. The calculated diffusion barriers along the most-favored pathway were 0.026, 0.027, 0.027, and 0.083 eV for Li+, Na+, K+, and Mg2+, respectively. While there were significant differences in motility of metal ions in Mo2CrC2 materials, the structural stability is due to their variation in electron localization distribution with Na+ or Mg2+ ions during adsorption in Mo2CrC2. The AIMD simulation results confirm that the Mo2CrC2 monolayer can maintain its structural stability up to 1500 K. Therefore, Mo2CrC2 may find wide applications including in electric vehicles and metal ion batteries.
Acknowledgments Financial support from China Scholarship Council (CSC: 201808440416) of China and Research and the Arts (HMWK) of the Hessen state in Germany. The Lichtenberg high performance computer is gratefully acknowledged, we are also thankful to TU Darmstadt. We also thank LetPub (www.letpub.com) for its linguistic assistance during the preparation of this manuscript.
Conflict of Interest The authors declare that they have no conflict of interest.
References 1. Wick P, Louw-Gaume A E, Kucki M, et al. Classification Framework for Graphene-Based Materials [J]. Angewandte Chemie, 2014, 53(30):7714-8. 2. Anasori B, Lukatskaya M R, Gogotsi Y. 2D metal carbides and nitrides (MXenes) for energy storage [J]. Nature Reviews Materials, 2017, 2(2):16098. 3. Peng L, Zhu Y, Chen D, et al. Two‐Dimensional Materials for Beyond-Lithium-Ion Batteries[J]. Advanced Energy Materials, 2016, 6(11): 1600025. 4. Li Y, Wu D, Zhou Z, et al. Enhanced Li Adsorption and Diffusion on MoS2 Zigzag Nanoribbons by Edge Effects: A Computational Study [J]. Journal of Physical Chemistry Letters, 2012, 3(16):2221. 5. Peng J, Chen X, Ong W J, et al. Surface and heterointerface engineering of 2D MXenes and their nanocomposites: insights into electro-and photocatalysis[J]. Chem, 2018. 6. Shein I R, Ivanovskii A L. Graphene-like nanocarbides and nanonitrides of d metals (MXenes): synthesis, properties and simulation[J]. Micro & Nano Letters, 2013, 8(2): 59-62. 7. Kurtoglu M, Naguib M, Gogotsi Y, et al. First principles study of two-dimensional early transition metal carbides[J]. Mrs Communications, 2012, 2(4): 133-137. 8. Sun D, Hu Q, Chen J, et al. Structural transformation of MXene (V2C, Cr2C, and Ta2C) with O groups during lithiation: a first-principles investigation[J]. ACS applied materials & interfaces, 2015, 8(1): 74-81.
9. Er D, Li J, Naguib M, et al. Ti3C2 MXene as a high capacity electrode material for metal (Li, Na, K, Ca) ion batteries[J]. ACS applied materials & interfaces, 2014, 6(14): 11173-11179. 10. Wang X, Kajiyama S, Iinuma H, et al. Pseudocapacitance of MXene nanosheets for high-power sodium-ion hybrid capacitors.[J]. Nature Communications, 2015, 6:6544. 11. Feng J, Sun X, Wu C, et al. Metallic Few-Layered VS2 Ultrathin Nanosheets: High Two-Dimensional Conductivity for In-Plane Supercapacitors [J]. Journal of the American Chemical Society, 2011, 133(44):17832-17838. 12. Wang D, Liu Y, Meng X, et al. Two-dimensional VS2 monolayers as potential anode materials for lithium-ion batteries and beyond: first-principles calculations[J]. Journal of Materials Chemistry A, 2017, 5(40): 21370-21377. 13. Guo X, Zhang X, Zhao S, et al. High adsorption capacity of heavy metals on two-dimensional MXenes: an ab initio study with molecular dynamics simulation[J]. Physical Chemistry Chemical Physics, 2016, 18(1): 228-233. 14. Henkelman G, Uberuaga B P, Jónsson H. A climbing image nudged elastic band method for finding saddle points and minimum energy paths[J]. The Journal of chemical physics, 2000, 113(22): 9901-9904. 15. Hafner J. Ab‐initio simulations of materials using VASP: Density‐functional theory and beyond[J]. Journal of computational chemistry, 2008, 29(13): 2044-2078. 16. Berland K, Cooper V R, Lee K, et al. van der Waals forces in density functional theory: a review of the vdW-DF method[J]. Reports on Progress in Physics, 2015, 78(6): 066501. 17. Giovannetti G, Khomyakov P A, Brocks G, et al. Substrate-induced band gap in graphene on hexagonal boron nitride: Ab initio density functional calculations[J]. Physical Review B, 2007, 76(7): 073103. 18. Sun G, Kürti J, Rajczy P, et al. Performance of the Vienna ab initio simulation package (VASP) in chemical applications[J]. Journal of Molecular Structure: THEOCHEM, 2003, 624(1-3): 37-45. 19. Hafner J. Materials simulations using VASP—a quantum perspective to materials science[J]. Computer physics communications, 2007, 177(1-2): 6-13. 20. Xiao Y, Zhang F C, Han J I. Electrical structures, magnetic polaron and lithium ion dynamics in three transition metal doped LiFe1− xMxPO4 (M= Mn, Co and La) cathode material for Li ion batteries from density functional theory study[J]. Solid State Ionics, 2016, 294: 73-81. 21. Zhang W, Wang Y, Wang J, et al. Theoretical prediction of the structural, electronic, mobility, and dynamic properties of Mo3N2T2 (T= H, O and OH) with Li adsorption feature[J]. Solid State Ionics, 2018, 325: 238-246. 22. Burns L A, Mayagoitia Á V, Sumpter B G, et al. Density-functional approaches to noncovalent interactions: A comparison of dispersion corrections (DFT-D), exchange-hole dipole moment (XDM) theory, and specialized functionals[J]. The Journal of chemical physics, 2011, 134(8): 084107. 23. Xie Y, Dall’Agnese Y, Naguib M, et al. Prediction and characterization of MXene nanosheet anodes for non-lithium-ion batteries[J]. ACS nano, 2014, 8(9): 9606-9615. 24. Novoselov K S, Mishchenko A, Carvalho A, et al. 2D materials and van der Waals heterostructures[J]. Science, 2016, 353(6298): aac9439. 25. Bo T, Liu P F, Xu J, et al. Hexagonal Ti2B2 monolayer: a promising anode material offering high rate capability for Li-ion and Na-ion batteries[J]. Physical Chemistry Chemical Physics, 2018, 20(34): 22168-22178. 26. Xiao Y, Wang J, Wang Y, et al. A new promising catalytic activity on blue phosphorene nitrogen-doped nanosheets for the ORR as cathode in nonaqueous Li–air batteries[J]. Applied Surface Science, 2019, 488: 620-628. 27. Hong J, Pan Y, Hu Z, et al. Direct imaging of kinetic pathways of atomic diffusion in monolayer molybdenum disulfide [J]. Nano Letters, 2017, 17(6). 28. Zhou Y, Zu X. Mn2C sheet as an electrode material for lithium-ion battery: A first-principles prediction [J]. Electrochimica Acta, 2017. 29. Tang Q, Zhou Z, Shen P. Are MXenes promising anode materials for Li ion batteries? Computational studies on electronic properties and Li storage capability of Ti3C2 and Ti3C2X2 (X= F, OH) monolayer[J]. Journal of the American Chemical Society, 2012, 134(40): 16909-16916. 30. Yang S, Li D, Zhang T, et al. First-principles study of zigzag MoS2 nanoribbon as a promising cathode material for rechargeable Mg batteries[J]. The Journal of Physical Chemistry C, 2011, 116(1): 1307-1312. 31. Zhang X, Yu Z, Wang S S, et al. Theoretical prediction of MoN2 monolayer as a high capacity electrode material for metal ion batteries[J]. Journal of Materials Chemistry A, 2016, 4(39): 15224-15231. 32. Liang Y, Feng R, Yang S, et al. Rechargeable Mg batteries with graphene‐like MoS2 cathode and ultrasmall Mg nanoparticle anode[J]. Advanced Materials, 2011, 23(5): 640-643. 33. Xiao Y, Liu Y, Frederick N, et al. Ab initio study of the electronic structure, magnetic and dynamics properties in LiMO2 and NaMO2 (M=V and Cr) as electrode cathode material for batteries[J]. Solid State Ionics, 2017, 307: 26-34. 34. Li Y, Wu D, Zhou Z, et al. Enhanced Li adsorption and diffusion on MoS2 zigzag nanoribbons by edge effects: a computational
study[J]. The journal of physical chemistry letters, 2012, 3(16): 2221-2227. 35. Zhang X, Yu Z, Wang S S, et al. Theoretical prediction of MoN2 monolayer as a high capacity electrode material for metal ion batteries[J]. Journal of Materials Chemistry A, 2016, 4(39): 15224-15231. 36. Zheng J, Ren Z, Guo P, et al. Diffusion of Li+ ion on graphene: A DFT study[J]. Applied Surface Science, 2011, 258(5): 1651-1655. 37. Kuritz N, Murat M, Balaish M, et al. PFC and Triglyme for Li–Air Batteries: A Molecular Dynamics Study[J]. The Journal of Physical Chemistry B, 2016, 120(13): 3370-3377. 38. Eom T H, Xiao Y, Han J I, et al. Electrical structure, magnetic polaron and lithium ion dynamics in four mixed-metal oxide multiple-phase electrode cathode material for Li ion batteries from density functional theory study[J]. Computational Materials Science, 2017, 132: 92-103. 39. Xiao Y, Zhang F C, Han J I. Synthesis, characterization and lithium-ion migration dynamics simulation of LiFe 1− x T x PO 4 (T= Mn, Co, La and Ce) doping cathode material for lithium-ion batteries[J]. Applied Physics A, 2016, 122(11): 980. 40. Berdiyorov G R, Mahmoud K A. Effect of surface termination on ion intercalation selectivity of bilayer Ti3C2T2, (T=F, O and OH) MXene [J]. Applied Surface Science, 2017, 416. 41. Zhang X, He T, Meng W, et al. Mn2C Monolayer: Hydrogenation/Oxygenation Induced Strong Ferromagnetism and Potential Applications[J]. The Journal of Physical Chemistry C, 2019.12.12. 42. Guo X, Zhang X, Zhao S, et al. High adsorption capacity of heavy metals on two-dimensional MXenes: an ab initio study with molecular dynamics simulation.[J]. Physical Chemistry Chemical Physics, 2015, 18(1):228. 43. Gandi A N, Alshareef H N, Schwingenschlögl U. Thermoelectric Performance of the MXenes M2CO2 (M = Ti, Zr, or Hf) [J]. Chemistry of Materials, 2016, 28(6). 44. Yu Y X. Prediction of Mobility, Enhanced Storage Capacity, and Volume Change during Sodiation on Interlayer-Expanded Functionalized Ti3C2 MXene Anode Materials for Sodium-Ion Batteries [J]. Journal of Physical Chemistry C, 2016, 120(10). 45. Wang G. A Theoretical Prediction of the Intrinsic Half-Metallicity in Surface-Oxygen-Passivated Ti2N MXene[M]// Introduction to statistics for the behavioral sciences /. Saunders, 2013:464-478. 46. Wang D, Gao Y, Liu Y, et al. First-Principles Calculations of Ti2N and Ti2NT2 (T = O, F, OH) Monolayers as Potential Anode Materials for Lithium-Ion Batteries and Beyond [J]. Journal of Physical Chemistry C, 2017, 121(24). 47. Er D, Li J, Naguib M, et al. Ti3C2, MXene as a High Capacity Electrode Material for Metal (Li, Na, K, Ca) Ion Batteries [J]. Acs Appl Mater Interfaces, 2014, 6(14):11173-11179. 48. Banerjee S, Periyasamy G, Pati S. Possible application of 2D-boron sheets as anode material in lithium ion battery: A DFT and AIMD study [J]. Journal of Materials Chemistry A, 2014, 2(11):3856-3864. 49. Chen X, Kong Z, Li N, et al. Proposing the prospects of Ti3CN transition metal carbides (MXenes) as anodes of Li-ion batteries: a DFT study.[J]. Physical Chemistry Chemical Physics Pccp, 2016, 18. 50. Wang D, Gao Y, Liu Y, et al. Investigation of Chloride Ion Adsorption onto Ti2C MXene Monolayers by First-principles Calculations[J]. Journal of Materials Chemistry A, 2017, 5(47). 51. Eklund P, Rosen J, Persson P O Å. Layered ternary M n+1AXn, phases and their 2D derivative MXene: an overview from a thin-film perspective [J]. Journal of Physics D Applied Physics, 2017, 50(11). 52. Hu T, Wang J, Zhang H, et al. Vibrational properties of Tir3C2 and Ti3C2T2 (T =O, F, OH) monosheets by first-principles calculations: a comparative study [J]. Physical Chemistry Chemical Physics, 2015, 17(15):9997-10003. 53. Xie Y, Naguib M, Mochalin V N, et al. Role of surface structure on Li-ion energy storage capacity of two-dimensional transition-metal carbides [J]. Journal of the American Chemical Society, 2014, 136(17):6385-94. 54. Ma Z, Hu Z, Zhao X, et al. Tunable Band Structures of Heterostructured Bilayers with Transition-Metal Dichalcogenide and MXene Monolayer [J]. Journal of Physical Chemistry C, 2014, 118(10):5593–5599. 55. Khazaei M, Arai M, Sasaki T, et al. Two-dimensional molybdenum carbides: potential thermoelectric materials of the MXene family [J]. Physical Chemistry Chemical Physics, 2014, 16(17):7841-9. 56. Zha X H, Yin J, Zhou Y, et al. Intrinsic structural, electrical, thermal, and mechanical properties of the promising conductor Mo2C MXene[J]. The Journal of Physical Chemistry C, 2016, 120(28): 15082-15088. Zha X H, Luo K, Li Q, et al. Role of the surface effect on the structural, electronic and mechanical properties of the carbide MXenes[J]. EPL (Europhysics Letters), 2015, 111(2): 26007.
Yi Xiao: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Writing - original draft, Project administration, Resources. Conceptualization, Data curation, Formal analysis, Writing - review & editing. Weibin Zahng: Funding acquisition, Software. Validation, Visualization.
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Adsorption Mechanisms of Mo2CrC2 MXenes as Potential Anode Materials for Metal-ion Batteries: A First-Principles Investigation Yi Xiao a*, Weibin Zhang b* a Institute b
of Materials Science, TU Darmstadt, 64287 Darmstadt, Germany
School of Physics and Optoelectronic Engineering, Yangtze University, Jingzhou, 434023, P. R. China
E-mail addresses: [email protected] (Y. Xiao), [email protected] (W. Zhang).
1. We designed a new 2D electrode material with a Mo2CrC2 monolayer for energy storage 2. It ensured excellent kinetic properties for electron transportation and ion diffusion for rechargeable batteries. 3. Mo2CrC2 may find wide applications including in electric vehicles and metal ion batteries. 4. MD simulations reveal that the Mo2CrC2 monolayer has good stability during the Li and Mg ion insertion processes.
S0169-4332(20)30639-5 https://doi.org/10.1016/j.apsusc.2020.145883 APSUSC 145883
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
12 August 2019 11 February 2020 23 February 2020
Please cite this article as: Y. Xiao, W. Zhang, Adsorption Mechanisms of Mo2CrC2 MXenes as Potential Anode Materials for Metal-ion Batteries: A First-Principles Investigation, Applied Surface Science (2020), doi: https:// doi.org/10.1016/j.apsusc.2020.145883
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2020 Published by Elsevier B.V.
Adsorption Mechanisms of Mo2CrC2 MXenes as Potential Anode Materials for Metal-ion Batteries: A First-Principles Investigation Yi Xiao a*, Weibin Zhang b* a Institute b
of Materials Science, TU Darmstadt, 64287 Darmstadt, Germany
School of Physics and Optoelectronic Engineering, Yangtze University, Jingzhou, 434023, P. R. China
E-mail addresses: [email protected] (Y. Xiao), [email protected] (W. Zhang).
ABSTRACT: Two dimensional MXene are attracting increasing interests as electrode materials for metal ion batteries (MIBs) because metal ions can diffuse in a 2D lattice surface. In this work, first-principles calculations were carried out to investigate the adsorption performance of monolayer Mo2CrC2 for Li, Na, and K. The results show strong storage ability for various metal cations. There are three layers of Li, two layers of Na, one layer of K, and four layers of Mg on both sides of the monolayer. The electrochemical and thermodynamics of the intercalation of metal ions on the Mo2CrC2 Mxene have been intensively investigated. There is no band gap at the Fermi level of any configuration indicating that it has excellent electronic conductivity leading to an asymmetric electron localization distribution. The binding energy of metal ions decreases as the ion concentration increases. The adsorption of metal ions on a mono-layer of Mo2CrC2 has been studied, and the results indicate that the metal ions obviously improved the diffusion of metal ions. The CI-NEB method was used to study the diffusion of a cation on a Mo2CrC2 surface and search for its lowest energy along the diffusion pathway. Their structural transformations were studied via ab initio molecular dynamic (AIMD) simulations at a series of temperatures. The metal ions gradually evolved via thermal motion with increasing temperature. These results suggest that Mo2CrC2 could be a promising electrode material for Li- and Na-ion batteries in terms of Mo2CrC2 specific capacity, diffusion dynamics, and structural stability.
Keywords: Mo2CrC2 MXene; Metal ion batteries; Dynamics simulation; Ion diffusion; Conversion reaction.
1. Introduction The 2D MXene materials have shown promise as new energy storage devices. [1-2] There are many electrode materials with MXene-layered nanostructured materials; MXene-layered materials have excellent environmental protection, low cost, and availability. [3-4] This work studied their compatibility with the specific capacity characteristics of rechargeable batteries. Their safety issues and widespread use as cathode materials for lithium-ion batteries deserve further study. 2D MXene materials have a high specific capacity, high safety alternatives, and
high-volume energy storage performance versus other common cathodes. [5] MXenes are widely used in electronic devices, catalysts, energy storage, or conversion materials.[6] In addition, the electrochemical performance of lithium-ion batteries is competitive with other electrode materials, and it is well known that the structure can be considered a monolayer with the general formula Mn+1Xn with M = Sc, V, Hf, Nb, Zr, Ta, Cr, and Mo; here, X is C or N, and n is set to 1, 2, or 3. [7-8] MXene usually have some surface-terminated functional groups such as H, F, O, or OH, and these terminated MXene species are defined by the general formula Mn+1XnTx. MXenes have been synthesized via experimental methods including Ti3C2, Ti2C, (Cr0.5Nb0.5)2C, Ta4C3, (V0.5Cr0.5)3C2, Ti3CN, V2C, and Nb2C. [9-10] These MXenes have attracted great attention and demonstrated excellent electrochemical properties. [11] A new promising MXene material called Mo2CrC2 has been developed for anode materials for metal-ion batteries. It exhibits promising electrochemical performances with large capacities, excellent kinetic properties, and good stabilities.[11] However, there are still many challenges before practical use. It is difficult to synthesize and suffers from thermodynamic instability with frequent charging and discharging. The results show that the Mo2CrC2 MXene monolayer has high performance stability and storage capacity: Layered Mo2CrC2 can improve Li+ ion mobility due to its high electronic conductivity and lithium-ion diffusion coefficient. Recently, there have been many efforts to replace the terminated group on the layered surface in Mo2CrC2. These have been theoretically investigated to explain the synthesis of other terminated MXenes as well as their physical and chemical properties.[12] To better understand the electrochemical properties of MXene as an electrode, it is essential to investigate its electronic structure, capacity, stability, and dynamics via simulation. This work combines first-principles density functional theory (DFT) calculations with ab initio molecular dynamics (AIMD) simulations. [13] We used molecular dynamics simulations (MD) to study the thermal stability of these materials and improve their first-principles calculations. The MD simulations explain the effect of structural disorder of Mo2CrC2A2 (A=Li, Na, K, and Mg). We then used the CI-NEB method [14] with bond valence force fields to study structural and lithium-ion transport properties. The calculations indicate that Mo2CrC2A2 increases the adsorption energy of the cations due to charge transfer and bonding. The basic structure of Mo2CrC2 in the monolayer is not obviously influenced by the concentration of adsorbed metal ions. The adsorption energy of cations is subjected to adsorption concentrations. We further show that metal ions tend to adsorb on both sides of Mo2CrC2 layer rather than only one side in two-dimensional materials. The stability of the lithiated structures will be studied via ab initio molecular dynamics. The calculated energy barriers of one Li-ion diffusion on the MXene surface are very small.
2. Simulation Method All calculations were performed with density functional theory (DFT) within the Generalized Gradient Approximation and Perdew Burke Ernzerh (GGA-PBE) method as implemented in the Vienna ab initio simulation package (VASP) code. [15-16] To consider the action of the interlayer, the van der Waals (VdW) density functional (vdWDF) of optB86b was calculated for all simulations. [17] A plane-wave cutoff energy at 500 eV confirmed this convergence (10-5 eV in energy and 0.01 eV in force) of the structural relaxation within the conjugate gradient (CG) method. The Brillouin zone was integrated using an 11×11×1 MonkhorstPack mesh k-point grid during the relaxation for the band structure and density of states (DOS) calculations. [18] A vacuum separation of the interlayer was set to 20 Å to avoid any interactions during the use of periodic boundary conditions. Metal ion adsorption was studied on a 2×2×1 MXene nanosheet with a Brillouin zone sampling of 4×4×1 k-mesh. The adsorption energies of the first layer (equation 1) and additional layers (equation 2) were calculated via the following equation:
Ead(ave) (EMXeneMm EMXene mEM ) / m (M Li, Na, K, Mg)
Ead(ave） ( EMXene Mn EMXene M ( n 1) mEM ) / m ( M Li, Na, K , Mg ) . The open-circuit voltage (OCV) was expressed by the following equation:
(1) （2）
V (EMXeneMm EMXene mEM ) / mze (M Li, Na, K, Mg) .
(3)
Here, EMXene+Mn is the total energy of a metal-treated nanosheet, EMXene is the total energy of a pristine MXene nanosheet, and EM and m are the total energy of bulk metal ions and the number of adsorbed ions in each layer (for a 2×2 supercell is 8 on both sides), respectively. Terms n and c represent the number of adsorption layers and the valence state of fully ionized cations, respectively; m is the number of adsorbed metal atoms. For example, z=1 for Li+, Na+, and K+; z=2 for Mg2+. The theoretical capacities [19] of metal ion batteries Q were calculated by
Q czF / 3.6MMo2CrC2Mx .
(4)
Here, F is the Faraday constant at 96,485.3383 C/mol, and M stands for the molar mass of the Mo2CrC2Mx in g/mol; c and z represent the number of metal ion and the valence state of fully ionized cations, respectively. To investigate the diffusion pathway and energy barriers, the minimum energy paths of metal diffusion were considered. We used the climbing image-nudged elastic band method (CI-NEB) to search the transition state. This was done via VASP code, and this method was within eight images as simulated between the initial and final states. The molecular dynamic simulations were performed at various temperatures (300, 900, and 1500 K) with the NVT ensemble and the Nose-Hooveŕ method via ab initio molecular dynamics (AIMD). [20] Trajectory analysis of metal ion migration was based on Einstein’s diffusion equations as follows:
MSD t (t ) r (0) D
2
1 1 MSD t (t ) r (0) 4t 4t
2
.
(5) The radial distribution function (RDF) was used to estimate the lengths of different types of bonds and distances between atoms; this is defined as the following equation:
G (r )
1
(r r ) i0
i
V
N 1 ( r ri ) N
4 r ( r ) (0) 4 r 2 dr .
(6) Here, ρ, V, and N are the density, the volume, and the number of particles of the system, respectively. Term r is the atomic coordinate. [21]
3. Results and Discussion 3.1 Adsorption and storage capacity The intercalating ions from Figure 1 show the structures of Mo2CrC2 and three high-symmetry sites in the Brillouin zone. We conducted binding energy calculations as a function of the metal ions located on the top of a Mo atom, a C atom, and a Cr atom on both sides of the MXene. Thus, a total of three potential configurations have been considered. Taking dispersive interactions into account, [22] the following ions were considered: Li+, Na+, K+, and Mg2+. To examine the energetic stability of metal ions on the monolayer Mo2CrC2 MXene, we calculated the average adsorption energy of Li+, Na+, K+, and Mg2+ at the various states; these metals ions have similar adsorption sites (Figure 1e). The binding energies of the metal ions top-on-M (Mo), top-on-C, and top-on-Cr sites were calculated via PBE functionals. As the metal ions adsorbed at the top-on-Mo site, they spontaneously moved to the top-on-Cr site. Thus, the top-on-Cr sites were discussed in this work. All adsorption energies are negative after being fully adsorbed on Li+, Na+, K+, and Mg2+. This suggests that these ions can be adsorbed by the Mo2CrC2 MXenes nanosheets. K, in particular, has higher adsorption energies than the other ions. This may be related to the strong interaction between them and the MXene surface.
Figure 1. The structures of Mo2CrC2 supercell (2×2×1) modeling. (a) Top view of Mo2CrC2, (b) side view of Mo2CrC2, (c) the considered adsorption sites on the surface of Mo2CrC2 monolayer (top view). (d) A schematic diagram of the top-view Mo2CrC2 with three high-symmetry sites. (e) Adsorption energy (Eav) of metal ions in the first layer is located at Mo, Cr, and C atom sites on both sides for Mo2CrC2 monolayer surface.
Nonetheless, the Li and Mg adsorption energies of the previous work show an average adsorption energy of -0.76 eV vs. Li/Li+ on Mo2CrC2 electrode and -0.51 eV/atom for Mg. These species may be physisorbed by the MXenes surface. Some adsorption energies calculations about other MXenes were listed for comparison (in Table S3, see Supporting Information) [23-24]. The adsorption behavior of the Mo-based Mo2CrC2 MXenes resembles that of the Cr3C2 materials. This similar structure nicely explains why the adsorption profiles of Mo2CrC2 are similar to Cr3C2 within the variation of metal ion concentrations. The theoretical capacity of Mo2CrC2M2 increases from 193.06 mAh g-1 for Li, 173.05 mAh g-1 for Na, and 343.19 mAh g-1 for Mg (Figure 2b). Some other MXenes are listed for comparison in Table S4 (see Supporting Information). According to the additional adsorption layers calculation, a double layer was adsorbed on the Na ion. A double layer of Na ions formed after being full adsorbed on the monolayer Mo2CrC2 MXene in Figure 2d. Similar to other alkali metal ions, adsorption capacity is limited by the concentration of ions. Metal ion adsorption on MXene nanosheets with these metals ions is at similar adsorption sites regardless of the cation. The total open-circuit voltage (OCV) was calculated via the adsorption energy of the Mo2CrC2Mx (M=Li, Na, K, and Mg) structure (7.16 eV for Mg) in Figure 2a. This value is still higher than those of other metal ions (0.59–5.13 eV) with monolayer adsorption. The adsorption energy first decreases from 1.83 to 2.10 eV and then increases slightly from 2.10 to 1.96 eV. This finding offers an intrinsic advantage for ion conductivity at high ionic concentration. This may facilitate the fundamental science and technological development of 2D materials in energy storage applications. The average open-circuit voltages of alkali metal ions were observed at ≈0.76 V for Li+/Li, ≈0.89 V for Na+/Na, ≈1.06 V for K+/K, and ≈0.51 V for Mg2+/Mg in Table S1. This suggests that lithium and magnesium ion batteries should be encouraged for compounds with average open-circuit voltage close to or below zero.
Figure 2. Variation of Eav as a function of metal ions contents in Mo2CrC2. Here, metal content x is defined as the number of adsorbed metal ions per Mo2CrC2Mx (M=Li, Na, K, and Mg) unit cell. (a) Total of OCV for monolayer adsorption, (b) Theoretical capacity for monolayer adsorption, (c) E adsorption for multilayer adsorption, and (d) Theoretical capacity for multilayer adsorption on a metal-coated Mo2CrC2 nanosheet.
Here, a 2×2×1 supercell of the Mo2CrC2 monolayer was adopted in the calculations. We focus on Mo2CrC2M2n compositions here (M=Li, Na, K, and Mg; n is the number of adsorption layers). Importantly, Li, Na, and Mg can form stable multilayers on bare Mo2CrC2 MXenes via our proposed criteria. As expected of multilayer adsorption, Mg and Li can form stable adsorption layers four or three layers thick on all bare MXenes. The Mg ion is as high as 927.50 mAh g-1 on Mo2CrC2 with four metal adsorption layers. These are the highest capacities predicted for the Mg-ion battery. The Li ions also show a high capacity of 519.49 mAhg-1 on Mo2CrC2. This is higher than the theoretical capacity of K and Na ions; the theoretical storage capacity of Na ion can reach as high as 297.91 mAh g-1. The charge-discharge process of the Mo2CrC2 monolayer is the common cell reaction A+/A: Mo2CrC2+xM++xe- Mo2CrC2Mx. Here, we evaluate the average adsorption energy (Eav) of Li/Na/K/Mg of Mo2CrC2 monolayer. This is multilayer adsorption in the interlayer expanded bare Mo2CrC2 monolayer MXenes. Furthermore, the theoretical capacity profiles of Li- and Na-adsorbed of Mo2CrC2 were similar to the profiles of VS2 and Ti2B2 reported in the literature [25] indicating the reliability of our DFT calculation results.
3.2 Electronic properties Periodic calculations in the solid phase (Figure 3) showed that Cr-C has a similar bond length range from 2.095 to 2.101 Å, and Mo-C has a similar bond length range from 2.191 to 2.195 Å. In Mo2CrC2Li2, the length of Cr–C and Mo–C bonds (2.095 and 2.193 Å) is a little shorter than the corresponding bonds (2.196 and 2.097 Å) in the Mo2CrC2 monolayer, while the Cr-C and Mo-C bond lengths in Mo2CrC2Mg2 are 2.191 and 2.101 Å, respectively. They are slightly longer than the corresponding bonds in the Mo2CrC2 monolayer. The vacuum of Mo2CrC2M2n (M=Li, Na, K, and Mg) monolayer is 20 Å with ion adsorption from atomic layer distances of 4.686, 5.168, 5.512, and 4.760 Å, respectively. We studied the electrical properties of the Mo2CrC2M2n (M=Li, Na, K, and Mg) monolayer via DFT calculations. Figures 4 and S3 (Supporting Information) illustrate the total density of state
(TDOS) and partial density of state (PDOS) of these systems.[26-27] The lowest total energy of the structure is achieved when the metal atoms in the top atomic layer of the MXene are located directly above the Cr atoms. The three compounds under investigation have similar electronic properties (Figure 4) because of their structural similarity.
Figure 3. Geometric structures for single layer adsorption on both sides of Mo2CrC2Mx (M=Li, Na, K and Mg) MXenes: (a) Li, (b) Na, (c) K, and (d) Mg ion adsorption on a metal-coated Mo2CrC2 nanosheet.
The partial density of states (PDOS) of all models was studied and shows full monolayer adsorption. Figure S3 presents the PDOS for metal ions, Li-s, and s,p orbitals of Na, K, and Mg for the first layer of metal atoms. These results clearly demonstrate that the PDOS of the adsorption layer does not change significantly by adding the metal atoms; thus, we showed only one Na layer. The strong hybridization between Mo-4d and Na-3s states in the valence region shows the bonding states between C-2p and Mo-4d in the first layer. Hybridization between Cr-3d and Na-3p in the conduction region shows the anti-bonding state between Mo and Na. In the K adsorption layer, the hybridization between Mo-4d and K-4s remains similar in the valence band region, but this is reduced in the conduction state region. The Mo2CrC2 monolayer has intriguing properties and promising applications in metal ion batteries. The previous results can be used for further analysis to determine the reproduction in the electronic properties of the density of states as illustrated in Figure 4. The highest occupied states mainly have Mo4d features. This explains the relatively fast recovery of the electronic structure of insulators or semiconductors, and more insights into the electronic properties and chemical bonding can be gleaned from their partial density of state (PDOS).
Figure 4. For clarity, the total DOS and PDOS of metal ions atom on a metal-coated Mo2CrC2 nanosheet: (a) Li, (b) Na, (c) K, and (d) Mg. MXenes with geometry structures. Dashed lines represent the Fermi level at 0/eV.
The DOS and PDOS of metal ions on a metal-treated Mo2CrC2 nanosheet (Figures 4 and S3) show that the valence band is mainly composed of Li from -1 to 6 eV, and the conduction band is formed mainly by Mo4d and Li-s orbitals. The valence electrons fill the C2p band that is mainly built by the electronic states with Mo4d localized at the metal atoms. The electronic charge is distributed mainly on Mo and C atoms; these show a strong spd hybridization between M4d and Cr3d with C2p bands. The ionic character contribution is the weakest between Mo and metal cations in Mo2CrC2M2. Various interactions of Mo-A form the major types of interactions and determine the metal cation migration capabilities. However, battery charge and discharge are determined by the metal ion migration capabilities.
3.3 Multilayer adsorption In general, the theoretical capacity and voltage are two important indicators that determine battery performance. The charge and discharge process of Mo2CrC2 is described via the following half-cell reaction versus Mn+/M0. [28] Our results show that the first layer was located at the most stable adsorption site (top Cr site), and the second adsorption layer had a stable adsorption site on the top of the C site. In real-world applications, it is valuable to explore the storage capacity of the electrode materials for the batteries. In our work, the average adsorption energies are used to study the storage capacity of metal ions on the Mo2CrC2 monolayer on both sides. We proposed a practical adsorption model in which 2×2 supercells are investigated and can be labeled as Mo2CrC2M2n (here M is Li, Na, and Mg) with n representing different adsorption layers (n=1, 2, 3, and 4). For the first-layer metal ion adsorption as shown in Figure 5, all of the different metal atoms are absorbed at the top of Cr sites; there are eight metal atoms adsorbed on both sides of the Mo2CrC2 MXene monolayer. The average adsorption energy of metal atoms for the first layer is -0.64, -0.28, -0.20, and -0.89 eV for Li, Na, K, and Mg: This indicates that these metal atoms can adsorb full coverage and remain stable. After adsorption of the first layer, it will form the second adsorbed layer without K atoms, and these metal atoms can be adsorbed at the top of C atom sites with Mo2CrC2Li4, Mo2CrC2Na4, and Mo2CrC2Mg4; there are 16 metal atoms for full coverage
adsorption. The corresponding average adsorption energies are -0.31, -0.04, and -0.42 eV for full adsorption of Li, Na, K, and Mg, respectively. However, according to Eq. (2), the adsorption energies for the second adsorption layers of metal atoms are 0.02 for Li, 0.20 for Na, and 0.025 for Mg ion. However, according to this theory, we deduced that the positive adsorption energies indicate that the two layers of these metal ions are not stable indicating weak repulsive interactions between Mo2CrC2 and Li, Na, and Mg; this decreases the adsorption energy. Meanwhile, the electrons of the metal ions are spread out in the metal layers and form a negative electron cloud with clustering—these can be adsorbed for full coverage layer-by-layer. According to our DFT calculations, the average adsorption energies are negative for full adsorption for Li, Na, K, and Mg. Metal adatoms can be adsorbed stably with the clustering effect. For example, the third layer for Li atoms is adsorbed at the top of Mo sites, and the adsorption energy is -0.02 eV, which means that the adsorption of two layers of Li is stable. Furthermore, the -0.27 eV value is seen with clustering. The second layer for K cations had a positive adsorption energy of 0.32 eV, which indicates that the K cations can only form one layer on the monolayer. This suggests that these metal cations could form stable layer-by-layer adsorption. Further calculations indicated that Mg atoms can accumulate up to the fourth layer, and Mg cations could be adsorbed on Mo2CrC2Mg8 with negative average adsorption energies of -0.18 eV for the outermost Mg cation absorbed layer. In summary, the Li cations of the three adsorption layers preferred the top-of-Mo site. The Na cations in the two adsorption layers preferred the top-of-C site, and Mg cations can achieve four adsorption layers on the top of the first Mg layer site. Based on these results, a fully adsorbed Mo2CrC2 monolayer for the different metal cations was Mo2CrC2Li6, Mo2CrC2Na4, and Mo2CrC2Mg8.
Figure 5. Geometric structures for multilayer adsorption on both sides of fully adsorbed Mo2CrC2Mx (M=Li, Na, and Mg) MXenes monolayers with three Li layers, two Na layers, and four Mg layers. (a) Mo2CrC2Li6, (b) Mo2CrC2Na4, and (c) Mo2CrC2Mg8. Top
(first row) and side (second row) views. The different colors of the number represent the average adsorption energy of each layer.
We also estimated the vertical distance of the metal cations and Mo2CrC2 monolayer after the adsorption. The vertical distance is 4.786 Å for the first layer, 6.018 Å for the second layer, and 7.264 Å for the third layer of Li adsorption. In Mo2CrC2Na4, 5.190 Å was seen in the first layer, and 7.174 Å was seen in the second layer. The vertical distance of the Mo2CrC2Na8 monolayer was for the single layer (Mo2CrC2Na2), two layers (Mo2CrC2Na4), and three layers (Mo2CrC2Na6) adsorption values of 4.772, 6.244, and 7.792 Å, respectively, for Mg cations arranged layer-by-layer up to four layers deep (8.690 Å). These results suggest that the Mo2CrC2 monolayer is a robust anode material in both lithium ion battery (LIBs) and magnesium ion battery (MIBs). These capacities were attributed to many factors such as stacking layers and side reactions under experimental conditions, [29] but these factors were not considered in our DFT calculations. The open-circuit voltage (OCV) of Mo2CrC2 monolayers at different metal cation adsorption layers was calculated with equation (3). The theoretical discharge (Mn+/M0) profile voltage was obtained via the average adsorption energy. Figure 6(b) shows that all of the specific capacities were obtained in a zero-voltage window indicating that the Mo2CrC2 monolayers were suitable anode materials for Li-ion and Mg-ion batteries. According to Figure 6, the corresponding theoretical capacity and OCV of the Mo2CrC2Li6 are 519.49 mAhg-1 and -0.21 eV, respectively. Meanwhile, the theoretical capacity and open-circuit voltage (OCV) of the Mo2CrC2Mg8 are 927.50 mAhg-1 and 0.18 V, respectively. This increases the adsorbed Mg atoms from 6 to 24 in 2×2 supercell of Mo2CrC2 monolayer. Therefore, the calculated theoretical capacity of the Mo2CrC2Na2 of sodium-ion battery (NIBs) is 297.91 mAhg-1, and the average open-circuit voltage is 0.04 V. The differences in electron density obviously change for the four systems: The electron density evolved between metal cations and the Mo2CrC2 surface with a simultaneous decrease for layer-by-layer assembly of Mo2CrC2Li6, Mo2CrC2Na4, and Mo2CrC2Mg8. The electron localization function (ELF) [30] maps showed a distribution of electronic localization in the two-dimensional nanomaterial surface between the cation ions and the surface in the nanosheets. This indicates strong bonding between them suggesting excellent chemical stability for the Mo2CrC2 monolayers as electrodes for Li and Mg ion batteries in practical applications.
Figure 6. Calculated voltage profiles and capacity of Mo2CrC2 monolayers at different cation adsorption for (a) voltage and (b) theoretical capacity.
Next, we studied the adsorption behavior of different metal cations on Mo2CrC2 monolayers by electron localization function (ELF) (Figure S4). This is similar to a homogeneous electron gas; the value of the isosurface level is set to 0.0–0.8 a.u. Values of 0.8 and 0.0 correspond to fully localized and fully delocalized electrons, respectively. Figure S4 maps the electron localization functions (ELF) on the (110) planes for all of the configurations of the Mo2CrC2M2 (M=Li, Na, K, and Mg) monolayer with one adsorption layer. This showed that the valence electrons of the adatoms were fully ionized and spread out in the metal cations layer forming a negative electron cloud. Therefore, the K cations can form only one layer of adsorption on the Mo2CrC2 monolayer. This
indicates that the resulting ionic bonds are stronger than the metallic bonds. The local regions of electron density are usually the main contributors to the atomic and electron transfer between the metal cations. The electron density covers an entire localized region. Molybdenum and lithium atoms are usually regarded as electron donors and do not bind to either ionic or covalent processes regardless of electron gain or loss. They are left with an overall positive or negative charge. There are substantial concentrations of electrons around the outermost metal cations layer like the free electron gas localized on the Mo2CrC2 MXene surface. The results show an ionic bond character. However, there is almost no free electron delocalization [32, 44] in the internal layers of Mo2CrC2 MXene. The electron density of Mo2CrC2Li6, Mo2CrC2Na4, and Mo2CrC2Mg8 is analyzed based on Figures 5 and S4. The results show the electron density (red region) around the metal cations illustrating that the electron density around the metal cations increases. This implies partial electron transfer to the cations (red region) from adjacent Mo2CrC2 surfaces (blue region). Therefore, the charge transfer can significantly improve the electronic conductivity and theoretical specific capacity of the materials.
3.4 Ion diffusion The ion diffusion properties are a critical metric of metal ion batteries. They determine the charge/discharge rate of rechargeable batteries. Based on the previous analysis, the top of the Cr atoms was the most favorable adsorption site for all adatoms. There are three possible diffusion pathways between the two neighboring favorable adsorption sites (Figure 7). In pathway 1, a cation moves from the top of one Cr site (A) to an adjacent Cr site (A`) across the top of a C site (B). This was defined as A→B→A`. In pathway 2, the cations migrate from one A site and move directly to the nearest neighboring A` site. Pathway 3 is from the A site to the A` site across the top of a Mo site (C) or A→C→A`. The diffusion barrier profiles (Figure 8) of Mo2CrC2 monolayers for the three pathways and all cations show that pathway 3 has the largest diffusion barrier: There is only one saddle point in pathway 1. This illustrates that the diffusion of activated metal cations diffusing in the Mo2CrC2 layer occurs via one transition state (TS) via pathway 3. In comparison, the energy barrier along A→C→A` was significantly smaller. According to the Arrhenius equation, the diffusion coefficient (DLi) of the cations can be estimated by the following equation: DLi=d2ν0exp-Ea/kBT. (8). Here, Ea is the energy barrier (activation energy), kB is the Boltzmann constant, T is room temperature, and ν0 is the attempt frequency—a constant for each chemical reaction that relates to collision theory in the correct orientation. The calculated diffusion coefficient in Mo2CrC2A2 (A=Li, Na, K, and Mg) at room temperature (300 K) ranges from 10-9 up to 10-7 cm2/s. The Mo2CrC2 can provide better kinetic properties than the other 2D materials. [33]
Figure 7. A schematic diagram of the top and side views for possible pathway of a sodium ions diffusion on a 2×2×1 supercell of (a) Li, (b) Na (c) K, or (d) Mg ion adsorption on a metal-coated Mo2CrC2 nanosheet Mxenes.
Figure 8. Energy profiles of metal ion diffusion on a 2×2×1 supercell of (a) Li, (b) Na (c) K, and (d) Mg ion diffusion on Mo2CrC2 nanosheet Mxenes.
For all adsorbed cations, a local energy minimum was observed at the intermediate C site. There are two saddle points in the middle of the A → C → A` and A → A` bridges for all metal cations without K cation. The calculated diffusion barriers along pathway 1 were 0.026, 0.027, and 0.021 eV for Li+, Na+, and K+, respectively. The obtained diffusion barrier for Mg2+ was consistent with our previous adsorption energies of metal cations results. For Mg2+, path 2 (A→A` bridges) has the smallest diffusion energy barrier versus other pathways (0.083 eV). There is a much smaller diffusion barrier value than in the other 2D energy storage materials: TMD (MoS2: 0.49 eV for Li)[34] and (MoN2: 0.78 eV for Li, 0.56eV for Na and 0.49eV for Mg) [35] and graphene (0.32 eV for Li) [36]. In addition, the diffusion barriers for Mg2+ were larger than those for Li+, Na+, and K+. Therefore, the diffusion barrier followed the ordered sequence from lowest to highest: Mg2+ < Li+ < Na+ < K+ along the most favorable diffusion pathways in the three pathways. The diffusion barrier was similar to the diffusion path of Li+, Na+, and K+. In addition, the adsorption distance of the Cr–Li bonds (4.69 Å at A site and 3.45 Å at B site) in Mo2CrC2Li2 is shorter than that in Mo2CrC2Na2 (5.17 Å for A site and 4.24 Å for B site); these values were also calculated for Mo2CrC2K2 (5.51 Å for A site and 4.18 Å for B site) from Figure 7. Therefore, after layer-by-layer adsorption, cations migrate on one layer of the blocks, and the metal cation diffusion makes this more difficult. Our work determined the kinetic pathways for metal cations. The vacancy migration is consistent with the most-favored pathway calculated via the CI-NEB method; this work also presents the barrier energy involved. The atomic arrows indicate the motion of neighboring favored adsorption site atoms (Figure 8). The TS state has a barrier energy of 0.0210.083 eV relative to that of the most stable cations vacancy state. The lower energy barrier usually leads to faster metal-ion transport in rechargeable batteries. Path 1 has a lower energy barrier indicating that spatial effects play an important role in accelerating the metal-ion migration—especially that of K (0.021 eV). To the best of our knowledge, alkaline earth metals (Mg2+) have a high diffusion barrier versus other metal adatoms. Indeed, we found that the diffusion of Mg2+ across the top of C or Mo atoms must overcome a barrier energy of about 0.376 eV in the A→C→A` direction or 0.085 eV in the A→B→A` direction.
3.5 MD simulation
Metal ion diffusion occurs in the cation layer via vacancy jumping because of the layered structure of 2D Mo2CrC2 MXene materials. This work primarily studied the thermal motion of metal ions within a supercell (2x2x1) with four formula units via MD simulations at 300, 900, and 1500 K within 10 ps simulation times. Figure 10 shows the mean square displacements (MSD) [37-40] of Mo2CrC2M2 (M= Li, Na, K, and Mg) at various temperatures (300, 900, and 1500 K). The metal cation MSD values are given preference in Figure 9 suggesting that MSD values should be fairly constant. These were calculated via the Einstein diffusion equation for four complexes along with MD calculated displacements. We used linear fitting of the MSD values and favored cationic displacements for the metal ions to obtain the self-diffusion coefficient of the cations. This provides insight into how the potential can be improved during the 10-ps simulation. This contributes to the self-diffusion coefficient caused by the relative motion of metal ions calculated by equation 5, i.e., the computer simulation of the MSD of a cation considered as implemented in the NVT ensemble. The value of the self-diffusion coefficient was calculated from the linear relation of the MSD curve with simulation time. In Mo2CrC2A2, the self-diffusion coefficients are 5.12×10-7, 1.32×10-7, 1.37×10-7, and 5.71×10-7 m2/s for Li+, Na+, K+, and Mg2+ ions at room temperature, respectively.
Figure 9 Mean square displacements between the metal ion sites of (a) Li, (b) Na, (c) K, and (d) Mg ion diffusion on Mo2CrC2 nanosheet Mxenes at 300, 900, and 1500 K. (a) Li, (b) Na (c) K, and (d) Mg ion diffusion on Mo2CrC2 nanosheet Mxenes.
In general, the radial distribution function (RDF) g(r) can give the probability of finding one particle in the interatomic separation (r) from another nanoparticle. [40-43] Significant results have been obtained for cations (Li, Na, K, and Mg).[44-47] Here, we studied cations with potentially significant interactions between metals ions and the Mo2CrC2 surface according to RDF calculations. The plot of RDF demonstrates a series of peaks corresponding to the evolution of particle separation within nearest neighbor distances. As temperature increases, the peak widths gradually become broader, and the peak height gradually reduced due to the thermal motion of the metal ions. The RDFs of Mo-C and Cr-C are all shown in Figure 10. To further analyze the intensity of the first peak positions, we found that the contribution order of cations is K+ > Li+> Na+> Mg2+ indicating the order of the binding strength between transition metals (TMs) and carbon. This study shows that the RDFs of the Mo2CrC2 surface with metal ion layer had a prominent difference for the interatomic separation. The Mo2CrC2Li2 results on RDF of the nearest atomic distance of Mo-C (around the position of 2.1 Å) and Cr-C (around the position of 2.2 Å) pair functions are consistent with prior experimental studies. As the temperature increases, the difference in the RDF of the Li-ions gradually becomes less clear; the peaks of RDFs of the Mo-C pair finally overlap [see Figure 10(d)]. The differences between calculated values of the self-diffusion coefficient are obvious. There is an obvious first peak in all configurations. The characteristic of the RDF is determined by the interatomic separation (Mo-C and Cr-C) (from 2.1 to 2.3 Å).
Figure 10. The radial pair distribution function g(r) at 300, 900, and 1500 K. (a) Li, (b) Na, (c) K, and (d) Mg ion diffusion on Mo2CrC2 nanosheet Mxenes.
Figure 11. The motion of ions at 300 K: (a) Li, (b) Na, (c) K, and (d) Mg on Mo2CrC2 nanosheet Mxenes.
Figure 11 shows that by using the widely varying simulation level with NVT ensemble conditions at room temperature, all of these composites found common features of metal ion motion with long (3−5 ps) dynamics. This was seen in the solid phase at room temperature by exploring the motion displacement of ions during the atomic level dynamics. [48-50] Our calculations highlight that Li ions are most likely to migrate with obvious localization in supercells in the Z directions; see Figure 11a for a random snapshot extracted from the simulated time evolution. This is in line with the thermally activated carrier recombination indicating a cation-induced shifting of the supercell surface in different spatial regions. This might underlie possible ionic transport mechanisms and could be explored via cation dynamics.[51-54] The snapshot of MD simulations results in Figure S6 shows that the most energetically stable configuration is used as the initial structure for AIMD. The simulation temperature is set to 300, 900, and 1500 K. At 300 K, the structural transformation of Mo2CrC2Li2 and Mo2CrC2Mg2 did not differ significantly during the simulation time. However, a structural transformation was seen with Mo2CrC2Na2 and Mo2CrC2K2 at 300 K. This leads to stronger kinetic properties than Li and Mg ions on Mo2CrC2. The diffusion of cations is quite dependent on the diffusion path. The simulation temperature is then set to 900 and 1500 K with significantly high temperature conditions. There are obvious structural transformations at all configurations. Our simulation model is not sufficiently large, and the simulation time is limited. [55-56] The simulation results show that both Mo2CrC2Na2 and Mo2CrC2K2 begin
to transform into the corresponding disordered structures at 900 and 1500 K (see Figure S5). We selected Mo2CrC2Mg2 as a representative species for MD simulations at 300 K. The structure retains its original state, and the structural transformation is not observed by Mg ion monolayer adsorption. The MD simulations reveal that the Mo2CrC2 monolayer has good stability during the Li and Mg ion insertion processes. [57]
4. Conclusion In summary, we designed a new 2D electrode material with a Mo2CrC2 monolayer for energy storage. The promising properties of the Mo2CrC2 monolayers as the electrode for Li+, Na+, K+, and Mg2+ ions batteries were investigated using first-principles calculations. Li ions have three adsorption layers, and Na cations have two adsorption layers with a preference for the top of the Mo2CrC2 surface. Importantly, Mg ions can achieve four adsorption layers—this leads to high theoretical capacities of fully adsorption system, i.e., 519.49 mAh g-1 for Li, 297.91 mAh g-1 for Na, 154.88 mAh g-1 for K, and 927.51 mAh g-1 for Mg during the multilayer adsorption. The promising conductivity and low diffusion barriers of Mo2CrC2 monolayers ensured excellent kinetic properties for electron transportation and ion diffusion for rechargeable batteries. The calculated diffusion barriers along the most-favored pathway were 0.026, 0.027, 0.027, and 0.083 eV for Li+, Na+, K+, and Mg2+, respectively. While there were significant differences in motility of metal ions in Mo2CrC2 materials, the structural stability is due to their variation in electron localization distribution with Na+ or Mg2+ ions during adsorption in Mo2CrC2. The AIMD simulation results confirm that the Mo2CrC2 monolayer can maintain its structural stability up to 1500 K. Therefore, Mo2CrC2 may find wide applications including in electric vehicles and metal ion batteries.
Acknowledgments Financial support from China Scholarship Council (CSC: 201808440416) of China and Research and the Arts (HMWK) of the Hessen state in Germany. The Lichtenberg high performance computer is gratefully acknowledged, we are also thankful to TU Darmstadt. We also thank LetPub (www.letpub.com) for its linguistic assistance during the preparation of this manuscript.
Conflict of Interest The authors declare that they have no conflict of interest.
References 1. Wick P, Louw-Gaume A E, Kucki M, et al. Classification Framework for Graphene-Based Materials [J]. Angewandte Chemie, 2014, 53(30):7714-8. 2. Anasori B, Lukatskaya M R, Gogotsi Y. 2D metal carbides and nitrides (MXenes) for energy storage [J]. Nature Reviews Materials, 2017, 2(2):16098. 3. Peng L, Zhu Y, Chen D, et al. Two‐Dimensional Materials for Beyond-Lithium-Ion Batteries[J]. Advanced Energy Materials, 2016, 6(11): 1600025. 4. Li Y, Wu D, Zhou Z, et al. Enhanced Li Adsorption and Diffusion on MoS2 Zigzag Nanoribbons by Edge Effects: A Computational Study [J]. Journal of Physical Chemistry Letters, 2012, 3(16):2221. 5. Peng J, Chen X, Ong W J, et al. Surface and heterointerface engineering of 2D MXenes and their nanocomposites: insights into electro-and photocatalysis[J]. Chem, 2018. 6. Shein I R, Ivanovskii A L. Graphene-like nanocarbides and nanonitrides of d metals (MXenes): synthesis, properties and simulation[J]. Micro & Nano Letters, 2013, 8(2): 59-62. 7. Kurtoglu M, Naguib M, Gogotsi Y, et al. First principles study of two-dimensional early transition metal carbides[J]. Mrs Communications, 2012, 2(4): 133-137. 8. Sun D, Hu Q, Chen J, et al. Structural transformation of MXene (V2C, Cr2C, and Ta2C) with O groups during lithiation: a first-principles investigation[J]. ACS applied materials & interfaces, 2015, 8(1): 74-81.
9. Er D, Li J, Naguib M, et al. Ti3C2 MXene as a high capacity electrode material for metal (Li, Na, K, Ca) ion batteries[J]. ACS applied materials & interfaces, 2014, 6(14): 11173-11179. 10. Wang X, Kajiyama S, Iinuma H, et al. Pseudocapacitance of MXene nanosheets for high-power sodium-ion hybrid capacitors.[J]. Nature Communications, 2015, 6:6544. 11. Feng J, Sun X, Wu C, et al. Metallic Few-Layered VS2 Ultrathin Nanosheets: High Two-Dimensional Conductivity for In-Plane Supercapacitors [J]. Journal of the American Chemical Society, 2011, 133(44):17832-17838. 12. Wang D, Liu Y, Meng X, et al. Two-dimensional VS2 monolayers as potential anode materials for lithium-ion batteries and beyond: first-principles calculations[J]. Journal of Materials Chemistry A, 2017, 5(40): 21370-21377. 13. Guo X, Zhang X, Zhao S, et al. High adsorption capacity of heavy metals on two-dimensional MXenes: an ab initio study with molecular dynamics simulation[J]. Physical Chemistry Chemical Physics, 2016, 18(1): 228-233. 14. Henkelman G, Uberuaga B P, Jónsson H. A climbing image nudged elastic band method for finding saddle points and minimum energy paths[J]. The Journal of chemical physics, 2000, 113(22): 9901-9904. 15. Hafner J. Ab‐initio simulations of materials using VASP: Density‐functional theory and beyond[J]. Journal of computational chemistry, 2008, 29(13): 2044-2078. 16. Berland K, Cooper V R, Lee K, et al. van der Waals forces in density functional theory: a review of the vdW-DF method[J]. Reports on Progress in Physics, 2015, 78(6): 066501. 17. Giovannetti G, Khomyakov P A, Brocks G, et al. Substrate-induced band gap in graphene on hexagonal boron nitride: Ab initio density functional calculations[J]. Physical Review B, 2007, 76(7): 073103. 18. Sun G, Kürti J, Rajczy P, et al. Performance of the Vienna ab initio simulation package (VASP) in chemical applications[J]. Journal of Molecular Structure: THEOCHEM, 2003, 624(1-3): 37-45. 19. Hafner J. Materials simulations using VASP—a quantum perspective to materials science[J]. Computer physics communications, 2007, 177(1-2): 6-13. 20. Xiao Y, Zhang F C, Han J I. Electrical structures, magnetic polaron and lithium ion dynamics in three transition metal doped LiFe1− xMxPO4 (M= Mn, Co and La) cathode material for Li ion batteries from density functional theory study[J]. Solid State Ionics, 2016, 294: 73-81. 21. Zhang W, Wang Y, Wang J, et al. Theoretical prediction of the structural, electronic, mobility, and dynamic properties of Mo3N2T2 (T= H, O and OH) with Li adsorption feature[J]. Solid State Ionics, 2018, 325: 238-246. 22. Burns L A, Mayagoitia Á V, Sumpter B G, et al. Density-functional approaches to noncovalent interactions: A comparison of dispersion corrections (DFT-D), exchange-hole dipole moment (XDM) theory, and specialized functionals[J]. The Journal of chemical physics, 2011, 134(8): 084107. 23. Xie Y, Dall’Agnese Y, Naguib M, et al. Prediction and characterization of MXene nanosheet anodes for non-lithium-ion batteries[J]. ACS nano, 2014, 8(9): 9606-9615. 24. Novoselov K S, Mishchenko A, Carvalho A, et al. 2D materials and van der Waals heterostructures[J]. Science, 2016, 353(6298): aac9439. 25. Bo T, Liu P F, Xu J, et al. Hexagonal Ti2B2 monolayer: a promising anode material offering high rate capability for Li-ion and Na-ion batteries[J]. Physical Chemistry Chemical Physics, 2018, 20(34): 22168-22178. 26. Xiao Y, Wang J, Wang Y, et al. A new promising catalytic activity on blue phosphorene nitrogen-doped nanosheets for the ORR as cathode in nonaqueous Li–air batteries[J]. Applied Surface Science, 2019, 488: 620-628. 27. Hong J, Pan Y, Hu Z, et al. Direct imaging of kinetic pathways of atomic diffusion in monolayer molybdenum disulfide [J]. Nano Letters, 2017, 17(6). 28. Zhou Y, Zu X. Mn2C sheet as an electrode material for lithium-ion battery: A first-principles prediction [J]. Electrochimica Acta, 2017. 29. Tang Q, Zhou Z, Shen P. Are MXenes promising anode materials for Li ion batteries? Computational studies on electronic properties and Li storage capability of Ti3C2 and Ti3C2X2 (X= F, OH) monolayer[J]. Journal of the American Chemical Society, 2012, 134(40): 16909-16916. 30. Yang S, Li D, Zhang T, et al. First-principles study of zigzag MoS2 nanoribbon as a promising cathode material for rechargeable Mg batteries[J]. The Journal of Physical Chemistry C, 2011, 116(1): 1307-1312. 31. Zhang X, Yu Z, Wang S S, et al. Theoretical prediction of MoN2 monolayer as a high capacity electrode material for metal ion batteries[J]. Journal of Materials Chemistry A, 2016, 4(39): 15224-15231. 32. Liang Y, Feng R, Yang S, et al. Rechargeable Mg batteries with graphene‐like MoS2 cathode and ultrasmall Mg nanoparticle anode[J]. Advanced Materials, 2011, 23(5): 640-643. 33. Xiao Y, Liu Y, Frederick N, et al. Ab initio study of the electronic structure, magnetic and dynamics properties in LiMO2 and NaMO2 (M=V and Cr) as electrode cathode material for batteries[J]. Solid State Ionics, 2017, 307: 26-34. 34. Li Y, Wu D, Zhou Z, et al. Enhanced Li adsorption and diffusion on MoS2 zigzag nanoribbons by edge effects: a computational
study[J]. The journal of physical chemistry letters, 2012, 3(16): 2221-2227. 35. Zhang X, Yu Z, Wang S S, et al. Theoretical prediction of MoN2 monolayer as a high capacity electrode material for metal ion batteries[J]. Journal of Materials Chemistry A, 2016, 4(39): 15224-15231. 36. Zheng J, Ren Z, Guo P, et al. Diffusion of Li+ ion on graphene: A DFT study[J]. Applied Surface Science, 2011, 258(5): 1651-1655. 37. Kuritz N, Murat M, Balaish M, et al. PFC and Triglyme for Li–Air Batteries: A Molecular Dynamics Study[J]. The Journal of Physical Chemistry B, 2016, 120(13): 3370-3377. 38. Eom T H, Xiao Y, Han J I, et al. Electrical structure, magnetic polaron and lithium ion dynamics in four mixed-metal oxide multiple-phase electrode cathode material for Li ion batteries from density functional theory study[J]. Computational Materials Science, 2017, 132: 92-103. 39. Xiao Y, Zhang F C, Han J I. Synthesis, characterization and lithium-ion migration dynamics simulation of LiFe 1− x T x PO 4 (T= Mn, Co, La and Ce) doping cathode material for lithium-ion batteries[J]. Applied Physics A, 2016, 122(11): 980. 40. Berdiyorov G R, Mahmoud K A. Effect of surface termination on ion intercalation selectivity of bilayer Ti3C2T2, (T=F, O and OH) MXene [J]. Applied Surface Science, 2017, 416. 41. Zhang X, He T, Meng W, et al. Mn2C Monolayer: Hydrogenation/Oxygenation Induced Strong Ferromagnetism and Potential Applications[J]. The Journal of Physical Chemistry C, 2019.12.12. 42. Guo X, Zhang X, Zhao S, et al. High adsorption capacity of heavy metals on two-dimensional MXenes: an ab initio study with molecular dynamics simulation.[J]. Physical Chemistry Chemical Physics, 2015, 18(1):228. 43. Gandi A N, Alshareef H N, Schwingenschlögl U. Thermoelectric Performance of the MXenes M2CO2 (M = Ti, Zr, or Hf) [J]. Chemistry of Materials, 2016, 28(6). 44. Yu Y X. Prediction of Mobility, Enhanced Storage Capacity, and Volume Change during Sodiation on Interlayer-Expanded Functionalized Ti3C2 MXene Anode Materials for Sodium-Ion Batteries [J]. Journal of Physical Chemistry C, 2016, 120(10). 45. Wang G. A Theoretical Prediction of the Intrinsic Half-Metallicity in Surface-Oxygen-Passivated Ti2N MXene[M]// Introduction to statistics for the behavioral sciences /. Saunders, 2013:464-478. 46. Wang D, Gao Y, Liu Y, et al. First-Principles Calculations of Ti2N and Ti2NT2 (T = O, F, OH) Monolayers as Potential Anode Materials for Lithium-Ion Batteries and Beyond [J]. Journal of Physical Chemistry C, 2017, 121(24). 47. Er D, Li J, Naguib M, et al. Ti3C2, MXene as a High Capacity Electrode Material for Metal (Li, Na, K, Ca) Ion Batteries [J]. Acs Appl Mater Interfaces, 2014, 6(14):11173-11179. 48. Banerjee S, Periyasamy G, Pati S. Possible application of 2D-boron sheets as anode material in lithium ion battery: A DFT and AIMD study [J]. Journal of Materials Chemistry A, 2014, 2(11):3856-3864. 49. Chen X, Kong Z, Li N, et al. Proposing the prospects of Ti3CN transition metal carbides (MXenes) as anodes of Li-ion batteries: a DFT study.[J]. Physical Chemistry Chemical Physics Pccp, 2016, 18. 50. Wang D, Gao Y, Liu Y, et al. Investigation of Chloride Ion Adsorption onto Ti2C MXene Monolayers by First-principles Calculations[J]. Journal of Materials Chemistry A, 2017, 5(47). 51. Eklund P, Rosen J, Persson P O Å. Layered ternary M n+1AXn, phases and their 2D derivative MXene: an overview from a thin-film perspective [J]. Journal of Physics D Applied Physics, 2017, 50(11). 52. Hu T, Wang J, Zhang H, et al. Vibrational properties of Tir3C2 and Ti3C2T2 (T =O, F, OH) monosheets by first-principles calculations: a comparative study [J]. Physical Chemistry Chemical Physics, 2015, 17(15):9997-10003. 53. Xie Y, Naguib M, Mochalin V N, et al. Role of surface structure on Li-ion energy storage capacity of two-dimensional transition-metal carbides [J]. Journal of the American Chemical Society, 2014, 136(17):6385-94. 54. Ma Z, Hu Z, Zhao X, et al. Tunable Band Structures of Heterostructured Bilayers with Transition-Metal Dichalcogenide and MXene Monolayer [J]. Journal of Physical Chemistry C, 2014, 118(10):5593–5599. 55. Khazaei M, Arai M, Sasaki T, et al. Two-dimensional molybdenum carbides: potential thermoelectric materials of the MXene family [J]. Physical Chemistry Chemical Physics, 2014, 16(17):7841-9. 56. Zha X H, Yin J, Zhou Y, et al. Intrinsic structural, electrical, thermal, and mechanical properties of the promising conductor Mo2C MXene[J]. The Journal of Physical Chemistry C, 2016, 120(28): 15082-15088. Zha X H, Luo K, Li Q, et al. Role of the surface effect on the structural, electronic and mechanical properties of the carbide MXenes[J]. EPL (Europhysics Letters), 2015, 111(2): 26007.
Yi Xiao: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Writing - original draft, Project administration, Resources. Conceptualization, Data curation, Formal analysis, Writing - review & editing. Weibin Zahng: Funding acquisition, Software. Validation, Visualization.
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Adsorption Mechanisms of Mo2CrC2 MXenes as Potential Anode Materials for Metal-ion Batteries: A First-Principles Investigation Yi Xiao a*, Weibin Zhang b* a Institute b
of Materials Science, TU Darmstadt, 64287 Darmstadt, Germany
School of Physics and Optoelectronic Engineering, Yangtze University, Jingzhou, 434023, P. R. China
E-mail addresses: [email protected] (Y. Xiao), [email protected] (W. Zhang).
1. We designed a new 2D electrode material with a Mo2CrC2 monolayer for energy storage 2. It ensured excellent kinetic properties for electron transportation and ion diffusion for rechargeable batteries. 3. Mo2CrC2 may find wide applications including in electric vehicles and metal ion batteries. 4. MD simulations reveal that the Mo2CrC2 monolayer has good stability during the Li and Mg ion insertion processes.