Ab Initio Prediction and Characterization of Mo2C Monolayer as

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Ab Initio Prediction and Characterization of MoC Monolayer as Anodes for Lithium-Ion and Sodium-Ion Batteries Qilong Sun, Ying Dai, Yandong Ma, Tao Jing, Wei Wei, and Baibiao Huang J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b00171 • Publication Date (Web): 23 Feb 2016 Downloaded from http://pubs.acs.org on February 24, 2016

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The Journal of Physical Chemistry Letters

Ab Initio Prediction and Characterization of Mo2C Monolayer as Anodes for Lithium-Ion and Sodium-Ion Batteries Qilong Sun†, Ying Dai*, †, Yandong Ma‡, Tao Jing†, Wei Wei† and Baibiao Huang*, †

†

School of Physics, State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, People’s Republic of China

‡

Engineering and Science, Jacobs University Bremen, Campus Ring 1, 28759 Bremen, Germany

Email: [email protected] (Y.D.) Email: [email protected] (B.B.H.)

ACS Paragon Plus Environment


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ABSTRACT: Identifying suitable electrodes materials with desirable electrochemical properties is urgently needed for the next-generation of renewable energy technologies. Here, we report an ideal candidate material, Mo2C monolayer, with not only required large capacity but also high stability and mobility by means of first-principles calculations. After ensuring its dynamical and thermal stabilities, various low energy Li and Na adsorption sites are identified, and the electric conductivity of the host material is also maintained. The calculated minor diffusion barriers imply a high mobility and cycling ability of Mo2C. In addition, the Li-adsorbed Mo2C monolayer possesses a high theoretical capacity of 526 mAh·g-1 and a low average electrode potential of 0.14 eV. Besides, we find that the relatively low capability of Na-adsorbed Mo2C (132 mAh·g-1) arises from the proposed competition mechanism. These results highlight the promise of Mo2C monolayer as an appealing anode material both for lithium-ion and sodium-ion batteries. TOC GRAPHICS


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Rechargeable Lithium-ion batteries (LIBs), one of great successes of clean energy storage technologies, have attracted intensive attentions due to the combination of outstanding reversible capacity, high power density and long cycle life.1-7 Currently, LIBs are widely applied as portable power sources in electronic devices and to power electric vehicles.8-9 However, a breakthrough in terms of enhanced energy storage materials is urgently needed by the rapid development of the electronic market in recent years. So the footsteps seeking for new energetic materials to meet the demand for next generation LIBs have never stopped.10-12 The capability of LIBs is highly dependent on the performances of their electrode materials. Accordingly, to overcome these formidable challenges, one practicable option is to develop new enhanced electrode materials. Compared with the burgeoning improvement in cathode materials, the development of anode materials is much slower and limited to carbon related materials. Nowadays, the most widely used anode material is graphite because of its relatively good cycling stability and low cost.13 But the relatively low capacity (372 mAh/g) and poor rate capability restrict its further applications.14 Fortunately, with the development of material science, two-dimensional (2D) materials provide more promising choices as they often possess flat surfaces and high surface areas, which potentially allow for high-energy densities and high motilities.15-17 So far, many 2D materials, including graphene,18-19 transition metal dichalcogenides (TMD),20 and MXenes, have been identified to be superior candidates used as electrode materials.15, 20-23

On the other hand, the alternative option is the rechargeable Na-ion batteries

(NIBs), which are rapidly emerging as a plausible alternative to LIBs. NIBs possess



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not only the abundance of Na and the associated cost advantages, but also high theoretical capacities, suitable negative redox potentials (-2.71 V vs SHE), operational safety, and environmentally benign nature.24-26 Similar to Li-ion batteries, a significant part of research effort focuses on the development of negative electrodes (anode), such as MoS2/graphene,27 MXenes,28-29 and van der Waals-bonded layered materials24, 30. To this end, looking for a high-capacity host material that is available for the anodes of LIBs and also meets the requirements of the NIBs would be all that can be wished for or desired. Transition metal carbides (TMCs), which originate from the incorporation of carbon atoms into the metal lattice, are a large family of materials with many intriguing properties and applications. They can offer metallic conductivity combined with excellent catalysts capable of challenging noble metals. Recently, these TMCs have moved into the territory of 2D materials and joined this family. For example, Xu et al. obtained large-area high-quality 2D ultrathin α-Mo2C crystals by chemical vapor deposition (CVD) with low temperature superconductivity.31-32 Thus, this is truly an exciting opportunity that can lead to dozens of new 2D materials with unique properties as this versatile synthesis method can be extended to other carbides beyond Mo2C. Such stable high-quality clean 2D superconducting crystals not only provide a new platform to study 2D superconductor physics, but also exhibit desirable properties as electrode materials for LIBs and NIBs. Given that finding suitable electrodes for the rechargeable batteries is one of the glorious missions of material science and the experimental or theoretical studies on ultrathin Mo2C as anodes in


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batteries is still a gap, we then expect to find out whether the Mo2C nanomaterials are suitable for the next generation of LIBs and NIBs. Further, we would like to give powerful physical insights for the performance of Mo2C as electrodes. Inspired by the contents discussed above, in this work, we systematically investigate the electronic and Li/Na storage properties of proposed Mo2C monolayer by means of density functional theory (DFT). Firstly, phonon spectra and ab initio molecular dynamics (AIMD) simulations provide compelling evidence for thermal and dynamical stabilities of the Mo2C monolayer. Our results indicate that both Li and Na can adsorb on the surface of Mo2C monolayer maintaining the metallic nature of the adsorption systems, which is crucial for the anodes materials. Moreover, the bare Mo2C monolayer exhibits minimal diffusion barriers for both Li and Na atoms implying a superior diffusion mobility of Li/Na in LIBs and NIBs. In addition, we also predict a high specific Li capacity (526 mAh·g-1) for the Mo2C monolayer when used in LIBs. Besides, we find that the relatively low capability of Na adsorbed on the Mo2C can attribute to the proposals competition mechanism. At last, we identify the existence of free electron gas in the Li adsorbed systems through Electron localization functions (ELF). And the examination of volume of the batteries materials after adsorption is also very optimistic. Overall, our results show that Mo2C monolayers are promising electrode materials for LIBs and NIBs, simultaneously. The results are encouraging, and provide a promising candidate for the next generation portable batteries. The relaxed geometric structure of 2D Mo2C monolayer is shown in Figure 1a and



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1b. As depicted in Figure 1a,b, the Mo2C monolayer lattice structure exhibits a hexagonal unit cell. Similar to the transitional metal dichalcogenides (i.e. MoS2, WS2, MoSe2 and so on), each bare Mo2C monolayer consist of a triple layer of atoms stacked in the sequence of Mo-C-Mo with the thickness of 2.70 Å. As for the Mo2C monolayer, the equilibrium lattice parameters are found to be a = b = 2.85 Å with the length of Mo-C being 2.15 Å in the ground state.

Figure 1. Optimized geometry of the Mo2C monolayer (of 3×3 supercell). Top (a) and side (b) views of the Mo2C monolayer, which show a layered structure like a sandwich. (c) The considered adsorption sites on the 2D Mo2C surface.

Though the high-quality 2D ultrathin Mo2C has been achieved by chemical vapor deposition (CVD) in experiment, confirming the dynamical and thermal stabilities of the real sense of monolayer Mo2C would still be the top priority in our studies. As a starting point, AIMD simulations are performed to check the thermal stability of the Mo2C monolayer. The snapshots of structures calculated by AIMD are shown in Figure 2a. Meanwhile, for clarity, particular distorted details are enhanced in the


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enlarged inserts α and β. After running 1000 steps (3 ps) at the 300 K, we notice that no broken bonds and geometric reconstructions are found in our result. As shown in the insert (α) in Figure 2a, the most significant deformation is only a minor movement for the Mo atom in the lower atomic layer, suggesting that the structure of the Mo2C monolayer will be stable even at a temperature of 300 K. In addition, the evolution of free energy for the Mo2C monolayer during AIMD simulation at 300 K is shown in Figure 2b, which also confirm that the Mo2C monolayer is thermodynamically stable at the temperature of 300 K. On the other hand, the phonon dispersion relations, which are defined as the k wave vector dependence of the frequencies

of the

normal modes for all branches j and selected directions in the crystal, are widely used to demonstrate the structural stability in many other literatures.33 Hence, we perform phonon dispersion calculations to further examine the lattice dynamics, and the corresponding results are shown in Figure 2c. The phonon dispersion curves clearly show that its optical and acoustical branches are well separated and all branches have positive frequency. Two acoustical branches are with linear dispersions as the Γ→ 0, while the lowest transverse branch displays a quadratic dispersion near Γ point. In this respect, those results as well indicate that 2D periodic honeycomb structure of Mo2C is stable. Consequently, the present analysis, referring to the AIMD simulations and the calculated phonon dispersion curves, may offer a stringent proof for the stability of honeycomb structure of Mo2C monolayer generally. Moreover, the excellent thermal and chemical stability of the 2D ultrathin Mo2C surface, which owning the same coordination of the surface Mo atoms as our Mo2C monolayer, has also been



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identified in the experiments, and dispelled the concerns of the negative effects from surface dangling bonds.31 This also guarantees the structural stability of the Mo2C monolayer and the scientific rigor of subsequent studies. However, there are maybe several complicated factors to the origin structural stability, which should be studied systematically in the future.

Figure 2. (a) Snapshots from the AIMD simulation of geometrical structures for the Mo2C monolayer at the temperature of 300 K. The inserts α and β show the enhanced local details of the geometric distortions. (b) Variation of the free energy in the AIMD simulation at 300 K during the timescale of 3 ps. (c) The calculated phonon dispersion curves of the Mo2C monolayer.

To systematically study Li/Na intercalation in Mo2C monolayer, we first examine adsorption sites for an isolated Li and Na atom. Considering the symmetry of the Mo2C monolayer, there are four typical adsorption sites. These binding sites (denoted as S1-S4) are schematically illustrated in Figure 1c. We performed a full geometry optimization for the Li (Na) incorporation into Mo2C by considering the four


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mentioned adsorption configurations. As different supercell sizes would not change the ranking of the obtained adsorption energy, we used the similar supercell size as previous studies in order to facilitate comparison.34-35 To quantitatively evaluate the adsorption behaviors, the energetically most favorable adsorption sites of Li/Na ion on Mo2C monolayer are discussed by calculating the adsorption energies. The adsorption energy is defined as the difference between the total energy of the Li/Na adsorbed system and the sum of the total energy of the isolated metal ions on the isolated Mo2C monolayer.36-38

E Ad = EM @ Mo 2C − EMo 2 C − EM where EM is the energy of per atom for the bulk metal (Li/Na), EMo 2C and

EM @ Mo 2 C represent the total energy of the isolated Mo2C monolayer and metal-ion adsorbed monolayer system, respectively. A negative value of adsorption energy indicates that the corresponding atom prefers to adsorb on the Mo2C monolayer, instead of forming a metal cluster. The corresponding results are depicted in Figure 3. First, we can see that the Na adsorbed configurations are more stable than the case of Li in general. The negative adsorption energies for both Li and Na atom adsorbing on the Mo2C monolayer irrespective of adsorption sites mean the Li/Na storage is exothermic and spontaneous. This characteristic is fundamental and favorable for the Li/Na-ion batteries applications. On the other hand, the S1 adsorption site is identified to be more energetically favorable than the other adsorption sites for both cases of Li and Na. And the values of adsorbing above the S3 sites for both Li and Na are the biggest. In addition, we notice that the systems with adsorbed Li/Na on S2 sites



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possess the same adsorption energies as the cases of adsorbed on S4 sites. The reason is that the energy barrier between the two configurations of Li/Na adsorbing on S2 and S4 sites is so small that the adsorbed Li/Na above the Mo-C bond (S4 site) have shifted to the top of C atom (S2 site) after geometry optimization. To obtain a further understanding of the intrinsic attributes of the Li/Na adsorbed systems, we calculated the total and projected density of states (DOS) for the Li/Na adsorbed Mo2C monolayer and the results are plotted in Figure 3b,c, respectively. At the first glance, we can see that the DOS at the Fermi level for both systems are dominated by Mo 3d orbitals. In addition, it is noteworthy that the systems maintain their metallic nature after the adsorption of Li or Na atom as the Fermi level locating at the peak of the PDOS of C and Li/Na. The metallic character for both pristine Mo2C monolayer and its Li/Na intercalated state ensures the good electronic conduction. This feature indicates an advantage of Mo2C monolayer and is essential to its application as battery electrodes.

Figure 3. (a) Calculated adsorption energy for Li/Na based on the four distinct binding sites on the surface of Mo2C monolayer. (b) and (c) Total (T) and projected density of states of the corresponding Li@Mo2C and Na@Mo2C, respectively. The


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horizontal dashed line represents the Fermi level.

As the discussed above, the Mo2C monolayer, bare or adsorbed, exhibit metallic characteristics, which show great potential for energy storage application. Further, one of the crucial characteristics for evaluating suitability of an electrode material for rechargeable batteries is the charge-discharge rate, which depends on the mobility of the intercalating ion.39 So we then turn our attention to the motion of metal ions on the Mo2C monolayer. And we consequently identify its diffusion paths and calculate the corresponding diffusion barriers using the nudged elastic band (NEB) method in order to provide more insights into the diffusion properties of Li/Na on the surface of the Mo2C monolayer. Here, three possible pathways with high structural symmetry between the nearest neighboring low-energy adsorption sites were considered. The top and side views of the migration pathways are illustrated in Figure 4a. The different pathways are marked by red, blue and black (denoted as P1, P2 and P3, respectively). For Li on the Mo2C surface (Figure 4b), the calculated lowest diffusion barrier is about 0.035 eV in P1 along with the path length of 2.88 Å. Meanwhile, the path of P3, which cover the high symmetry site above the Mo atom, possess the highest barrier of about 0.133 eV. This result is in good agreement with our estimations of the adsorption energy. On the other hand, we find that the obtained diffusion barriers of the Na adsorbing system decreases obviously compared with the Li adsorbing system in aggregate. Therefore, we can conclude that the Mo2C monolayer will be easier for Na ion to diffuse. As shown in Figure 4c, P2 has the lowest diffusion barrier of about 0.015 eV and the corresponding path is about 3.34 Å, while P3 also represent the



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highest barrier (0.071 eV). Additionally, P1 is also a low energy Na diffusion path with a barrier of 0.025 eV. To our surprise, the obtained low barriers for both Li and Na system are comparable with the cases of other anode materials even considering the deviation between different approaches, such as MXenes40-41, Ga2N42, graphene system8, 34, 43, MoS224 and so on.27, 38, 44-48 That is to say, Mo2C monolayer could own the fast charge-discharge capability for Li and Na. Moreover, our results suggest that 2D Mo2C should be of one promising electrode materials for high power application. Here, we also notice that P1 and P2 possess similar energy (12 meV) at the middle of the diffusion path, while corresponds to the adsorption sites S2 and S, respectively. This is probably the reason why the optimized geometry of S4 turn out to be the same as S2.

Figure 4. (a) Considered ion migration pathways and corresponding energy barriers


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of (b) Li and (c) Na on Mo2C monolayer.

For practical application, the storage capacity of the batteries is the key indicator for the electrode materials. Hence, the average adsorption energies are calculated to investigate the storage capacity of Li/Na on the Mo2C monolayer. Considering that multilayer Li/Na ions adsorption provides a mechanism to greatly enhance the capacities for the 2D anode materials, we adopt the Mo2C monolayer with the increase of adsorbed Li/Na ions on both sides, simultaneously. Here, we employ the 2×2×1 supercell of Mo2C as host model for extra ions-storage layer by layer. As a start, we assume the charge-discharge processes follow the common half-cell reaction in aqueous solution:41

Mo2C + xLi + + xe − ↔ Lix Mo2C Mo2C + xNa + + xe − ↔ Nax Mo2C

The average adsorption energies are defined as the difference in total energies before and after Li/Na intercalation. It should be noted that the volume and entropy effects are usually negligible during the reaction. For Li/Na intercalation on the Mo2C monolayer, the first layer of metal atoms is adsorbed at the S1 sites, which are the most stable site for Li/Na adsorption and above the center of the honeycomb lattice. When the adsorption sites of the first layer are occupied completely, the other Li/Na will form the second adsorbed layer at the S2 site above the C atoms (see Figure 1c). To evaluate the interaction between the Li/Na layer and the host monolayer material, we calculate the average adsorption energy in each layer (Eave), as defined by:



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Eave =

( EMo2CM 8 n − EMo2CM 8( n−1) − 8 EM ) 8

( M = Li, Na )

where EM is the cohesive energy in metal M (Li, Na), EMo2CM 8 n and EMo2CM 8( n−1) are the total energies of 2D Mo2C with n and (n-1) adsorbed Li/Na layers. The number “8” in the formula represents eight adsorbed Na atoms in each layer (for a 2×2×1 supercell on both sides). For Li adsorbed Mo2C monolayer, the calculated average adsorption energies in the first layer and second layer are -0.26 and -0.01 eV. And the average adsorption energy will become to be a positive value as other additional layers are adsorbed. It means that these Li atoms would tend to get together forming clusters. So the maximum number of Li storage on the Mo2C monolayer is 16, and the corresponding chemical stoichiometry is Li4Mo2C. Therefore, the corresponding theoretical specific capacity of Mo2C as electrodes is about 526 mAh·g-1. This value is more superior compared with some other 2D materials such as MoS2/graphene (~338 mAh·g-1)27, Ti3C2 monolayer (~320 mAh·g-1)22, and graphite (372 mAh·g-1). Further, we also estimate the average voltage (Vave) by the equation expressed as:49-51 Vave =

( EMo2C + ELi − ELix Mo2C ) xzF

where F is the Faraday constant, z is the electronic charge of Li ions in the electrolyte (z = 1). As the increase of the adsorbed Li concentration from 8 to 16 atoms on the Mo8C4 monolayer, the calculated average open-circuit voltage decreases from 0.26 eV to 0.14 eV. Based on the analyses above, we can conclude that Mo2C monolayer should be a good candidate for the application of as an electrode material in LIBs. Next, we turn our focus on the case of Na adsorbed Mo2C system. We find that


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both of the calculated average adsorption energies are positive for the first and second layer (0.13 eV and 0.42 eV, respectively). Since the Na atoms cannot occupy 8 adsorption sites on the surface of Mo2C monolayer, we examine the adsorption number of 2, 4, and 6, instead. The calculated results are -0.52, -0.17 and 0.01 eV for the Na2Mo8C4, Na4Mo8C4, and Na6Mo8C4, respectively. Therefore, the maximum capability of Na storage of the Mo2C monolayer is 4 Na atoms (NaMo2C). The corresponding theoretical specific capacity and average open-circuit voltage turn to be 132 mAh·g-1 and 0.166 eV. This obtained capacity is closed to the case of reported layered MoS2, which possesses a maximum theoretical capacity of 146 mAh·g-1.24 However, it should be noticed that the predicted diffusion barrier of Mo2C are much lower than MoS2 (0.015 VS 0.28 eV). So the proposed Mo2C material could have even better performance than the MoS2. As mentioned above, the single Na adsorbed Mo2C systems are more stable than the case of Li on a whole. So why the adsorption concentration of Na on the surface of Mo2C is much lower than that of Li? The reason for this issue probably is the large exchange interaction between the Na atoms. For one randomly selected Na atom, the interactions mainly originate from two aspects: the surrounding Na atoms and the Mo2C monolayer. But these two kinds of intersections are in a state of competitive mechanism. The interactions between the Na atoms will be weak as few Na atoms are adsorbed on the Mo2C. At this point, the adsorption energies are negative as illustrated above. Once the adsorption number come up to 6, the dominant interactions will be the self-interaction between the Na atoms, and the adsorption energies become a positive value. In general, we still have



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reason to believe that the Mo2C monolayer would be a considerable candidate material for the NIBs while further continuous studies on the surface of Mo2C are placed high hopes on. At last, in order to obtain insights on the physical origin of the predicted multilayer adsorption behavior, we analyze the electronic structure of Mo2C with one and two Li layers as the main example. Figure 5 shows the electron localization functions (ELF) of (110) section. Apart from the number of adsorbed layers, substantial concentration of electrons is spread out in the outer metal layers like the free electron gas, forming a negative electron cloud.40, 44, 52 Meanwhile, the inner atoms referring to Mo and C show ionic bond character for more highly localized electrons around the atoms. Through the comparison of Figure 5a,b, we find that the electrons transfer form inner Li layer to outer layer. However, in contrast to the reported MXene nanosheets, there is still a negative electron cloud distribution in the gaps between Li and Mo atoms.40 Moreover, it should be noted that the dispersive free electron gas between the adsorbed Li layers can play a role in stabilizing the metal-ions adsorption through the screen effect between various positive metal-ions near the surface. This also implies a great advantage of Mo2C monolayers for electrodes of metal-ion batteries.


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Figure 5. Electron localization functions (ELF) of (110) section with (a) one layer and (b) two layers of Li atoms. Red represents the electrons that are highly localized and blue signifies the electrons with almost no localization.

Additionally, we also check the geometries after the adsorption of layered Li atoms to estimate the varying of volume. As a result, the calculated thicknesses of the host Mo2C monolayer are 2.75 and 2.71 Å for the one and two layer adsorbed Li configurations, respectively. Compared with the bare monolayer (2.70 Å), this volume value for the fully occupied Mo2C (16 Li atoms adsorption system) just rose slightly to 0.4%, while the bond length of Mo-C also has very little change. At this point, this negligible structural distortion may explain the reason why the Mo2C monolayer possesses significantly lower Li/Na diffusion barriers. The smooth surface undoubtedly minimizes the resistance during the transport process of metal ions compared with the case of graphene system. Additionally, the dispersive free electron gas between Mo and Li atoms could further promote the mobility while avoiding the



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formation of ionic bonds. Therefore, this characteristic can further facilitate the applications of Mo2C as superior electrode materials. In this work, the geometric and electronic properties and Li and Na-ion storage capacities of the 2D Mo2C monolayer have been systematically studied on the basis of first-principles simulations. It is demonstrated that the proposed Mo2C monolayer can be promising electrode materials not only for the LIBs but also for the new born NIBs. Through the calculated phonon spectra and AIMD simulation, we firstly confirmed the thermal and dynamical stabilities of the Mo2C monolayer, which is the precondition of our studies followed. It is found that both Li and Na atoms can adsorb steadily on the surface of Mo2C monolayer and the metallic nature of the adsorption systems is still maintained. Moreover, our results showed that the bare Mo2C monolayer exhibits minimal diffusion barriers of 0.035 and 0.015 eV for Li and Na atoms, respectively. This means Mo2C monolayer could exhibit an excellent diffusion mobility and high charge and discharge rates for Li and Na atoms in the LIBs and NIBs. In addition, our work reveals high first cycle electrochemical capacity of 526 mAh·g-1 with respect to total weight of the electrode with excellent cyclability of Li ions. Regarding to Na ions, the predicted capacity is up to 132 mAh·g-1, which can be further improved. Besides, it is found that the relatively low capability of Na adsorbed on the Mo2C can attribute to the proposals competition mechanism. At last, we identified the existence of free electron gas in the Li adsorbed systems through Electron localization functions (ELF). Our identification of possible electrode materials highlights the urgent need to discover suitable electrolytes. And these


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findings are encouraging for further experimental and theoretical research. Computational Methods: The first-principles DFT calculations were performed in conjunction with the projector augmented wave (PAW) scheme, as implemented in the plane-wave basis code VASP (Vienna ab initio simulation package).53-54 The generalized

gradient

approximation

(GGA)55

as

formulated

by

Perdew–Burke–Ernzerhof (PBE)56 has been used for exchange and correlation contributions. A cutoff energy of 450 eV was chosen for the plane-wave expansion of wave functions and the Monkhorst–Pack scheme of k-point sampling was adopted for the integration over the first Brillouin zone.57 A 7×7×1 grid for k-point sampling was used for geometry optimization, while 9×9×1 for the static total energy calculations. All structures are fully optimized until the residual forces are less than 0.01 eV/Å. The convergence criteria for energy of 10-5 eV was met. We applied periodic boundary conditions and a vacuum space of 15 Å along the z direction in order to avoid the interactions between two slabs in the nearest-neighbor unit cells.37, 58 Besides, AIMD simulations were carried out to examine thermal stability by using 5×5 supercells at 300 K within each time step of 3 fs. The adopted supercells of Mo2C has been identified to be reliable, while using to describe the corresponding properties.8, 40, 42 In addition, the storage capacities (C) of the batteries were also estimated by the equation expressed as: C=

χ N Ae εM

where χ is the adsorbed electric charges (in mol) with per Mole electrodes materials, N A is the Avogadro Constant, and e is the elementary charge. ε is the ratio for



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conversion of mAh to coulombs ( ε = 3.6), and M is the molar mass of the electrodes material.

AUTHOR INFORMATION Corresponding Author Ying Dai (Y.D.) [email protected] Baibiao Huang (B.B.H.) [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work is supported by the National Basic Research Program of China (973 program, 2013CB632401), National Natural Science foundation of China under Grant 11374190 and 21333006, and the Taishan Scholar Program of Shandong Province, and 111 Project B13029. We also thank the High Performance Computing Centre of Shandong University for providing high performance computation.

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Ab Initio Prediction and Characterization of Mo2C Monolayer as

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