Adsorption and Diffusion of Magnesium on Nitrogen Doped Mo2C Monolayer

The Mg adsorption and diffusion behaviors on nitrogen doped (N-doped) Mo 2 C monolayer have been systematically investigated by the rst principles based on density functional theory (DFT). The adsorption energies of Mg on pristine Mo 2 C and Mo 2 C 1 − x N x (x = 0.0625, 0.125, 0.1875 and 0.25) have been studied. The adsorption energies of Mg on N-doped Mo 2 C are lower than that of pristine Mo 2 C. Especially, the adsorption energies of Mg are − 1.639 eV and − 1.625 eV on T C1 and H 2 sites for Mo 2 C 0.875 N 0.125 , which have decreased by 16.49% and 18.43%. Furthermore, the Mg diffuses along H 3 -B-H 4 and H-B-H with the barriers of 0.021 eV and 0.028 eV, which indicate that Mo 2 C 0.875 N 0.125 exhibits fast diffusion properties. Additionally, the partial density of states (PDOS) reveals the interaction between Mg and Mo 2 C 0.875 N 0.125 . The PDOS results indicate that nitrogen doping causes the PDOS peaks transfer to a lower energy level, which is benet for the bonding between Mg and MoC 0.875 N 0.125 . These results suggest that the adsorption and diffusion behaviors of Mg are enhanced by nitrogen doping.


Introduction
Lithium ions batteries (LIBs) are widely used to in phones, laptops, digital cameras, and other portable devices [1][2][3]. However, safety, high costs and resource shortages have restricted the development of lithium batteries [4,5]. With the development of the intelligent electronic applications such as new energy vehicles, energy storage plant and arti cial satellite, which require high battery storage and stable cycle capacity. MIBs have been considered as the potential alternatives to LIBs, due to the natural abundance, low cost, safety and high volumetric energy density (3832 mAh cm − 3 ) [6,7]. However, it is well-known that the performance of rechargeable batteries depends on their anode or cathode materials. Hence, a great deal of efforts has been carried out to search for novel anode materials for MIBs.
As lager surface area and excellent elctrochemistry, extensive investigations have been focused on exploring 2-dimensional materials as anodes for MIBs [8]. For example, Sibari et. Al have demonstrated that phosphorene is a good anode material for MIBs with a high capacity of 315.52 mAh.g − 1 and diffusion barriers value of 0.05 eV along the zigzag direction [9]. The monolayer black P as anode for MIBs has also been researched by Jin et. Al based on rst-principles, which exhibits excellent properties, such as large adsorption energies of Mg (-1.09 eV on H adsorption sites) and low diffusion barriers of 0.08 eV along the zigzag directions [5]. Li et al. have found that the g-Mg 3 N 2 is a promising anode material for MIBs with high capacities storage (531 mAh g − 1 ) [10].
Recently, a new family of two-dimensional materials, Mxenes, such as WS 2 , Sr 2 C, TiS 2 , has attracted extensive attention in the application of anode materials due to good conductivity, high reversible capacity and high power density [8,[11][12][13][14]. Mo 2 C, a representative of two-dimensional Mxenes materials, has superconductivity and low diffusion barriers as anode materials [15][16][17][18]. In addition, Xu et. Al have succeeded in synthesizing the large-area high-quality 2D α-Mo 2 C [16]. Fan et. Al have reported that Mo 2 C monolayer is a potential anode material for MIBs [19].
However, the development of two-dimensional materials is restricted by the high diffusion barriers and the low cycling stability [9,20]. Doping metal elemental, such as Cr [21], Ru [22], Zn [23], Sr [24], and nonmetal elemental, such as C [25], N [26], B [27] are typical strategies to enhance the properties of electrode materials [28]. Moreover, nitrogen doping is an effective way to enhance metal-semiconductor transition and electronic conductivity due to strong electronegativity and similar atomic radius to carbon [29][30][31]. Daula et. Al indicate that Si 2 BN as anode materials exhibits excellent theoretical capacity of 647.896 mAh g − 1 and low migration energy barriers between 0.08 eV and 0.35 eV for MIBs by calculations [30]. Zhang et. Al have reported that the incorporation of nitrogen into graphene like C 2 N exhibits high theoretical capacities of 588.4 mAh g − 1 as anode for MIBs [4]. In present work, the rst principle calculations are implemented to investigate the adsorption and diffusion behaviors of Mg on the N-doped Mo 2 C with different nitrogen doping concentrations.

Computational Methods
All the calculations have been carried out by using SIESTA code by the rst-principles [32,33]. Within SIESTA code, the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof functional (PBE) is widely applied to describe the electron exchange correlation term [34,35] accomplish different doping concentrations 6.67%, 14.29%, 23.08% and 33.33%, respectively. Hence, we consider one nitrogen atom, two nitrogen atoms, three nitrogen atoms and four nitrogen atoms to replace the carbon atoms in 4×4×1 supercell.
The four typical adsorption sites on pristine Mo 2 C have been considered: hollow (H), top (T C and T Mo ) and bridge (B) sites as shown in Fig. 1(a). Meanwhile, the adsorption energies of Mg  Fig. 1(b), and the results are summarized in Table 1. As shown in Table 1 and Mo-4d states are plotted in Fig. 3. As shown in Fig. 3, the Mg-3s(no), C-2p(no) and Mo-4d(no) states represent the PDOS for Mg adsorption on the pristine Mo 2 C. The Fermi level has been depicted by vertical dashed line in Fig. 3. Obviously, it can be seen that the Fermi level locates at the peak of Mg-3s orbits, which suggest that the Mg adsorption on pristine Mo 2 C and MoC 0.875 N 0.125 is stable [37]. Additionally, the major electron contribution of Mo 2 C and MoC 0.875 N 0.125 is Mo-4d orbits, which is similar to Li, Na and K adsorption on Mo 2 C [15,17]. Furthermore, it is noticed that the C-2p,