1、 1 2200 汉字， 1500 单词 出处： Jing Y, Zhou Z, Cabrera C R, et al. Metallic VS2 Monolayer: A Promising 2D Anode Material for Lithium Ion BatteriesJ. Journal of Physical Chemistry C, 2013, 117(48):25409-25413. 毕业设计（论文）外文译文 学生姓名： 学号： 专业名称：新能源材料与器件 译文标题（中英文）： Metallic VS2 Monolayer: A Promising 2D Anode Mater
2、ial for Lithium Ion Batteries 金 属 的 VS2 单分子层 : 一种有希望成为锂离子电池的 2D 阳极材料 译文出处：物理化学杂志 指导教师审阅 签名 ： 2 文译文正文： ABSTRACT:By means of density functional theorycomputations, we systematically investigated the adsorption and diffusion of lithium on the recently synthesized VS2 monolayer, in comparison with MoS2
3、monolayer and graphite.Intrinsically metallic, VS2 monolayer has a higher theoretical capacity (466 mAh/g), a lower or similar Li diffusion barrier as compared to MoS2 and graphite, and has a low average opencircuit voltage of 0.93 V (vs Li/Li+). Our results suggest that VS2 monolayer can be utilize
4、d as a promising anode material for Li ion batteries with high power density and fast charge/discharge rates. 1. INTRODUCTION As one of the most important energy storage devices, lithium ion batteries (LIBs) are playing an indispensable role in modern society. To meet the demand of LIBs with better
5、performances, it is urgent to develop advanced electrode materials that can provide satisfactory specific capacity, cyclic stability, high-rate capability, and safety. To this end, researchers have been devoted to improving traditional electrode materials as well as innovating new candidates. Graphe
6、ne, a single layer of carbon atoms tightly packed in a honeycomb sublattice, has been a subject of extensive studies ever since its experimental realization, due to its excellent properties, such as ultrahigh surface area, good conductivity, ultrafast intrinsic carrier mobility, and mechanicalflexib
7、ility.As compared to graphite, graphene can accommodate Li on both sides, and thus is attractive as a LIB anode.However, experimental results on the Li storage performance of graphene are rather controversial. Several experimental studies suggest that graphene can present a higher Li capacity than g
8、raphite；in contrast, in situ Raman spectra revealed that the amount of Li adsorbed on graphene would be significantly reduced due to the repulsive interaction between Li cations.Quite recently, by means of density functional theory (DFT) computations, Liu et al. examined feasibility of lithium stora
9、ge on graphene and its derivatives, and concluded that pristine graphene is actually not an ideal Li storage material due to Li clustering and phase separation, although the C3B monolayer, its derivative, is a promising electrode material.Both theoretical and experimental studies have demonstrated t
10、he importance of edge effects on graphene for Li storage,and the observed superior Li storage performance of graphene should be attributed to the presence of edges as well as defects.Two-dimensional (2D) nanomaterials are not limited to graphene. Many noncarbon monolayers, such as BN, MoS2, and WS2,
11、 have also been experimentally realized, and their applications to electronics, energy storage, etc., have been explored. Among these 2D materials, MoS2 is a very promising electrode material for LIBs. Experimentally, it has been demonstrated that MoS2 has good performance as LIB anode； especially t
12、he specific capacity is rather high. Theoretical studies revealed that Li can be stably adsorbed on MoS2 monolayer with a low diffusion barrier；however, MoS2 monolayer is semiconducting with a considerable band gap of 1.80 eV,and this lacking in good conductivity would essentially limit its electroc
13、hemical performances. Although it has been demonstrated theoretically that cutting 2D MoS2into zigzag MoS2 nanoribbons can convert them into metallic,and such zigzag MoS2 nanoribbons have a remarkably enhanced binding interaction with Li without sacrificing the Li mobility,at present the production
14、of MoS2 nanoribbons in large scale remains a big challenge.Recently, Feng et al.have successfully exfoliated bulk VS2 flakes into ultrathin VS2 nanosheets via a unique ammoniaassisted strategy.Different from MoS2,VS2 monolayer is metallic with a spin-polarized ground state.The in-plane supercapacito
15、rs utilizing VS2 nanosheets as electrodes exhibited high specific capacitance and excellent cyclic stability, which are attributed to the significant metallic behavior and high specific surface area of VS2 monolayer.In principle, the metallicity of VS2 nanosheets could also facilitate its performanc
16、e as LIB anode. In this work, we performed systematic DFT computations to explore the feasibility of using VS2 nanosheets as anode materials for LIBs. Our results revealed that VS2 monolayer can provide higher Li binding strength, faster or similar Li mobility, and higher theoretical capacity than M
17、oS2 counterpart and graphite.All of these characteristics suggest the great potential of utilizing VS2 monolayer as anode material for LIBs. 3 2. COMPUTATIONAL DETAILS Our DFT computations were performed using an all-electron method within a generalized gradient approximation (GGA) for the exchange-
18、correlation term, as implemented in the DMol code. The double numerical plus d functions (DND) basis set and PerdewBurkeErnzerhof (PBE) functional were adopted.Especially, to accurately account for the long-range electrostatic interactions between Li atoms in high concentrations, we adopted the PBE+
19、D2 method with the Grimme vdW correction.Self-consistent field (SCF) computations were performed with a convergence criterion of 10 6 au on the total energy and electron density. To ensure high-quality numerical results, we chose the real-space global orbital cutoff radius as high as 5.1 in all comp
20、utations. The Brillouin zones were sampled with 4 4 1kpoints. The transition states were located by computing the minimum-energy path (MEP) for the Li diffusion processes using the nudged elastic band (NEB) method, which starts by inserting a series of image structures between the initial and final
21、states of the reaction. An artificial spring force then is introduced between all nearest-neighboring image structures.The MEP can be obtained by optimizing these image structures simultaneously as the true force on the image structures has a zero projection in the direction perpendicular to the pat
22、h. 3. RESULTS AND DISCUSSION 3.1. Single Li Atom Adsorption and Diffusion on VS2 Monolayer.Similar to other transition metal dichalcogenides (TMD), VS2 monolayer presents the sandwich-like structure with the V layer sandwiched between two S layers. Generally, there are two polymorphs of VS2, includi
23、ng trigonal (T) phase and hexagonal (H) phase, both of which are sensitive to the change of temperature and the variation of VS2 layers. At room temperature, VS2 monolayer prefers to crystallize in H-phase.Therefore, our investigations are based on the H-phase structure. In the optimized structure o
24、f VS2 monolayer in H configuration (Figure 1a), a unit cell contains one V atom and two S atoms with the lattice parameters ofa=b= 3.17 .The V S bond lengths are uniformly 2.36 , and the VS V bond angles are 84.44 . In good agreement with previous studies,our computations showed that VS2 monolayer h
25、as a spin-polarized ground state, which is 28 meV lower in energy than the unpolarized state. Next, we studied the adsorption of one Li atom on the surface of VS2 monolayer. To safely avoid the interaction between two Li atoms, we used a 4 4 supercell of VS2 monolayer. The Li binding energy (Eb) was
26、 defined as: = EE E bVSLiVSLi 22 whereEVS2 LiandEVS2 are the total energies of Li-adsorbed VS2 monolayer and VS2 monolayer, respectively. Li is the chemical potential of Li and is taken as the cohesive energy per atom of bulk Li. According to our definition, a more negative binding energy indicates
27、a more favorable exothermic reaction between VS2 and Li. There are two stable adsorption sites for Li adsorption on VS2 monolayer (Figure 1b), the hollow site (H) above the center of the hexagon and the top site (T) directly above one V atom. We also examined the other possible adsorption site, that
28、 is, on the top of S atom； however, it is not a local minimum site for Li adsorption, as the Li atom on this site moved to the neighboring T site after full relaxation. Our computations show that Li atom prefers to be adsorbed at the T site with a binding energy of 2.13 eV, and the distance from the surrounded S atoms is 2.42 . According to Hirshfield charge population analysis,there is about 0.37 |e| charge transfer from Li to VS2 monolayer. For lithiation at the H site, the binding energy is 2.01 eV with a mean Li S distance of 2.44 , and Li possesses a 0.36|e| positive charge.
