1、 1 2000汉字， 1300单词 学生毕业设计（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 Material 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 外文译文正文： Metallic VS2 Monolayer: A Promising 2D Anode Material for Lithium Ion Batteries
2、 Yu Jing, Zhen Zhou, Carlos R. Cabrera,and Zhongfang Chen 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 monolayer and graphite. Intrinsically metal
3、lic, 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 utilized as a promising anode material for Li ion
4、 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 performances, it is urgent to develop adva
5、nced 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. Graphene, a single layer of carbon atoms tightly
6、 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 mechanicalflexibility.As compared to graphite, graphene ca
7、n 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 graphite；in contrast, in situ Raman spectra
8、 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 storage on graphene and its derivatives, and co
9、ncluded 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 the importance of edge effects on graphene
10、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, have also been experimentally realized, a
11、nd 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 the specific capacity is rather high. Theor
12、etical 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 electrochemical performances. Although it has been
13、 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 of MoS2 nanoribbons in large scale remains
14、 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 supercapacitors utilizing VS2 nanosheets as electrodes
15、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 performance as LIB anode. In this work, we performed
16、 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 MoS2 counterpart and graphite.All of these
17、characteristics 3 2. COMPUTATIONAL DETAILS Our DFT computations were performed using an all-electron method within a generalized gradient approximation (GGA) for the exchange-correlation term, as implemented in the DMol code. The double numerical plus d functions (DND) basis set and PerdewBurkeErnze
18、rhof (PBE) functional were adopted.Especially, to accurately account for the long-range electrostatic interactions between Li atoms in high concentrations, we adopted the PBE+D2 method with the Grimme vdW correction.Self-consistent field (SCF) computations were performed with a convergence criterion
19、 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 computations. The Brillouin zones were sampled with 4 4 1kpoints. The transition states were located by computing the minimum-ene
20、rgy 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 states of the reaction. An artificial spring force then is introduced between all nearest-neighboring image structures.The ME
21、P 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 path. 3. RESULTS AND DISCUSSION 3.1. Single Li Atom Adsorption and Diffusion on VS2 Monolayer.Similar to other transition metal
22、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, including trigonal (T) phase and hexagonal (H) phase, both of which are sensitive to the change of temperature and the variation of
23、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 of VS2 monolayer in H configuration (Figure 1a), a unit cell contains one V atom and two S atoms with the lattice parameters o
24、fa=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 has a spin-polarized ground state, which is 28 meV lower in energy than the unpolarized state. Next, we studied the adsorption
25、 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 defined as: = EE E bVSLiVSLi 22 whereEVS2 LiandEVS2 are the total energies of Li-adsorbed VS2 monolayer and VS2 monolayer, r
26、espectively. 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 a more favorable exothermic reaction between VS2 and Li. There are two stable adsorption sites for Li adsorption on VS2 monol
27、ayer (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 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 th
28、e 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.
