Monolayer: A Promising 2D Anode Material for Lithium Ion Batteries

Nov 22, 2013 - Department of Chemistry, Institute for Functional Nanomaterials, University of Puerto Rico, Rio Piedras Campus, San Juan 00931,. Puerto...
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Metallic VS2 Monolayer: A Promising 2D Anode Material for Lithium Ion Batteries Yu Jing,†,‡ Zhen Zhou,*,‡ Carlos R. Cabrera,† and Zhongfang Chen*,† †

Department of Chemistry, Institute for Functional Nanomaterials, University of Puerto Rico, Rio Piedras Campus, San Juan 00931, Puerto Rico ‡ Tianjin Key Laboratory of Metal and Molecule Based Material Chemistry, Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Computational Centre for Molecular Science, Institute of New Energy Material Chemistry, Synergetic Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, People’s Republic of China S Supporting Information *

ABSTRACT: By means of density functional theory computations, we systematically investigated the adsorption and diffusion of lithium on the recently synthesized VS2 monolayer, in comparison with MoS2 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 utilized 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 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.1−4 Graphene, 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 mechanical flexibility.5−9 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 graphite;10−12 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.13 Quite recently, by means of density functional theory (DFT) computations, Liu et al. examined feasibility of lithium storage 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,14 although the C3B monolayer, its derivative, is a promising electrode material.14,15 Both theoretical and experimental studies have demonstrated the importance of edge effects on graphene for Li storage,16,17 © 2013 American Chemical Society

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, and their applications to electronics, energy storage, etc., have been explored.18 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.19−23 Theoretical studies revealed that Li can be stably adsorbed on MoS2 monolayer with a low diffusion barrier;24 however, MoS2 monolayer is semiconducting with a considerable band gap of ∼1.80 eV,25 and this lacking in good conductivity would essentially limit its electrochemical performances. Although it has been demonstrated theoretically that cutting 2D MoS2 into zigzag MoS2 nanoribbons can convert them into metallic,26 and such zigzag MoS2 nanoribbons have a remarkably enhanced binding interaction with Li without sacrificing the Li mobility,24 at present the production 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.27 Different from MoS2, VS2 monolayer is metallic with a spin-polarized ground state.27−29 The in-plane supercapacitors utilizing VS2 nanosheets as electrodes exhibited high specific capacitance and excellent cyclic stability, which are Received: November 7, 2013 Published: November 22, 2013 25409

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attributed to the significant metallic behavior and high specific surface area of VS2 monolayer.27 In principle, the metallicity of VS2 nanosheets could also facilitate its performance 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 MoS2 counterpart and graphite. All of these characteristics suggest the great potential of utilizing VS2 monolayer as anode material for LIBs.

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 DMol3 code.30,31 The double numerical plus d functions (DND) basis set and Perdew−Burke−Ernzerhof (PBE) functional were adopted.32 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.33 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 Å in all computations. The Brillouin zones were sampled with 4 × 4 × 1 k points. The transition states were located by computing the minimum-energy path (MEP) for the Li diffusion processes using the nudged elastic band (NEB) method,34 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 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 path.

Figure 1. Schematics of optimized free VS2 monolayer (a) and Li adsorbed VS2 monolayer (b) in top and side views, respectively.

where EVS2−Li and EVS2 are the total energies of Li-adsorbed VS2 monolayer and VS2 monolayer, respectively. μLi 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 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 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 Å. According to Hirshfield charge population analysis,35 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 Å, and Li possesses a 0.36 |e| positive charge. For comparison, we also studied the adsorption of Li on MoS2 monolayer by using a same 4 × 4 supercell. Li on MoS2 monolayer also favors the T site with a binding energy of −0.6 eV (see Figure S1a, Supporting Information). Therefore, Li adatoms can be more favorably stabilized on the T site of VS2 monolayer as compared to that on MoS2. Generally, electrical conductivity is a vital factor to decide the electrochemical performance of an electrode. However, including MoS2, most TMD monolayers are semiconductors with rather band gaps, implying poor electrical conductivity as electrode materials. We computed the density of states (DOS) and partial DOS (PDOS) of VS2 (Figure 2) and DOS of MoS2 monolayer (Figure S2a, Supporting Information). VS2 monolayer is metallic with considerable electronic states at the Fermi level (Figure 2a), and the metallic states are contributed by both V-3d and S-3p states according to PDOS analysis (Figure 2b and c). In contrast, MoS2 monolayer is semiconducting with a band gap of 1.77 eV and no electronic states at the Fermi level (Figure S2, Supporting Information), which agrees with previous investigations.25,26a,36 Therefore, the metallic VS2 monolayer stands out to be a highly desirable electrode material for LIBs. The rate performance of an electrode material is mainly determined by the Li mobility; thus it is necessary to estimate the diffusion of Li atom on the surface of VS2 monolayer. Obviously, the diffusion of Li on VS2 monolayer can occur by

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, including 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.29 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 of a = b = 3.17 Å. The V−S bond lengths are uniformly 2.36 Å, and the V−S−V bond angles are 84.44°. In good agreement with previous studies,27−29 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 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: E b = E VS2 − Li − E VS2 − μLi 25410

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good agreement with previous reports.37,38 According to the Arrhenius equation, the diffusion constant (D) is proportional to exp(−Ea/kBT), where Ea and kB are the activation energy and Boltzmann constant, respectively. Therefore, we can estimate that at room temperature the Li mobility on VS2 monolayer can be more than 3 times faster than that on MoS2 monolayer and can be as fast as that in graphite. Therefore, once used as the anode material in LIBs, good high-rate performances can be expected for VS2 monolayer. 3.2. Li Storage Capacity of VS2 Monolayer and Average Open Circuit Voltage. Next, we explored the Li storage capacity of VS2 monolayer because it can directly determine whether VS2 monolayer can be efficiently applied to LIBs. First, we constructed a number of configurations with stoichiometry of LixVS2 (x = 0.125, 0.222, 0.5, 0.667, 1, and 2) by putting two Li atoms in a 4 × 4, 3 × 3, 2 × 2, √3 × √3, √2 × √2, and 1 × 1 supercell, respectively. In all of these configurations, Li atoms were distributed evenly on the T sites of both sides of VS2 monolayer (Figure 4a). We then computed the Li binding energy for all of these configurations by full geometry optimization. As shown in Figure 4b, the Li binding energy of LixVS2 decreases gradually with the increase of x. This is because with increasing Li concentration, the distance between neighboring Li atoms becomes smaller and smaller, raising more and more pronounced repulsive electrostatic interactions between Li cations. Encouragingly, VS2 monolayer can provide a Li binding energy of −0.93 eV even at x = 2, indicating that Li atoms can still be stably adsorbed on VS2 monolayer and the phase separation problem can be safely avoided at such a high concentration. In comparison, MoS2 monolayer can also hold Li atoms in Li2MoS2 with a binding energy of −0.26 eV. As Li2VS2 represents the highest Li storage capacity, we can easily deduce that VS2 monolayer has a theoretical capacity of 466 mAh/g. As a comparison, MoS2 monolayer can also store Li atoms up to Li2MoS2, but the theoretical capacity (335 mAh/g) is much lower than that of VS2 monolayer due to the higher molecular weight. For comparison, we also explored the theoretical capacity of graphite. According to our computations, graphite can achieve a maximum Li capacity of LiC6 with a binding energy of −0.11 eV. Further increasing Li concentration would turn the binding energy into unfavorable positive, due to the strong repulsion interactions between Li ions. Thus, the theoretical capacity of graphite is 372 mAh/g, agrees well with the literature.14 Therefore, VS2 monolayer also can provide a higher power density than graphite and MoS2 monolayers as LIB anode materials. Finally, we computed the average open circuit voltage (OCV) for Li intercalation on VS2 monolayer. Because the charge/discharge processes of VS2 follow the common half-cell reaction vs Li/Li+:

Figure 2. (a) DOS for VS2 monolayer, and PDOS for V (b) and S (c) in VS2 monolayer.

migrating from a T site to another, while passing through an H site. As shown in Figure 3, when Li moves between two T sites,

VS2 + x Li+ + x e− ↔ LixVS2

Figure 3. Lithium diffusion pathway (T1−H−T2) on the surface of VS2 monolayer and Li motion barrier from site T to site H.

With volume and entropy effects both neglected, the OCV for Li intercalation in VS2 monolayer can be computed from the energy difference based on the equation below:39−41

it only needs to overcome a small energy barrier of 0.22 eV. For comparison, the Li diffusion barrier on MoS2 monolayer is 0.25 eV computed at the same theoretical level (Figure S1b, Supporting Information). Moreover, we also explored the Li mobility in graphite, which is the commercialized anode material presently. Interestingly, the energy barrier for Li diffusion in graphite (0.22 eV) is the same as that of VS2 monolayer. Our result on Li diffusion in graphite also achieves

OCV ≈ [E Lix1VS2 − E Lix2VS2 + (x 2 − x1)E Li]/(x 2 − x1)e

where ELix1VS2, ELix2VS2, and ELi are the total energies of Lix1VS2, Lix2VS2, and metallic Li, respectively. According to this equation, when VS2 monolayer reaches the highest Li capacity, corresponding to the case of x = 2 in Li2VS2, the computed 25411

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circuit voltage, and excellent electrical conductivity, VS2 monolayer should have a great potential to be applied as anode material for LIBs. Encouragingly, VS2 few-layers have been realized experimentally, and we strongly believe that 2D VS2 anodes with excellent electrochemical performances can be achieved in the near future.

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ASSOCIATED CONTENT

S Supporting Information *

Geometric structures of Li adsorption on MoS2 monolayer, DOS and band structure of single-layered MoS2, and energy profiles for Li diffusion on MoS2 monolayer. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support in the U.S. by the Department of Defense (Grant W911NF-12-1-0083) and in China by NSFC (21073096) and 111 Project (B12015) is gratefully acknowledged.

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Figure 4. (a) Schematics of LixVS2 (top view) and (b) the variation of binding energy with increasing Li content in VS2 and MoS2 monolayer.

average OCV is 0.93 V, which is between those of commercial anode materials, 0.11 V for graphite (in computations) and 1.5−1.8 V for TiO2.42−44 Thus, applicable OCV provides significant feasibility for monolayer to be applied as LIB anodes.

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