MoS2@VS2 Nanocomposite as a Superior Hybrid Anode Material

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MoS@VS nanocomposite as a superior hybrid anode material Abdus Samad, and Young-Han Shin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07161 • Publication Date (Web): 08 Aug 2017 Downloaded from http://pubs.acs.org on August 8, 2017

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ACS Applied Materials & Interfaces

MoS2@VS2 nanocomposite as a superior hybrid anode material Abdus Samad and Young-Han Shin∗ Department of Physics, University of Ulsan, Ulsan 44610, Republic of Korea E-mail: [email protected]

Abstract Using density functional theory, MoS2 @VS2 nanocomposite is reported as a hybrid anode with upgraded electronic conductivity and Li/Na storage capacity. The chemically active monolayer VS2 can be stabilized in energy and phonon vibrations by using the monolayer MoS2 as a substrate. Stability of the chemically active monolayer VS2 is attributed to the interfacial charge accumulation between the monolayer MoS2 and VS2 . Maximum specific capacity of the nanocomposite has been enhanced to 584 mAh/g both for Li and Na storage. We attribute the high enhancement in Li/Na storage capacity of MoS2 @VS2 nanocomposite to the charge redistribution in the formation of the nanocomposite. The lithiation/sodiation open circuit voltage range of the nanocomposite is quite feasible to be used as anode. Diffusion barriers of Li/Na ions on the surfaces of the nanocomposite are comparable to the barriers on corresponding monolayers while at the interface the barriers are lower than that for bulk MoS2 . This study utilizes different aspects of the two different materials in a hybrid anode with highly enhanced electrochemical performance.

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ACS Paragon Plus Environment


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Keywords MoS2 @VS2 nanocomopsite, Li/Na ion batteries, charge redistribution, stability of monolayer VS2 , enhanced Li/Na storage capacity of MoS2 @VS2 nanocomposite

1

Introduction

Clean, safe, and efficient storage of electrical energy, both at large and small scales is one of the worldwide most important issues. Rechargeable Li ion batteries (LIBs) are effectively serving in small portable electronic devices for the last two decades. Its use in electric vehicles is however less efficient because of its competition with gasoline/diesel powered vehicles. Although an electric vehicle does not pollute and is more economical but it is costly, requires longer time for recharge, and enables only short-distance movement. In a hybrid vehicle some of these problems have been partially controlled. A neverending research on safety improvements, increasing rate capability, storage capacity, cycle life, and voltage of the cell of a rechargeable metal ion battery is still in progress. On the other hand to take hold of the commercial market, searching for cheap, abundant, and environmentally friendly ingredients is also important. 1–4 MoS2 has shown a leading role in manufacturing the electronic devices and emerging physics. 5–10 Weak van der Waals interlayer interactions and strong in-plane covalent bonding in MoS2 allows enough room between the layers to host the small Li/Na ions without large volume variations. Layered crystalline structures with large interlayer openings such as transition metal dichalcogenides have remained with researchers who are focusing on finding intercalation compounds to store Li/Na ions. 11–14 The prevailing graphite anode needs to be replaced by high capacity anode materials in the next generations of rechargeable metal ion batteries. The Li storage capacity of bulk MoS2 (340 mAh/g) is even less than that of graphite (372 mAh/g). However, the Na storage capacity of MoS2 (146 mAh/g) is much higher than graphite because of its larger interlayer space. Li storage capacity of bilayer 2


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MoS2 (300 mAh/g) is lower than bulk MoS2 . 15 The adsorption energy of Li/Na ions on monolayer MoS2 is so low that for reasonable density of ions, metallic clusters are formed when the ions come close to the metallic distance during diffusion. The Li/Na ions involved in the clusters do not take part in the reversible reactions at the anode and cathode. Vacancies in monolayer MoS2 have shown a significant increase in the binding strength of Li/Na ions. Similar tactics have worked well in graphene and silicene as well. The high Li storage capacity of MoS2 nanoribbons can be ascribed to the edge effects. However, the diffusion energy barrier of a Li ion on the surface of defect-mediated monolayer MoS2 (0.42 eV) goes higher than that on pristine monolayer MoS2 (0.1 eV), which results in reduced rate capability and increased charging time of the cell. 15–19 Another important issue in MoS2 as anode is its high electronic band gap which blocks the recovery of Li/Na ions. Desired properties from a layered material can be easily achieved by contacting it with another layered material with a careful selection. Chowdhury et al. 20 reported that capping of black phosphorene with h-BN enhanced the Li/Na binding energy from 1.8 eV to 2.8 eV. In our previous study, we showed that large-area expansion and insulating nature of monolayer SnS2 during the sodiation process can be solved by the addition of a graphene sheet. 21 Similar strategies have been used for phosphorene, 22 MoS2 , 23 silicene, 24 and some other insulating materials. The common feature of graphene addition was reported as the enhancement of electronic conductivity while some authors claimed increased Li/Na storage capacity of the electrode as well. 25–30 However, it is well known that pristine graphene in flat structure is almost inert to the adsorption of metal ions. Here, instead of graphene, we proclaim the significance of monolayer VS2 as metallic additive for monolayer MoS2 . In this article, the insulating monolayer MoS2 is presented as a poor adsorber of Li/Na ions and the metallic monolayer VS2 is shown to be capable of high density Li/Na storage. However, the monolayer VS2 has not been synthesized up to date. We show that monolayer VS2 can be grown on monolayer MoS2 to make MoS2 @VS2 nanocomposite. The combining 3



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process of monolayer MoS2 and monolayer VS2 is exothermic enough to stabilize monolayer VS2 . The charge density difference of the combined and individual monolayers MoS2 and VS2 predicts the accumulation of a low-density interfacial charge cloud, donated by the V atom of monolayer VS2 . The charge redistribution during the growth of metallic monolayer VS2 on monolayer MoS2 boosts the electrical conductivity and Li/Na storage capacity of the nanocomposite. It also has the lithiation/sodiation voltage range feasible for anodes.

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Computational details

All our calculations are performed by using the spin density functional theory, implemented in the Vienna Ab initio Simulation Package. 31 Generalized gradient approximation with the parameterization scheme of Perdew-Burke-Ernzerhof 32 is applied for the electron-electron exchange correlation processes whereas the electron-ion interactions are processed by the projector augmented wave method. Cutoff energy of 500 eV is considered for the plane-wave expansion of the electronic eigenfunctions, for the MoS2 @VS2 nanocomposite. The force criterion used for the structure relaxation is 0.001 eV/Å. The interlayer interactions have been weakened by producing a minimum vacuum of 30 Å. The van der Waals energy correction has been considered in the nanocomposite using the semiempirical correction scheme of Grimme (DFT-D2). 33 The Γ-point sampling with 6×6×1 k -points of the nanocomposite is used for the integration of Brillouin zone. Phonon modes are calculated using the finite displacement method implemented in the Phonopy program. 34 A 5×5×1 supercell of the MoS2 @VS2 nanocomposite with the atomic displacements of 0.01 Å is considered for the phonon calculations. A 3×3×1 supercell of the MoS2 @VS2 nanocomposite is examined for the adsorption and diffusion of Li/Na ions. Nudged elastic band method 35 is used for the optimization of minimum energy paths for the diffusion of Li/Na ions on both surfaces and interface of the MoS2 @VS2 nanocomposite. Bader charge analysis 36 is used to scale the charge transferred by the Li/Na atoms.

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Results and discussion

Table 1: Binding energy (Eb ), charge transferred (q), and distance from nearest S atom of a Li/Na ion (d(Li/Na)−S ) at the energetically favorable adsorption sites of the monolayer MoS2 and monolayer VS2 .

E-E o (eV)

Na Li Adsorption site Eb (eV) q (|e|) dLi−S (Å) Eb (eV) q (|e|) dNa−S (Å) Mosur 0.07 0.87 2.36 0.05 0.85 2.71 0.08 0.84 2.71 MoH 0.08 0.88 2.36 Vsur 1.97 0.87 2.36 -1.85 0.85 2.71 -1.85 0.85 2.70 VH 1.88 0.88 2.36 0.25

MoS2

Li Na

0.2 0.15

0.235 eV

0.1

H-site

0.05 T-site

E-E o (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0 0.2

0.102 eV

VS2

0.15 0.2 eV

0.1 T-site

0.05

H-site

0.09 eV

0 0

0.2

0.4

0.6

0.8

1

Reaction coordinate

Figure 1: Relative energy profiles through the minimum energy paths for the diffusion of Li/Na ions on the surface of (a) monolayer MoS2 from Mosur site to MoH site and (b) monolayer VS2 from the Vsur site to the VH site

3.1

Adsorption and diffusion of Li/Na ions on MoS2 , VS2 monolayers

Monolayer MoS2 has been confirmed theoretically and synthesized and characterized experimentally. 5,7,37 Monolayer VS2 is a strongly correlated material and most of its properties depend upon the effective Hubbard parameter (Ueff ). Stable phonon modes of vibrations can be achieved by selecting Ueff ≤ 2.5 eV. Although the freestanding monolayer VS2 was studied theoretically and there are some experimental reports about a few-layered VS2 . There is no 5



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report on successful synthesis of monolayer VS2 . 38–41 Similar to the literature our calculations show that 2H is the lowest energy phase, both for monolayer MoS2 and monolayer VS2 . 23,38 The lattice parameters are a = b = 3.183 Å for monolayer MoS2 and a = b = 3.174 Å for monolayer VS2 . Top and side views of the geometry of these monolayers are shown in Fig. S1. Monolayer MoS2 has a direct band gap of about 1.67 eV while monolayer VS2 is metallic and has a magnetic moment of 1 µB per unit cell. All these results are in good agreement with the previous reports. 16,40,42 Here for the sake of convenience we present a brief summary of adsorption and diffusion of Li/Na ions on the individual monolayers of MoS2 and VS2 whereas our theme is to report the superior electrochemical performance of MoS2 @VS2 nanocomposite. A 3×3×1 supercell of the 2H phase of monolayer MoS2 and monolayer VS2 is considered to search out the energetically stable adsorption sites for Li/Na ions. Binding energies (Eb ) of Li/Na ions with the monolayer MoS2 or monolayer VS2 are defined as: Eb = EMoS2 /VS2 + nEbulk(Li/Na) − EMoS2 /VS2 +n(Li/Na) , where EMoS2 /VS2 is the energy of a 3×3×1 supercell of pristine monolayer MoS2 or monolayer VS2 , Ebulk(Li/Na) is the energy for Li/Na ions in its body-centered cubic structure, and EMoS2 /VS2 +n(Li/Na) is the energy of a 3×3×1 supercell of monolayer MoS2 or monolayer VS2 plus n Li/Na adatoms. Positions right above the Mo atom of MoS2 (the so-called Mosur site) and V atom of VS2 (Vsur site) are found as energetically the most favorable sites for the adsorption of Li/Na ions. The next favorable site after the Mosur /Vsur is the position above the center of Mo-S hexagon (MoH site) or V-S hexagon (VH site). Schematics of both these sites have been shown in Fig. S1. The adsorption informations of a single Li/Na ion on these sites are summarized in Table 1, which shows that Li/Na binding with the monolayer MoS2 is so weak that if concentration of the adsorbed ions is slightly increased, Li/Na ions starts metallic clustering and these ions will not be able to take part in the reversible electrochemical reactions at anode or cathode. Thus the pristine monolayer MoS2 is not a good choice as anode for Li/Na ion batteries. Another problem for monolayer VS2 is that it is still not confirmed experimentally. 6


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Nudged elastic band method is used to optimize the minimum energy path for the diffusion of Li/Na ions by the linear interpolation of three images between the neighboring Mosur /Vsur and MoH /VH sites. Relative energy profiles for the diffusion of Li/Na ions over the surface of monolayer MoS2 and monolayer VS2 are plotted in Fig. 1. The diffusion energy barriers are in good agreement with the previously reported results. 43–45 The diffusion energy barrier for Na is lower than that of Li, which is due to the comparatively weak Coulombic interaction of Na ions with the monolayer because of its larger ionic radius. Similar trends have been found in other studies as well. 46–49 More explanations about the adsorption and diffusion of Li/Na ions on monolayer MoS2 and VS2 can be found in references. 15,16,43–45 (a)

0.189

(b)

6.67 6.13

0.198

(c)

6.13

0.198

(d)

0.199

6.07

Figure 2: Top and side views of (a) AA stacking, (b) AB stacking, (c) S atom of MoS2 above the V atom of VS2 , and (d) S atom of VS2 above the Mo atom of MoS2 of monolayer MoS2 and VS2 . Values given with the top views show the stacking energy in eV/atom, values given with the side views show the vertical distance between Mo and V in Å. The dark blue and yellow spheres represent the S atoms of monolayer VS2 and monolayer MoS2 , while the red and gray spheres are for V and Mo, respectively. The stacking order shown in (d) is energetically the most stable stacking.

3.2

Stacking stability of MoS2 @VS2 nanocomposite

As monolayer MoS2 and VS2 have almost the same lattice parameters and both are stable in the hexagonal geometry (2H phase), the calculated lattice mismatch between them is so small (∼0.28%) that monolayer VS2 completely covers monolayer MoS2 as shown in Fig. 2(a). The equation: Estack = (EMoS2 + EVS2 − EMoS2 @VS2 ) /N defines the stacking energy (Estack ) (unit eV/atom) of the nanocomposite where EMoS2 @VS2 is the energy of the 7



Figure 3: Top and side views of charge density difference (∆ρ) of the MoS2 @VS2 nanocomposite and monolayer MoS2 and monolayer VS2 . Electron gain is indicated by yellow at the isosurface of 2.5×10−4 |e|/Å3 and loss by light blue at the isosurface of 2.5×10−4 |e|/Å3 .

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Frequency (THz)

12 10 8 6 4 2 0 Γ

M

Γ

K

Figure 4: Phonon dispersion modes for the MoS2 @VS2 nanocomposite

2 1

Energy (eV)

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0 -1 -2 Γ

M

K

Γ

DOS

Figure 5: Electronic band structure and total density of states for the MoS2 @VS2 nanocomposite

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Table 2: Binding energy (Eb ) and charge (q) transferred from Li/Na atoms intercalated in a 3×3×1 supercell of the MoS2 @VS2 nanocomposite at interface site below V (Vi ), interface site below Mo (Moi ), surface site above V (Vsur ), surface site above Mo (Mosur ), surface site above the center of V-S hexagon (VH ), surface site above the center of Mo-S hexagon (MoH ), and interface site below the V-S hexagon (VHi ).

Li Na

Eb Vi Moi Vsur 2.52 1.79 2.28 1.90 1.29 2.19

(eV) Mosur 1.10 1.00

VH 2.17 2.19

MoH 0.94 0.96

VHi 2.04 1.53

Vsur

Vi 0.84 0.78

Moi 0.83 0.81

Vsur 0.87 0.85

q (|e|) Mosur VH 0.87 0.88 0.85 0.85

MoH 0.88 0.85

VH VHi

Vi

Mo i

MoH

Mosur

Figure 6: A schematic view of the Li/Na adsorption sites on the surfaces and interface of the MoS2 @VS2 nanocomposite MoS2 @VS2 unit cell, EMoS2 is the energy of the unit cell of monolayer MoS2 , EVS2 is the energy of the unit cell of monolayer VS2 , and N is the number of atoms. Stacking energies and interlayer distances for the four different stacking orders of MoS2 @VS2 nanocomposite are shown in Fig. 2. Estack of the MoS2 @VS2 is much higher than that of the graphene/SnS2 (0.12 eV/atom) 21 and graphene@MoS2 (0.03 eV per C atom) nanocomposite, 23 which is due to the fact that free-standing monolayer VS2 is chemically very active while graphene and monolayer MoS2 both are stable. Therefore, the interaction of monolayer MoS2 with the monolayer VS2 is much stronger than that with the graphene. To explain the release of high amount of energy in the formation of MoS2 @VS2 nanocomposite, the charge density difference (∆ρ = ρMoS2 @VS2 − ρMoS2 − ρVS2 ) of the nanocomposite and monolayer MoS2 and monolayer VS2 for the stacking order of Fig. 2(d) is shown in Fig. 3. In this stacking, the interfacial S atoms of the two layers are at a distance of 3.55 Å and electrons are accumulated at the interface between MoS2 and VS2 , which induces strong interlayer coupling. If Estack

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VHi 0.83 0.80


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is attributed to the electron cloud at the interface S atoms only, then the energy released per unit bond is 1.194 eV. However, density of the interfacial charge is too low and distance between the nearest S atoms of the two layers is also larger compared to V-S or Mo-S bond lengths therefore it is not a chemical bond but a van der Waals interaction. The side view of Fig. 3 depicts that V are charged more highly than any other atom of the MoS2 @VS2 nanocomposite, which means that the interfacial charge is due to the out-of-plane spreading of the V orbitals. The interfacial electron cloud induces a dipole moment in monolayer MoS2 . This redistribution of charge in the MoS2 @VS2 nanocomposite contributes to stabilize the monolayer VS2 . Further we show the phonon dispersion modes in Fig. 4 for the most stable stacking order [See Fig. 2(d)] to confirm the dynamic stability of the nanocomposite. As mentioned earlier, the experimental synthesis of freestanding monolayer VS2 has not been realized till today, but the computational investigations revealed that it could have stable phonon modes for Ueff ≤2.5 eV. 38–40 In Fig. 4, however, the stable phonon modes for the MoS2 @VS2 nanocomposite show that monolayer VS2 can be stabilized dynamically, by using monolayer MoS2 as a substrate. Furthermore, the in-plane elastic stiffnesses of the monolayer MoS2 , monolayer VS2 , and MoS2 @VS2 nanocomposite are studied. The in-plane elastic stiffness is 124.04 N/m for monolayer MoS2 , 93.18 N/m for monolayer VS2 , and 219.50 N/m for MoS2 @VS2 composite. It shows that the elastic stiffness of the nanocomposite has been enhanced by 2.28 N/m due to the synergic effects. To investigate the temperature effects on the stability of the nanocomposite, a snapshot of the MoS2 @VS2 nanocomposite after running 1500 steps of ab initio molecular dynamics simulations with a time step of 3 ps at 300 K is shown in Fig. S3. The small deviations of atoms from their lattice sites at equilibrium show that the nanocomposite is stable at room temperature. One of the most desired properties of the electrode materials is their high electronic conductivity without which full recovery of Li/Na ions is not possible. The strategy of graphene insertion in the electrode materials has been broadly applied for the enhancement of elec-

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tronic conductivity. 21–24 Reports over Li/Na adsorption on pristine graphene reveal that graphene itself is not a good choice for anode because of its poor Li/Na adsorption capability. Fig. 5 shows the electronic band structure and total density of states for MoS2 @VS2 nanocomposite. It reveals that combining semiconducting monolayer MoS2 with metallic monolayer VS2 results in a metallic nanocomposite. Similar results have been found in blue phosphorene/M S2 (M =Nb,Ta) heterostructure. 50 The nanocomposite has a ferromagnet order of the magnetic dipoles (0.907 µB per unit cell) which can be studied in detail in a separate article.

3.3

Adsorption and diffusion of Li/Na ions in MoS2 @VS2 nanocomposite

Total binding energy (Eb ) of Li/Na adsorption on surfaces and interface of the MoS2 @VS2 nanocomposite is systematically studied. Here Eb is defined as: Eb = EMoS2 @VS2 + nELi/Na − EMoS2 @VS2 +n(Li/Na) where EMoS2 @VS2 +n(Li/Na) is the total energy of the MoS2 @VS2 nanocomposite with n number of Li/Na atoms adsorbed on surface or interface sites of the nanocomposite, EMoS2 @VS2 energy of the nanocomposite without Li/Na adatom, and ELi/Na is energy per atom of bulk Li/Na (in body-centered cubic phase). Eb of a single Li/Na ion is calculated on a 3×3×1 supercell of the MoS2 @VS2 nanocomposite to find out energetically the most favorable adsorption sites. Eb and charge transferred by a Li/Na adatom at different adsorption sites of a 3×3×1 supercell of MoS2 @VS2 nanocomposite are shown in Table 2. These sites are shown schematically in Fig. 6. Comparing Eb of a Li/Na ion at the MoS2 surface or interface of MoS2 @VS2 (given in Table 2) with that on the monolayer MoS2 (given in Table 1) shows that monolayer MoS2 is made capable for higher density storage of Li/Na by making the MoS2 @VS2 nanocomposite. The enhanced Eb of Li/Na ions in the nanocomposite is due to the charge redistribution in the formation of the nanocomposite. To calculate the Li/Na adsorption capacity of the MoS2 @VS2 nanocomposite, number of Li/Na adatoms is increased by filling the adsorption sites in descending Eb sequence. For the same capacity, 11



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filling different adsorption sites results in different Eb as shown in Fig. 7 but we consider the configurations with highest Eb for each case. The filling sequence of the adsorption sites in descending Eb order is as Vi , Vsur , Mosur , VH , Moi , MoH . Li intercalation in the MoS2 @VS2 nanocomposite for the highest capacity can be seen in Fig. 8. Intercalation of Li/Na ions has a very small effect on the Mo-S and V-S bond lengths but incredibly increases the MoS2 -VS2 interlayer distance. Increase in the interlayer distance for Na intercalation is larger than Li intercalation for the same capacity, which is due to the larger ionic size of Na. For example, the interlayer distance for Li intercalation shown in Fig. 8 is 9.055 Å while for the same concentration of Na intercalation it is about 9.488 Å. Fig. 7 shows that the nanocomposite is energetically stable even after high-density Li/Na intercalation while Fig. 8 shows that neither the MoS2 layer nor the VS2 layer of the nanocomposite decompose to LiS2 and Mo or V therefore this anode is expected to have high recyclability and long life. The highest possible specific capacity of the MoS2 @VS2 nanocomposite without considering the mass of Li/Na ions is 584 mAh/g which is much higher than that of monolayer MoS2 . By this way the two materials of the nanocomposite cover weakness of each other. That is, the MoS2 layer stabilizes the VS2 layer and the VS2 layer upgrades the Li/Na adsorption capacity of the MoS2 layer. Open circuit voltage of the nanocomposite with increasing Li/Na adsorption concentration is shown in Fig. 9. The voltage (V = Eb /n|e|) profile of the MoS2 @VS2 nanocomposite with the increasing Li/Na concentration is similar to graphite, bulk MoS2 , and other anode materials. 12,51 The charging voltage range (1.8-0.5 V for lithiation and V for sodiation) of the nanocomposite is suitable to be used as anode material. The diffusion of a Li/Na ion from a stable adsorption site to the next stable adsorption site on the VS2 surface, MoS2 surface, and interface of MoS2 @VS2 nanocomposite is studied one by one, using the nudged elastic band method. As shown in Table 2, Vsur is energetically the most favorable adsorption site both for Li and Na ions on the VS2 surface of MoS2 @VS2 while VH is the second favorable adsorption site on the same surface. Therefore, energy 12


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4 3.5

Li Na

Eb (eV)

3 2.5 2 1.5 1 0.5 0 100

200 300 400 500 Specific capacity (mAh/g)

600

Figure 7: Total binding energy of Li/Na adsorption as a function of increasing specific capacity

9.055

Figure 8: Top and side views of maximum possible Li storage (584 mAh/g) in MoS2 @VS2 nanocomposite. Both layers of the heterostructure maintain their structure shape but the interlayer distance is increased by 2.99 Å. 2

Li Na

1.8 1.6

Voltage (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.4 1.2 1 0.8 0.6 0.4 100

200 300 400 500 Specific capacity (mAh/g)

600

Figure 9: Lithiation/sodiation open circuit voltage of MoS2 @VS2 nanocomposite as a function of specific capacity 13



0.3 (a)

S

V

(b)

Li Na

Li

0.25 Relative energy (eV)

VH

MoS2 surface

VS2 surface

MoH

Mo

0.2

Vsur S

0.15

0.27

Mo sur

0.24

MoH

0.1 VH

0.10

0.05

0.13 Mosur

Vsur

0 0

0.1

0.2 0.3 Reaction coordinate

0.4

0

0.1


0.4

0.5

Figure 10: Minimum energy paths and saddle points for the diffusion of a Li/Na ion (a) from a Vsur site to a nearby VH site on the VS2 surface of MoS2 @VS2 nanocomposite and (b) from a Mosur site to a nearby MoH site. The insets of plot (a) and (b) show the pathways. Energies of the saddle points are given in eV.

0.6

Li Na

0.5 Relative energy (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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MoS2@VS2 interface VH i

0.4 0.3 0.44

0.45 VHi

0.2 0.1

Vi

Vi

0 0

0.1


0.4

0.5

Figure 11: Minimum energy path for the diffusion of a Li/Na ion at the interface from a Vi site to a nearby VHi site. The inset shows the diffusion path schematically.

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paths for the diffusion of a Li/Na ion from a Vsur site to a nearby VH site on the VS2 surface of MoS2 @VS2 nanocomposite is analyzed on a 3×3×1 supercell and the relative energy is plotted in Fig. 10(a). Here the diffusion energy barriers for Li (0.24 eV) and Na (0.10 eV) are only slightly higher than their diffusion barriers on monolayer VS2 shown in Fig. 1(b). The energy barrier of a Na ion diffusion is lower than that of a Li ion which is similar to the diffusion energy trend of Li/Na on monolayer MoS2 and monolayer VS2 . The relative energy profile for the diffusion of a Li/Na ion on the MoS2 surface of the MoS2 @VS2 nanocomposite from a Mosur site to a nearby MoH site is shown in Fig. 10(b). Here again the diffusion energy barriers of Li/Na ions are slightly larger than those on pristine monolayer MoS2 (shown in Fig. 1). The diffusion energy barriers for Li/Na migration over both the surfaces are less than their diffusion energy barriers on pristine and defected graphene and silicene. 19,52 Diffusion of Li/Na ions at the interface of MoS2 @VS2 nanocomposite is also studied. Table 2 shows that the most favorable adsorption site for Li/Na ion on a 3×3×1 supercell of MoS2 @VS2 nanocomposite is the Vi site and VH is the second favorable one. We therefore study the diffusion of a Li/Na ion from a Vi site to a VH site as shown in Fig. 11. Since the Vi -to-VH path is periodic because of the structural symmetry of the nanocomposite, the Vi -to-VH diffusion of the ions is sufficient to provide a complete insight in the diffusion of the ions at the interface. The energy barrier of the diffusion for Li at the interface is 0.45 eV while for Na it is 0.44 eV. The diffusion energy barrier at the interface is thus higher than that at the surfaces, which is the case with other well-known electrode materials as well. 15,21,53 Some authors recently reported that considering the zero point energy and quantum tunneling effects, diffusion of the light Li ions at 300 K can be faster by 15.4 % than that calculated at 0 K. However, these effects are not expected dominantly for the heavier Na ions. 20,54

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Conclusions

Calculations based on spin density functional theory showed that the monolayer VS2 can be stabilized in energy and phonon vibrations on monolayer MoS2 . Growth of the unstable monolayer VS2 over monolayer MoS2 results in high energy release. The charge density difference shows that there is electron accumulation at the interface between the MoS2 layer and the VS2 layer of the MoS2 @VS2 nanocomposite. Phonon dispersion modes confirm the dynamic stability of the MoS2 @VS2 nanocomposite. Electronic band structure and density of states show that the MoS2 @VS2 nanocomposite is metallic. Formation of the MoS2 @VS2 nanocomposite enables the MoS2 surface to adsorb a high density of Li/Na ions. The maximum Li/Na storage capacity of the MoS2 @VS2 nanocomposite is as high as 584 mAh/g. The energy barriers for the diffusion of Li/Na ions over the surfaces of the heterostructure are comparable with those on the surface of monolayer MoS2 and monolayer VS2 while at the interface it is comparable with the diffusion energy barrier in bulk MoS2 . The nanocomposite utilizes different properties of the two monolayers in a single hybrid anode with enhanced electrochemical performance.

Acknowledgement This work was supported by Basic Science Research Program (NRF-2014R1A2A1A11050893), Nano-Material Technology Development Program (NRF-2014M3A7B4049367), Basic Research Laboratory (NRF-2014R1A4A1071686), and Priority Research Center Program (20090093818) through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning.

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The nanocomposite from the monolayer VS2 and MoS2 overcomes the weak points when it is separated into the monolayer state, and takes over the merits for using as an anode material for ion batteries. 67x25mm (300 x 300 DPI)


MoS2@VS2 Nanocomposite as a Superior Hybrid Anode Material

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