Two-Dimensional Transition Metal Dichalcogenide Monolayers as

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Two-Dimensional Transition Metal Dichalcogenide Monolayers as Promising Sodium Ion Battery Anodes Eunjeong Yang, Hyunjun Ji, and Yousung Jung* Graduate School of Energy, Environment, Water and Sustainability (EEWS), Korea Advanced Institute of Science and Technology (KAIST), Yuseong-gu, Daejeon 34141, Republic of Korea S Supporting Information *

ABSTRACT: A family of transition metal dichalcogenide (TMD) nanosheets has recently shown its potential as negative electrodes in lithium ion batteries (LIBs). Herein, Na ion adsorption and migration properties as well as the possibility of phase transition induced by the Na adsorption on TiS2, VS2, CrS2, CoTe2, NiTe2, ZrS2, NbS2, and MoS2 are predicted using first-principles calculations. In terms of average voltage and capacity, M = Ti, Zr, Nb, and Mo are found to be suitable as anodes for sodium ion batteries (SIBs) with voltages of 0.49−0.95 V and theoretical capacities of 260−339 mA h g−1. Among the latter four screened TMDs, in particular, TiS2 and NbS2 are expected to maintain the same configurational phase upon sodiation (favorable kinetics) with Na ion migration barriers of 0.22 and 0.07 eV, respectively, suggesting that these TMD compounds could be promising for high-power energy storage applications. It is shown that a proper treatment of phase transitions during sodiation, though often neglected in the literature, is critical in an accurate theoretical description and interpretation of these two-dimensional materials.

1. INTRODUCTION Sodium ion batteries (SIBs) have emerged as an appealing alternative to lithium ion batteries (LIBs), especially for largescale energy storage systems (ESSs), on account of a high abundance and low cost of the sodium metal. Moreover, the accumulated knowledge and development of LIBs could be directly applied to SIBs since Li and Na have similar chemical and physical properties. However, whereas diverse cathode materials have been suggested for SIBs,1−3 identifying an appropriate anode material still remains as one of the main hindrances obstructing the realization of the Na full cells. Since graphitic carbon, which is a common negative electrode material in LIBs, does not store a meaningful amount of sodium between its layers, non-graphitic carbon nanostructures have been considered, but a limited cycle life caused by electrolyte decomposition is problematic.4−7 Ti-based oxides and phosphates as well as various alloying/conversion compounds have also been investigated, but it was found that they suffer from capacity degradations or volume changes during cycling.8−11 In this regard, it is essential to discover suitable anode materials in developing advanced SIBs. The unique electronic and mechanical properties of a twodimensional (2D) material, graphene, motivated significant recent interests in other related 2D nanomaterials as well, such as silicene, phosphorene, and transition metal dichalcogenides (TMDs). Many TMDs are formed in layered structures in the bulk state, and thus the single-layered or few-layered 2D slabs whose general composition is MX2 can be relatively easily obtained by exfoliating them. Each slab in bulk TMDs consists of a hexagonally close-packed sheet of transition metal atoms of © 2015 American Chemical Society

groups 4−10 (M) sandwiched between the two sheets of chalcogen atoms (X). While the intraslab M−X bonds are covalent, the MX2 slabs are stacked by van der Waals forces; thus the TMDs are readily exfoliated using various methods including mechanical cleavage,12−14 sonication in solvents,15,16 and electrochemical Li intercalation and subsequent exfoliation.17,18 Not only it is fairly easy to make TMDs into 2D slabs in large scale using these liquid-phase procedures, but also TMDs are naturally abundant and mechanically strong with exceptionally high Young’s modulus values and flexibility,19,20 which makes them attractive candidates for electrochemical energy storage platforms. Recently, TiS2 and VS2 were theoretically suggested as promising anode materials for LIBs.21,22 The Li and Na adsorption properties on the MoS2 monolayers were also investigated via first-principles calculations.23−25 In these predictions, it is noted that electrochemical performances including the average voltages and ion migration barriers were evaluated within a fixed coordination approximation for the MX2 slab model in which the coordination environment of the TMs is assumed to remain the same before and after the alkali ion insertions. The lithiation or sodiation, however, can often induce the structural transformations for TMDs, for example, as typically found in MoS2.26−29 Herein, we systematically explore the Na adsorption and diffusion characteristics for the TMD slab materials for a set of transition metals (TiS2, VS2, CrS2, CoTe2, NiTe2, ZrS2, NbS2, Received: October 10, 2015 Published: November 3, 2015 26374

DOI: 10.1021/acs.jpcc.5b09935 J. Phys. Chem. C 2015, 119, 26374−26380

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The Journal of Physical Chemistry C

chalcogen atoms and lower case letters denote the metal atoms, respectively), placing the metal atom in a trigonal prismatic site. On the other hand, the stacking order in the octahedral phase (T phase) is AbC and the coordination of a metal is octahedral. Since the nonbonding d bands of the TMDs are located between the bonding and antibonding bands of the M−X bonds, the coordination geometry of a transition metal is closely related to the electronic structure of TMDs. A typical example is 2H-MoS2 (2 indicates the number of MX2 slabs in the unit cell), which is known to be semiconducting with a direct band gap while the 1T-MoS2 phase is metallic.37,38 Though the 1T phase is metastable and thus transforms into the thermodynamically stable 2H phase or distorted 1T (1T′) phase, it has been shown that the 1T phase could be obtained upon a Li or Na intercalation,26−29 and previous theoretical studies have reasoned that this phase transition is induced by effective electron transfers from the intercalated alkali ions.39−42 In addition, the T phase can be further stabilized by going through a periodic lattice distortion to form a charge density wave (CDW) state or the T′ phase.28,39,42−44 Supposing that a similar phase transition may be found in TMDs other than MoS2, evaluating the relative stability of each phase as a function of the amount of intercalated alkali ions could be an important factor in determining which TMD is suitable for electrode applications since repeated structural transformations, if they occurred, can surely affect the cycle performance of rechargeable cells. We evaluate these phase behaviors before and after the sodium ion insertion in section 3.3. 3.2. Na Ion Adsorption and Average Open Circuit Voltage. Since a relatively large amount of TMD nanosheets can be obtained by the top-down exfoliation procedures in the liquid phase and these nanosheets can also accommodate possible structural changes induced by an alkali ion insertion, TMDs have been considered as promising candidates for electrode materials.45−48 In this work, we extend the exploration of the TMDs as a possible Na anode material by systematically considering TMDs with a transition metal substitution. In order to examine the electrochemical performance of the exfoliated TMDs including the alkali ion storage capacity and the average open circuit voltage (OCV), the key attributes for energy storage applications, we first determined which phase among H, T, and T′ is the most stable for each MX2 slab without Na atoms. Upon comparing the total energy of the MX2 monolayers after full geometry relaxations, the H phase was found to be the most stable configuration for VS2, NbS2, and MoS2 whereas the T phase was the most stable for TiS2, NiTe2, and ZrS2. The results for MoS2 are consistent with experiments.49 For CrS2 and CoTe2, the T′ phase was more stable than the others (Table S1). The calculated lattice parameters and the M−X bond lengths are listed in Table 1 and compared with experiments when available. Next, the Na-adsorbed TMD monolayer configurations were determined by finding the most favorable adsorption sites as a function of the number of adsorbed Na atoms. For the H phase, Na atoms were placed either above the M atoms (denoted as the “above-M site”) or above the vacant space between the M atoms, whereas for the T and T′ phase the above-M sites were compared to the face-centered-cubic (fcc) hollow sites (Figure 2). Different amounts of Na in NaxMX2 were considered for each metal substitution where x = 0, 0.5, 1.0, 1.5, and 2.0, corresponding to the bare MX2 (x = 0) up to the 1 monolayer Na coverage (x = 2.0) on each side of the MX2 slab. For all x, the most stable adsorption site was determined by comparing

and MoS2) by taking their phase transition behaviors into account upon sodiation. The eight TMD slabs considered here were chosen based on the screening results of free-standing 2D dichalcogenides found by Lebègue et al.,30 but with two additional criteria: (a) disulfides are preferable to diselenides or ditellurides as they weigh less than the others; (b) considering that the electric conductivity of TMDs should be better than, or at least comparable to, that of MoS2 in order to be applied to electrodes, TMDs with a band gap smaller than 1.6 eV, which is the value for MoS2 monolayer, are selected as candidates for anode materials.

2. COMPUTATIONAL METHOD The ab initio calculations were carried out within the density functional theory (DFT) framework using the hybrid functional of Heyd−Scuseria−Ernzerhof (HSE06).31 The projectoraugmented wave (PAW)32 pseudopotentials were used as implemented in the Vienna Ab-initio Simulation Package (VASP).33 The hybrid functional was chosen instead of generalized gradient approximation (GGA) functionals such as Perdew−Burke−Ernzerhof (PBE),34 since the GGA functionals tend to significantly underestimate the band gaps, thereby potentially leading to an incorrect description of electronic structure of the subject materials. The Hubbard U correction solely cannot mitigate the effect of change in electronic structure due to oxidation/reduction, and for the sake of accuracy regarding the phase transition to be discussed in detail, we assumed that it is safe to use the HSE06 functional. The geometry optimizations were performed by the conjugated gradient method with the convergence threshold of 10−5 eV in energy and 0.005 eV/Å in force. The 3 × 3 × 1 kpoint grids were generated by the Monkhorst−Pack scheme and the plane wave energy cutoff of 500 eV was applied. The -D3 correction of Grimme35 was used to take dispersion interactions into account. Energy barriers and the lowest energy pathways for the Na ion migration were estimated by employing the nudged elastic band (NEB) method with a climbing image scheme and PBE exchange−correlation functional. The structures were visualized by the VESTA software.36 3. RESULTS AND DISCUSSION 3.1. Crystal Structure of TMDs. A monolayer MX2 slab can be either trigonal prismatic or octahedral phase according to the coordination environment of M with X as shown in Figure 1. In the trigonal prismatic coordination, two chalcogen sheets forming a slab can be stacked directly above each other (AbA stacking, or H phase, where capital letters represent the

Figure 1. Different structures of TMD monolayers: (a) side and top views of the H phase where the transition metal (M) is in a trigonal prismatic coordination with chalcogens (X); (b) side and top views of the T phase where M is in an octahedral coordination with chalcogens; (c) side and top views of the distorted T phase (T′ phase) where the coordination environment of M is the same as T phase. 26375

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The Journal of Physical Chemistry C Table 1. Calculated and Experimental Lattice Parameters (a) along the a-Axis and the M−X Bond Length (dM−X) of the Bare TMD Slabs in the Most Stable Phase HSE06-D3 H-VS2 H-NbS2 H-MoS2 T-TiS2 T-ZrS2 T′-CrS2 T′-CoTe2 T′-NiTe2

experiment (bulk)30,56,57

a (Å)

dM−X (Å)

a (Å)

dM−X (Å)

3.12 3.31 3.12 3.35 3.64 3.11 3.42 3.56

2.33 2.47 2.38 2.40 2.55 2.34 2.52 2.54

3.22 3.31 3.16 3.39 3.66 3.47 3.80 3.84

2.35 2.47 2.41 2.42 2.57 2.38 2.57 2.58

Figure 3. Calculated average voltages of different MX2 monolayers. Data points inside the shaded area exhibit voltages lower than 1.0 V vs Na+/Na.

studied here can accommodate up to two Na ions per formula unit (1 monolayer coverage on each side of the slab) and these values are comparable to the capacities of hard carbon anodes.52,53 We note that, without considering the phase transformation that occurs during the Na adsorptions, one would obtain 0.20 V as an OCV for MoS2 (instead of 0.49 V with the correct description of the phase information), showing the importance of reliable configuration samplings. In addition, the use of conventional PBE functional (instead of hybrid HSE06) predicts the OCV for MoS2 to be 1.0 V.23 These results suggest that the effects of a density functional and the phase transition should both be carefully taken into consideration in the study of dichalcogenides. 3.3. Phase Transition of Monolayer TMDs Associated with Na Adsorption. As described above, lithiation or sodiation of the TMD materials induces a phase transition in the compounds, which is closely related to their electronic structures including the d-electron count. Since a recurring structural transformation in electrode materials can degrade the performance of rechargeable energy storages over time, whether a certain TMD compound shows this structural change upon Na adsorption or not is another factor to be considered when selecting the appropriate material for electrode applications. Therefore, we evaluated the relative stability of three polymorphs for the Na adsorbed versions of TiS2, ZrS2, NbS2, and MoS2, whose calculated OCVs are lower than 1.0 V, by comparing their formation energies. For each MX2 slab, we defined the formation energy of NaxMX2 with respect to its most stable phase of bare MX2 as follows:

Figure 2. Possible Na adsorption sites on (a) H-phase MX2 slabs, where Na atoms can be placed either above the M atoms (above-M) or above the vacant spaces (above-vac), and (b) T- or T′-phase MX2 slabs, where Na atoms can be located in either the above-M or facecentered-cubic hollow sites (fcc hollow).

the total energies of each configuration. The results indicate that Na atoms have a preference for the fcc hollow sites for all cases except for M = Co, Nb, and Mo in which the above-M sites are the lowest energy sites. It is generally accepted that a voltage in the range of 0.0−1.0 V vs alkali metal is appropriate for negative electrodes to maximize the energy density and prevent the formation of alkali metal dendrites. The average potential can thus be used as a simple first-order discriminator in screening the suitable anode material. This OCV or equilibrium potential, VNa, with respect to Na+/Na can be estimated by calculating the difference in total energy between the bare and Na-adsorbed TMD slabs as follows.50 VNa = [E(MX 2) + xE(Na) − E(NaxMX 2)]/xF

where E(MX2), E(Na), and E(NaxMX2) are the total energy of bare TMD monolayer, Na metal, and sodiated TMD host, respectively. Here x represents the amount of adsorbed Na and F is the Faraday constant. Figure 3 shows the calculated average voltages of MX2 monolayers. The total energy of the most stable configuration among the H, T, and T′ types for each NaxMX2 was used for computing the voltage values. For instance, the OCV of MoS2 was estimated from the total energy difference between HMoS2 and T′-Na2MoS2 while that of TiS2 was evaluated from the difference between T-TiS2 and T-Na2TiS2. In this way we were able to obtain the calculated OCV of MoS2 (0.49 V) which is in good agreement with the measured voltages around 0.53−0.69 V.27,47,51 Considering a desirable voltage range for a sodium anode of 0.0−1.0 V, we thus identified four potentially suitable materials, namely, TiS2 (0.93 V), ZrS2 (0.74 V), NbS2 (0.95 V), and MoS2 (0.49 V). These materials also show theoretical capacities of 260−339 mA h g−1 since the MX2 slabs

Ef = E(NaxMX 2) − E(MX 2) − xE(Na)

where E(NaxMX2), E(MX2), and E(Na) correspond to the total energy of sodiated TMD, host TMD, and Na (bcc), respectively, where x is the amount of adsorbed Na. According to the present definition, a negative value of formation energy implies that the sodiated compound is thermodynamically stable. The formation energies of various NaxMX2 compounds as a function of x are summarized in Figure 4. It is clearly seen that phase transformation is induced by a Na adsorption for some TMD monolayers. For instance, the H structure is stable for MoS2 when x < 0.5 but the stability of the T′ phase increases as the number of adsorbed Na increases when x > 0.5. The relative stability change between the polymorphs of MoS2 as well as the Na composition in which this change occurs shows a good agreement with the experimental findings.27 Our 26376

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Figure 4. Calculated formation energies for three (H, T, and T′) MS2 monolayer polymorphs (M = (a) Ti, (b) Zr, (c) Nb, and (d) Mo) with regard to the amount of adsorbed Na.

Figure 5. Calculated total density of states (DOS) and partial DOS (PDOS) of (a) TiS2 and (b) ZrS2 for T and H phases.

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The Journal of Physical Chemistry C calculation results suggest that, among the TMD slabs examined, TiS2 and NbS2 would not undergo potentially unwanted phase transformations and remain in the same structure before and after the Na adsorption, a useful property for long cycle lives in energy storage applications. Interestingly, the T to H phase transition is expected for ZrS2 when more than one Na is adsorbed per formula unit whereas T-TiS2 remains stable regardless of x even though the delectron count of transition metal is the same for both cases (d0 when x = 0 and d2 when x = 2), and it has been previously reported that the relative stability of the H phase is at its maximum when the number of d-electrons is around 2.54 In order to understand this, we compared the density of states (DOS) of TiS2 and ZrS2 monolayers with the T and H phases (Figure 5). In Figure 5, it can be seen that the band gap of TTiS2 (0.6 eV) is smaller than that of H-TiS2 (1.5 eV) while it is the opposite for ZrS2 (2.2 and 1.7 eV for T and H phases, respectively). Therefore, as the d-electron count increases from 0 to 2 by the Na adsorption on TiS2, it would be energetically more favorable for the additional electrons to occupy the lowlying conduction band minimum (CBM) state above the Fermi level in the T phase rather than occupying that of the H phase with the relatively high-lying CBM state. For ZrS2, additional electrons from the redox reaction would prefer to go to the CBM state of the H phase for the same reason. 3.4. Surface Migration of Na Ion on TMD Monolayers. Considering that a facile surface migration of ions leads to a fast charging and discharging of the electrode, the mobility of charge carriers is another important factor when screening and assessing the viability of the electrode materials. During the (de)sodiation process in 2D materials like TMDs, adsorbed ions should be rearranged via a surface migration in order to form the lowest energy configurations, and thus a low activation barrier for ionic motions is crucial for enabling high-power applications. Therefore, we investigated possible surface migration paths for Na ion and associated activation barriers on the selected TMD monolayers. TiS2 and NbS2 were chosen since they are expected to stay in the same structural phase before and after the Na adsorption, with average voltages below 1.0 V vs Na+/Na. For each TMD, three different nearest neighbor hopping paths were identified as shown in Figure 6: for path 1, the Na ion moves along one adsorption site to the top of S, and then to the neighboring site; for path 2, the ion moves from one adsorption site to another while passing through the above-M sites for TiS2 and above vacant space for NbS2; for path 3, the ion is allowed to move from one site to another directly in a linear way. After the energy optimization process, the straight route of path 3 became essentially the same as path 1 in both TiS2 and NbS2. Besides, path 2 in TiS2 is relaxed to the same path as path 1, indicating that the Na ion migration prefers to follow path 1 when hopping between neighboring sites occurs. The calculated activation barriers for a Na ion hopping on TiS2 and NbS2 were 0.22 and 0.07 eV, respectively. For comparison, the Na diffusion barrier on MoS2 was computed with the same method and it was found to be 0.15 eV (Figure S1), which agrees with the value previously reported in the literature.23 Although various factors including ion concentration and strain of the layer could influence the ionic diffusion on real 2D systems,55 we can expect that the TMDs examined herein would exhibit better electrode performance at high cycling rates as the ion migration barrier of TiS2 and NbS2 is lower than that of MoS2.

Figure 6. Possible Na ion migration pathways on (a) TiS2 and (b) NbS2 as well as their energy profiles.

4. CONCLUSIONS Whereas graphitic carbon based materials are representative negative electrodes in LIBs, SIBs do not currently have standard anode materials. Our present calculation results for a set of transition metal dichalcogenides (TiS2, VS2, CrS2, CoTe2, NiTe2, ZrS2, NbS2, and MoS2) suggest that these twodimensional TMD slabs could indeed be potential anode materials for SIBs with promising electrochemical properties, as summarized below. Among the eight materials considered here, TiS2, ZrS2, NbS2, and MoS2 are found to have a suitable open circuit voltage range as a Na anode material (0.49−0.95 V vs Na+/Na), with the theoretical capacities of 260−339 mA h g−1. Many of these 2D materials are predicted to undergo a phase transformation during the Na ion adsorption, an important aspect that must be taken into account when calculating the voltages and ion migration barriers but often neglected in theoretical considerations of similar materials in the literature. We showed that either neglecting the phase transition information or using conventional PBE functional can yield incorrect battery properties that depend on the lowest energy configurations. For example, for MoS2 with the experimental voltages estimated as 0.53−0.69 V in various measurements, the calculated open circuit voltage of 0.49 V vs 0.20 V was obtained with and without including the phase transition for MoS2, respectively. On the basis of the importance of phase transformation during alkali ion adsorption, we further identified TiS2 and NbS2 as the phase-maintaining TMDs upon sodiation, a favorable point for a stable cyclability in energy storage applications. These materials also showed Na ion migration barriers as low as 0.22 (TiS2) and 0.07 eV (NbS2), suggesting that these materials can be cycled at a fast rate with regard to an ionic mobility.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b09935. 26378

DOI: 10.1021/acs.jpcc.5b09935 J. Phys. Chem. C 2015, 119, 26374−26380

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Calculated total energy differences of TMD polymorphs and Na ion migration on MoS2 (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support from the National Research Foundation of Korea (NRF-2014R1A4A1003712 and NRF2010-C1AAA001-2010-0029031).

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