Intrinsic Charge Storage Capability of Transition Metal

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Intrinsic Charge Storage Capability of Transition Metal Dichalcogenides as Pseudocapacitor Electrode Xin Cong, Chuan Cheng, Yiming Liao, Yifei Ye, Changxu Dong, He Sun, Xiao Ji, Wenqiang Zhang, Peilin Fang, Ling Miao, and Jianjun Jiang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b07004 • Publication Date (Web): 26 Aug 2015 Downloaded from http://pubs.acs.org on August 29, 2015

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The Journal of Physical Chemistry

Intrinsic Charge Storage Capability of Transition Metal Dichalcogenides as Pseudocapacitor Electrode Xin Cong, Chuan Cheng, Yiming Liao, Yifei Ye, Changxu Dong, He Sun, Xiao Ji, Wenqiang Zhang, Peilin Fang, Ling Miao* and Jianjun Jiang School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan, HUBEI 430074, People’s Republic of China. *Email: [email protected]

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Abstract The intrinsic charge storage capability of a serial of TMDs (MS2, M = Sc Ti V Cr Mn Fe Co Ni Zr Nb Mo Tc Hf Ta W Re, and MoX2, X = S Se Te) are investigated using density functional theory calculations. A map for pseudocapacitor electrode is provided, depending on the demands of high conductivity and a remarkable peak of density of states (DOS) in the range of electrolyte window. The calculated DOS suggest that most of T phase structures are superior to H phase in electroconductibility. The charge storage capability is represented by the number of gaining or losing electrons calculated by integrating DOS in electrolyte window. MS2 (M=Ti V Cr Fe Nb Mo Tc) of T phase are conductive and gain electrons easily with considerable valance state change of TM atom, showing a redox pseudocapacitance character as a cathode. Meanwhile, CoS2-H and MoSe2-T are promising anode materials. Moreover, the chalcogen (S, Se, Te) with different electronegativity and work function will result in the changes of Fermi level and surface polarization of MoX2, leading to the shift of their DOS in electrolyte window. In another way, the metallic hydrogenated H phase of MoS2 (MoS2H2) with conductivity should behave enhanced advanced electrochemical performance.

Keywords: transition metal dichalcogenides, pseudocapacitor, charge storage, anode and cathode, hydrogenation


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Introduction Electrochemical capacitors (ECs) have drawn wide attention due to their high power density, fast charge-discharge, and long service life.1-3 Compared with electrical double-layer capacitors (EDLs), pseudocapacitors with fast reversible surface reactions, can store relatively more charges and have the potential to combine the advantages of lithium ion batteries (high energy density) and capacitors (high power density).4 Metal oxide,5 hydroxides,6 sulfides7-9 and chloride10-12 materials as well as graphene-based nanostructure13-16 have been widely explored as ECs electrode materials. For extending the specific surface area of electrochemical active materials, designing nanostructured ECs electrode materials may be an effective approach to improve their electrochemical performances.17 In two-dimensional (2D) transition metal dichalcogenides (TMDs), large surface area and the potential of undergoing redox processes could be both available at the same time.18 Since discovery of TMDs, some of them have been synthesized19, such as MoS220, MoSe221, MoTe2,22-23 WS2,24 etc. The experimental researches about its electrochemical performance of TMDs have a recent start. Feng et al. successfully assembled highly conductive ultrathin VS2 nanosheets for constructing the electrodes of in-plane ECs with a specific capacitance of 4760 μF/cm2.25 Muller et al. have found that the fewlayer TiS2 nanocrystals with intercalation pseudocapacitance exhibit both high energy and power density.26 Acerce et al.22 chemically exfoliated nanosheets of MoS2 of the metallic 1T phase achieving a high capacitance of ∼400 to ∼700 F cm−3. In addition, the redox charge storage of active 1T phase of WS2 is manifested in a highly reversible



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and ultrafast capacitive fashion.27 Meanwhile, the properties of TMDs such as phase engineering,28 electronic,18, 29 device application,30-31 are also extensively studied. On the contrary, the theoretical researches32 about electrochemical charge storage mechanism of TMDs are still limited. It is very necessary to systematically investigate the electronic and surface properties of TMDs, and reveal their corresponding electrochemical characters. In this work, we perform first principles calculations to explore a variety of TMDs (MS2, M = Sc Ti V Cr Mn Fe Co Ni Zr Nb Mo Tc Hf Ta W Re) and their performances as electrode materials for pseudocapacitor. The electrochemical characters are estimated by calculating electronic conductivity, Fermi level and the number of gaining/losing electrons in electrolyte windows. Then MoX2 (X = S Se Te) are compared with each other to estimate surface effect. Finally, we take MoS2 of H phase for example to explore the effect of hydrogenation strategy.

Computational details The density functional theory (DFT) calculations were performed using the Vienna Ab Initio Simulation Package (VASP).33-35 The electron-ion interaction was described by the Projector Augmented Wave (PAW) pseudopotentials. And the exchangecorrelation function is described by Perderw, Burke, and Ernzerhof (PBE) version of the generalized gradient approximation (GGA).36-38 The energy cut-off of 500 eV was used for the plane-wave basis set. The conjugated gradient method was performed to


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the geometry optimization. The convergence condition for the energy is 10-4 eV and the structures were relaxed until the force on each atom was less than 0.01 eV/Å. The Brillouin zone was sampled using Monkhorst-Pack special k-point meshes of 5×5×1, 15×15×1 and 25×25×1 for geometry optimization, electronic self-consistent computation and density of states (DOS) calculation, respectively. The periodic structures of TMDs monolayers had been decoupled by a vacuum thickness large than 40 Å. Here, all MX2 were calculated with spin (Figure S1). The results show MS2 (M = Sc V Cr Mn Fe Nb ) with significant spin polarization, and the other studied TMDs with little spin polarization. Some MS2 (M =Sc Ti V Cr Mn Fe Co Ni Zr Nb Hf Ta) were tested with LDA+U correction in the H and T structures. The values of U were set according to previous research.39 Compared with published work,18 the presented results of FeS2 and MnS2 were adopted with LDA+U correction.

Results and discussion The structure of TMDs is consisted of a packed layer of metal atoms sandwiched between two layers of chalcogen atoms, where different stacked layer forms two phases (MX2-H/T), as shown in Fig. 1a and 1b, respectively. All studied MX2 monolayers are presented in Fig. 1c. There are some TMDs (such as MoS2-T22 and WS2-T27) predicted not thermodynamically stable by previous DFT calculation,18 which have been successfully synthesized for electrode of ECs recently. So we take both H and T phase



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structures of above TMDs into investigation here. The relaxed lattice parameters (a), the M-X bonds (dM-X) and the layer thickness (dx-x) of above mentioned MX2 monolayers obtained from our calculations (shown in Table S1) are in good agreement with previous DFT results,18, 40 indicating that our computational procedure is capable of describing the properties of MX2 monolayers at an atomistic level.

Fig. 1 A schematic showing the structure of (a) H phase and (b) T phase with top view and side view. The metal atoms are presented by blue spheres and the chalcogen atoms are presented by yellow spheres. The unit cells for TMDs monolayers are also depicted. (c) Summary of the results of our analysis comprising 16 different MS2 compounds and MoX2 (X = S Se Te) with H and T structures.

The structure, PDOS, vacuum level, Fermi level and other properties of TMDs are


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systematically investigated in this study. To qualitatively evaluate the properties of TMDs for pseudocapacitor electrode, a criterion is indispensible. Corresponding with published works,32, 41 the judgement method usually requires two conditions, namely high conductivity and an apparent peak of PDOS in the range of electrolyte window. More details are discussed as follows.

Electronic conductivities of TMDs

Firstly, the electrode material should possess metallic band structure, which offers an intrinsic advantage in electric conductivity compared with semiconductor or insulator. We calculated the band structures of all studied MX2, and found the results in accordance with previous DFT calculation.18 The corresponding PDOS are shown in Figure S1, and their conductivity characters are summarized in Fig. 1(c). For most of them, structures of T phase are more conductive than that of H phase, such as metallic T phase and semi-conductive H phase of ScS2, TiS2, MoS2, except NiS2. It can be found in Fig. 2a that monolayers of MoS2-T, WS2-T, TiS2-T VS2-T are conductive and monolayers of MoS2-H, WS2-H are semi-conductive, in good agreement with reported experimental results.22, 25, 42-44



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Fig. 2 (a) Total DOS (yellow region), PDOS (blue lines) of p-orbitals of X (X= S Se Te) atoms, PDOS (green lines) and integral DOS (red lines) of d-orbitals of metal atoms (Ti V Ni W Mo) with respect to the vacuum level reference. The electrolyte window is indicated by light blue region, and the Fermi level is drawn by black dashed lines. Orbital projected band structures and crystal field splitting of d-orbitals of (b) MoS2 and (c) NiS2.

To compare the electronic conductivities of H and T phases, the monolayers of MoS2 and NiS2 are taken as examples, and their PDOS with respect to vacuum level and band structures are presented in Fig. 2. As we known, there are two types of dorbital splitting of TM in H and T structures due to the crystal field effect. For H phase,


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4d-orbitals split into 3 groups, while 4d-orbitals of T phase split into 2 degenerate states, as shown in Fig. 2b and 2c. The calculated orbital projected band structures of MoS2 and NiS2 illustrate this crystal field effect very well, and exhibit that nature of conductivity is mainly determined by the occupation of d-orbitals. Partially filled orbitals at Fermi level tend to show metallic behavior while complete filling is more likely to behave as semi-conductor.28 Therefore, six d-orbital electrons of NiS2 fully fill 4d-orbitals of T phase, which is different from MoS2 with three electrons of 4d-orbitals.

Capability of charge storage of TMDs

Furthermore, the second demand for suitable pseudocapacitor electrode material is significant DOS peak of TMDs existing in the range of electrolyte window, for storing electrons in charging-discharging process.32 All calculated PDOS of TMDs in Figure S1 are referenced to the vacuum level by calculating the electrostatic potential, and could be compared with external scan voltage directly, while the standard hydrogen electrode (vs. SHE) appears to lie at -4.5 eV with respect to the vacuum level.45-46 To represent the capability of charge storage of all studied MX2 structures, we define the number of gain/losing electrons (Qg/Ql) as the integrated PDOS of metal dorbitals in the range between the original Fermi level and external scan voltage as shown in Fig. 3. The peak of DOS between Fermi level and scan voltage correspond to pseudocapacitance characteristics showing in cycle voltammogram. The external scan voltage is limited by the electrolyte window, which is 0 ~ 1.29 eV for the aqueous solution referenced to SHE, and is regulated by value of pH.



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Fig. 3 Schematic of the framework associating the calculated DOS with the experimental charging discharging process.

According to the PDOS in Figure S1, Qg/Ql (values showing in Table. S2) for studied MX2 of H and T phases in acid (pH=1), neutral (pH=7) and alkaline (pH=14) solutions are calculated and presented in Fig. 4. And it is obvious that the number of gain electrons increase as the value of pH rises. TMDs with metallic characters and large number of gain/losing electrons (Qg/Ql) in the aqueous solution would possess good performance of pseudocapacitor, marked in Fig. 1c. It is obvious that T phase of MS2 (M=Ti V Cr Fe Nb Mo Tc) are conductive with low resistance, and gain electrons easily with unoccupied d-orbitals, thus they are suitable for pseudocapacitor cathode. Monolayers of CoS2-H and MoSe2-T are conductive with occupied d-orbitals, indicating good properties of anode. Meanwhile, some TMDs with poor conductivity


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may be not suitable for ECs electrode, and the other MS2 (CrS2-H MnS2-T FeS2-H TaS2T WS2-T ReS2-T) would display only EDL characters with small and flat PDOS of dorbitals, leading to no obvious redox reaction in electrolyte window.

Fig. 4 Qg/Ql of studied MS2 of H (left) and T (right) phases, in different aqueous solutions with pH=1, 7, 14. Red and olive green represent Qg and Ql respectively. (negative value meaning inverse electron gain or loss).

These theoretical predictions correspond well with the electrochemical performances of all synthesized TMDs mentioned in introduction section. According to the previous experimental researches, MoS2-T nanosheets achieve a capacitance ranging from ∼400 to ∼700 F cm−3 in a variety of aqueous electrolytes, by adsorbing H+, Li+, Na+ and K+ ions efficiently, meanwhile MoS2-H as electrode of ECs performs only EDL character,22, 42 while Qg of metallic MoS2-T and semi-conductive MoS2-H are 0.83 and 0 respectively in liquor of pH=1. Metallic few-layer VS2 ultrathin nanosheets preform a specific capacitance of 4760 μF/cm2,25 with 1.07 (pH=1), 1.79 (pH=7) and 2.57 (pH=14) of Qg. The few-layer TiS2 nanocrystals with charge storage characteristics of intercalation pseudocapacitance, exhibit high energy and power density, while a big



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Qg of TiS2-T in 1.18 to 3.04 range as value of pH varies from 1 to 14.26 Furthermore, the ECs with electrode of WS2-T (Qg and Ql in range of 0.46~1.59 and 0.64~0.07, respectively) displays EDL character.27

Fermi level adjustment of TMDs with different chalcogen

The ability of charge storage is represented by Qg/Ql. It should be noted that the value of Qg/Ql is determined by the position of Fermi level, which is the criterion for distinguishing the occupied and unoccupied orbitals. TMDs with unoccupied d-orbitals above Fermi level gains electrons and would suits cathode material. On contrary, the material with occupied d-orbitals below Fermi level may be suitable for anode. From previous work,18 the band structures of TMDs with same metal are similar. However, the chalcogen (S, Se, Te) with different electronegativity and work function will result in the changes of Fermi level and surface polarization of TMDs. Fermi energy is adapted to the electron energies in vacuum (Efv) by calculating the electrostatic potential, which depends on the surface polarity. MS2-H/T (VS2-H, TiS2H, ScS2-H/T, CoS2-T, HfS2/T, MnS2-H, ZrS2-H/T, TaS2-H) with lower Efv than SHE are investigated to demonstrate chalcogen effect. With chalcogen variation, their Efv increase, showing in Figure. S2. MoX2 (X = S Se Te) are taken for example to explore surface influence on Efv. Their Efv, vacuum level (Evl), Fermi level before adopting electron energy in vacuum (Ef0) and bonds length are presented in Table 1. From Fig. 2a and Table 1, Efv goes up in order of MoS2 MoSe2 MoTe2, and Evl has little change,


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indicating that Fermi level are observably influenced by chalcogen atoms. In comparison, Efv is influenced by Ef0 more largely than vacuum level, suggesting that bonds built between molybdenum and dichalcogenides atoms primarily shift Efv. From Table 1, dM-X increases as various chalcogen atoms due to the radium and electronegativity, leading to rise of Ef0.

Table 1 Fermi level (Ef0) and vacuum level (Evl) of MoX2-H/T MoS2-H

MoSe2-H

MoTe2-H

MoS2-T

MoSe2-T

MoTe2-T

Ef0 (eV)

-4.59

-4.07

-3.29

-3.67

-3.23

-2.50

Evl (eV)

1.35

1.26

1.60

1.36

1.29

1.65

dM-X(Å)

2.41

2.47

2.58

2.43

2.48

2.58

Improvement of electronic conductivity through hydrogenation

According to above discussion, a part of TMDs are not suitable for ECs electrode, due to the poor conductivity or small Qg/Ql. The performances of these TMDs could be optimized through the approaches of improving conductivity and introducing peak of DOS in the range of electrolyte window. Recently, hydrogenation strategy is applied to TiS2 to acquire ultrahigh conductivity by Lin et al43 experimentally. To exhibit influence of hydrogenation, all semi-conductive TMDs are compared with intrinsic TMDs in electric conductivity (Table S3). The conductivities of them may be promoted, and most of them become conductive after hydrogenating. Here, MoS2-H monolayer with



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semiconductor character and flat PDOS is taken example to provide insight into the mechanism of hydrogenation. As shown in Fig. 5a, there are three high symmetry sites for adsorption of hydrogen atoms on the two surfaces of MoS2. The distances between H and sulphur surface and absorption energies of optimized structures are listed in

Fig. 5 (a) stable structure of MoS2H2, and three high symmetry sites. (b) orbital projected band structure and PDOS, red spot and line represent hydrogen s-orbitals, where zero energy represents the Fermi level. (c) comparison between total DOS of MoS2 and MoS2H2 adopted electron energy in vacuum. (d) top and side views of Na absorbed MoS2H2 structure.

Table 2. Distances between adsorption ions and top surface (d) and absorption energies (Eb). H

Na

site d(Å)

Eb(eV)

d(Å)

Eb(eV)

S

1.42

1.77

1.97

1.11

M

0.24

1.10

1.74

1.31

h

0.80

1.57

2.34

1.35

. The absorption energy of a hydrogen atom is defined as Eb = –


1 2

(Etotal – Eion

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– Emonolayer). Here Etotal is the total energy of adsorbed MoS2 monolayer, Eion is the energy of adsorbing ion and Emonolayer is the energy of bare monolayer. 47 Thus larger Eb signifies more stable absorption on MoS2 monolayer. In comparison, the hydrogen at S site on MoS2-H (see Fig. 5a) is the most stable structure with largest adsorption energy, forming MoS2H2.

Fig. 5 (a) stable structure of MoS2H2, and three high symmetry sites. (b) orbital projected band structure and PDOS, red spot and line represent hydrogen s-orbitals, where zero energy represents the Fermi level. (c) comparison between total DOS of MoS2 and MoS2H2 adopted electron energy in vacuum. (d) top and side views of Na absorbed MoS2H2 structure.

Table 2. Distances between adsorption ions and top surface (d) and absorption energies (Eb). H

Na

site d(Å)

Eb(eV)

d(Å)

Eb(eV)

S

1.42

1.77

1.97

1.11

M

0.24

1.10

1.74

1.31

h

0.80

1.57

2.34

1.35

The orbital projected band structure and DOS of MoS2H2 are presented in Fig. 5b.



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And the contribution of hydrogen s-orbital is also presented to show effect of hydrogenation. For MoS2H2, the electrons from hydrogen fill the d-orbital of Mo. Comparing with band structure of MoS2-H, the former unoccupied bands are partly filled and Fermi level rises into the conduction bands as MoS2-H is hydrogenated. And corresponding peak of PDOS are induced leading to good conductivity. Furthermore, the orbitals appear in the range of electrolyte window to storage or provide electrons, showing in Fig. 5c, indicating better performance of pseudocapacitor. The stability of MoS2H2 as electrode material is investigated by calculating the absorption of Na. The absorption energies on three high symmetry sites (S, M and h in Fig. 5a) and the distances between absorbed Na+ ion and topmost surface are presented in Table 2. The most stable structure is MoS2H2 absorbed Na+ on the h sites. From Fig. 5d, Na+ ion has little influence on morphology while monolayer of MoS2H2 absorbs or desorbs Na+ ion. The absorption energy of Na+ ion is lower than that of hydrogen, indicating that MoS2H2 structure would not desorb hydrogen while desorbing Na+ ion and has a long cycle life for electrode material. On conclusion, hydrogenation strategy has an obvious improvement for the properties of TMDs, such as conductivity, DOS and etc. And the hydrogenated TMDs could also have a long cycle life, with little structure deformation in ion absorbing and desorbing process.

Conclusion In this work, the pseudocapacitance properties of a serial of TMDs (MS2, M = Sc


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Ti V Cr Mn Fe Co Ni Zr Nb Mo Tc Hf Ta W Re, and MoX2, X = S Se Te) have been systematically investigated to provide a map for pseudocapacitor application, by DFT methods. The judgement of pseudocapacitor electrode requires good conductivity and ability of electrons storage. On the authority of emerging results, conductivity of T phase are superior to that of H phase, such as metallic T phase and semi-conductive H phase of ScS2, TiS2, MoS2, except NiS2 due to different electrons filling situation and the crystal field effect. From projected band structure, 4d-orbitals of H phase split into 3 groups, while 4d-orbitals of T phase split into 2 degenerate states. The capability of charge storage is represented by the number of gain/losing electrons (Qg/Ql) in the range of electrolyte window. The electrolyte window is regulated by pH, and as value of pH raises, the number of gain electrons increase. The pseudocapacitor cathodes are conductive and gain electrons easily, such as T phase of MS2 (M=Ti V Cr Fe Nb Mo Tc). Some TMDs with conductive behavior and ability of losing electrons are suitable for pseudocapacitor anode, such as CoS2-H and MoSe2-T. Moreover, two strategies are studied to improve the charge storage capacity. Firstly, the chalcogen (S, Se, Te) with different electronegativity and work function will result in the changes of Fermi level where Efv goes up as order of MoS2 MoSe2 MoTe2 and the shift of their density of states in electrolyte window. Furthermore, the hydrogenation strategy is applied for a part of TMDs with poor conductivity or small Qg/Ql, and MoS2H is taken an example to have insight into hydrogen influence and improvement of pseudocapacitance performance. Hydrogenation on surface elevates Efv of MoS2 and



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introduces apparent peak of DOS in the range of electrode window. While it works as pseudocapacitance electrode in solution, absorption and desorption of Na+ ion with small absorption energy have little influence on hydrogenated structure, indicating that desorbing Na+ ion would not desorb hydrogen and the monolayer of MoS2H2 would have a long cycle life. To sum up, our results provide valuable direction for further exploring electrochemical properties of TMDs and appropriate strategy to enhance their performance.

Acknowledgment This research work is supported by National Natural Science Foundation of China (Grant No. 51302097 and 51571096).

Supporting Information Structure values, DOS, Qg/Ql (pH = 1, 7 and 14) of TMDs, Evl, Ef0 and Efv variation with different chalcogen, bandgap of intrinsic and hydrogenated TMDs. This information is available free of charge via the Internet at http://pubs.acs.org.

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Table of Contents (TOC)



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A schematic showing the structure of (a) H phase and (b) T phase with top view and side view. The metal atoms are presented by blue spheres and the chalcogen atoms are presented by yellow spheres. The unit cells for TMDs monolayers are also depicted. (c) Summary of the results of our analysis comprising 16 different MS2 compounds and MoX2 (X = S Se Te) with H and T structures. 251x254mm (120 x 120 DPI)


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(a) Total DOS (yellow region), PDOS (blue lines) of p-orbitals of X (X= S Se Te) atoms, PDOS (green lines) and integral DOS (red lines) of d-orbitals of metal atoms (Ti V Ni W Mo) with respect to the vacuum level reference. The electrolyte window is indicated by light blue region, and the Fermi level is drawn by black dashed lines. Orbital projected band structures and crystal field splitting of d-orbitals of (b) MoS2 and (c) NiS2. 407x372mm (120 x 120 DPI)



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Schematic of the framework associating the calculated DOS with the experimental charging discharging process. 300x315mm (120 x 120 DPI)


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Qg/Ql of studied MS2 of H (left) and T (right) phases, in different aqueous solutions with pH=1, 7, 14. Red and olive green represent Qg and Ql respectively. (negative value meaning inverse electron gain or loss). 300x113mm (120 x 120 DPI)



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(a) stable structure of MoS2H2, and three high symmetry sites. (b) orbital projected band structure and PDOS, red spot and line represent hydrogen s-orbitals, where zero energy represents the Fermi level. (c) comparison between total DOS of MoS2 and MoS2H2 adopted electron energy in vacuum. (d) top and side views of Na absorbed MoS2H2 structure. 560x183mm (120 x 120 DPI)


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Intrinsic Charge Storage Capability of Transition Metal

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