First-principles study of MoSSe_graphene heterostructures as anode for Li-ion batteries

Journal Pre-proofs First-principles Study of MoSSe_Graphene Heterostructures as Anode for Liion Batteries Zhou Sheng-Hua, Zhang Jing, Ren Zhen-Zhen, Gu Jia-Fang, Ren Yu-Rong, Huang Shuping, Lin Wei, Li Yi, Zhang Yong-Fan, Chen Wen-Kai PII: DOI: Reference:

S0301-0104(19)30685-8 https://doi.org/10.1016/j.chemphys.2019.110583 CHEMPH 110583

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Chemical Physics

Received Date: Revised Date: Accepted Date:

13 June 2019 21 August 2019 28 October 2019

Please cite this article as: Z. Sheng-Hua, Z. Jing, R. Zhen-Zhen, G. Jia-Fang, R. Yu-Rong, H. Shuping, L. Wei, L. Yi, Z. Yong-Fan, C. Wen-Kai, First-principles Study of MoSSe_Graphene Heterostructures as Anode for Li-ion Batteries, Chemical Physics (2019), doi: https://doi.org/10.1016/j.chemphys.2019.110583

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First-principles Study of MoSSe_Graphene Heterostructures as Anode for Li-ion Batteries ZHOU Sheng-Huaa GU Jia-Fangd

ZHANG Jinga

REN Zhen-Zhena

REN Yu-Ronge HUANG Shuping*a LIN Wei*a

LI Yi*a

ZHANG Yong-Fan*a CHEN Wen-Kai*a,b,c a(Department b(State

of Chemistry,

Fuzhou University, Fuzhou, Fujian 350116,

China)

Key Laboratory of Photocatalysis on Energy and Enviroment, Fuzhou, Fujian 350116, China)

c(Fujian

Provincial Key Laboratory of Theoretical and Computational Chemistry (FTCC) , Xiamen University, Xiamen, Fujian 61005, China)

d(Department

of Chemical Engineering, Zhicheng College, Fuzhou University, Fuzhou 350002, P. R. China)

e(School

of Materials Science and Engineering, Jiangsu Collaborative Innovation Center of

Photovolatic Science and Engineering, Changzhou University, Changzhou 213164, Jiangsu, China)

1

ABSTRACT: By using means of density functional theory calculations, we reported the heterostructure consisting of monolayer MoSSe and graphene, as a promising anode material for lithium ion batteries. We investigated the adsorption and diffusion of lithium atoms in the MoSSe and MoSSe_graphene heterostructures, and we found that the combination with graphene makes the lithium atoms’ adsorption more stable, meanwhile, the diffusion barriers on the surface are lower than that at the interface, which are comparable to the barriers on the corresponding monolayers. The maximum lithium storage capacity of the heterostructure is enhanced to 390 mAh/g. Our work made a comparison with all the typical structures of MoSSe and MoSSe_graphene, and suggests that the S side of MoSSe-2 combined with graphene and the Se side of MoSSe-2 combined with graphene are the promising materials for their higher lithium capacity and charge/discharge rates.

2

1.

INTRODUCTION

Lithium ion batteries (LIBs) are preferred in these years’ investigations as the most up-and-coming energy storage systems (ESSs). They are widespreadly applied to the electronic equipment over the past decades on account of the high reversible specific capacity and the environmentally friendly performance. The anode materials is vital for the development of high-performance batteries[1-3]. Graphite as anode materials for lithium ion batteries has been well commercialized due to its good conductivity, high crystallinity and fine cycle performance[4-6]. However, the instability of the graphite structure and the low Lithium storage capacity hinders its further development[7]. With the increasing demand of the high-performance electronics, better electrochemical properties’ batteries are imperative to be designed[8, 9]. With the rapid development of graphene research and sustaining innovation of manufacturing materials technology, other materials with two-dimensional layered structural characteristics, such as transition-metal dichalcogenides (TMDCs)[10-16], have been gradually entered the scope of people's research due to their good mechanical flexibility and thermal stability, no surface dangling bonds, as well as compatibility with silicon CMOS processes[17-20]. Compared with graphene, TMDCs have higher theoretical Lithium storage capacity and lower diffusion barrier. As a promising TMDC, MoS2(1-x)Se2x has been demonstrated that it has a good performance on the hydrogen evolution reaction as well as exhibits excellent photocatalytic properties[21-23]. However, the exploration of applying MoS2(1-x)Se2x to be a kind of anode material is relatively rare. Recently, Ersan et al.[24] simulated the Li atom adsorption and diffusion on monolayer MoS2(1-x)Se2x with different ratios of sulfur and selenium and found that it was suitable to be a promising anode material. According to Wang et al.[22], a synthesized composite of rGO and MoS2(1-x)Se2x and the meso-GO/ MoS2(1-x)Se2x was investigated as a high-capacity anode material, and it came to the highest capacity when the ratio of sulfur and selenium was almost 1:1. One typical structure of MoS2(1-x)Se2x, the Janus monolayer MoSSe has got a large number of researches in these two years[25, 26]. Janus monolayer MoSSe shows good photocatalytic performance and electronic properties. Therefore, the Janus monolayer MoSSe is a promising electrode material for us to investigate. Nevertheless, TMDCs also have some deficiencies, such as low conductivity and structural instability[27-31]. Moreover, during the charge-discharge process, the volume expansion of the electrode is very serious, causing polarization of the electrode, resulting in serious degradation of the specific capacity. A frequently-used method is to construct complexes of TMDCs and carbon materials (graphene, amorphous carbon, etc.[32, 33]), and use the high conductivity and confinement of carbon materials to prevent the dissolution of polysulfide anions and increase their lithium storage capacity. In this way the VS2/graphene heterostructure was studied both in experiment and theory and was proved to have higher lithium capacity and wider voltage range compared with VS2[34, 35]. In like manner, Samad et al.[36] investigated MoS2/VS2 composite and demonstrated its excellent electrochemical 3

properties. Meanwhile, Yue et al.[37] synthesized MoS2(1-x)Se2x/graphene heterostructure and investigated its optical properties through experiments. Despite many efforts have been made to experimentally and theoretically rationalize the excellent properties of heterostrucutres anode materials,there are still several important issues to be addressed. Thus, in this paper, we used the first-principles to simulate the lithium adsorption and diffusion in the different structure of MoSSe monolayer. To make a comparison with the Janus structure of MoSSe, we found another typical MoSSe structure, the MoSSe-1, with sulfur and selenium atoms are arranged alternately on the same surface, and the Janus monolayer MoSSe is defined as MoSSe-2. Then, we combined the two different structures with the graphene and got 3 MoSSe_graphene heterostructures which are defined as structure SeSC (MoSSe-1 combined with graphene),SC (the S side of MoSSe-2 combined with graphene),SeC (the Se side of MoSSe-2 combined with graphene), we explored the lithium adsorption energies, diffusion barriers, the maximal possible lithium content as well as made a comparison with the monolayer MoSSe. The results suggest that MoSSe_graphene have the potential to be universal anode materials. 2.

COMPUTIONAL DETAILS

Spin-polarized calculations were performed using an all-electron method in the DMol3 code[38, 39] within the framework of density functional theory (DFT). We used the generalized gradient approximation (GGA) in the form of Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional[40]. The double numerical plus d functions (DND) basis set and the DFT+D2 method with the Grimme vdW[41] correction were adopted. We also use the dipole correction due to the different surface of the structure MoSSe-2. The Brillouin Zone (BZ) was sampled with 4 × 4 × 1 k points mesh for monolayer MoS2(1-x)Se2x and the heterostructures combined by the monolayer MoS2(1-x)Se2x and graphene. To weaken the interlayer interactions, we considered a vacuum spacing of 20 Å. A Methfessel-Paxton smearing of 0.005 Hartree was used, and the Self-consistent field (SCF) computations were set to a convergence criterion of 1×10-6 au. In all geometry optimizations, the maximal force on per atom is lower than 0.002 Ha/Å. The complete LST/QST[42] method was used to search the transition states, minization reaction pathway was less than 0.01 eV/Å−1, the activation energy barrier (Ea) is defined to be Ea =ETS − EIS, where IS and TS refer to the initial state and the transition state. Phonon modes are calculated in the Phonopy[43], with the atomic displacements are considered to be 0.01 Å. 3.

RESULTS AND DISSCUSION 3.1 The structures of MoSSe and MoSSe/graphene First of beginning, we optimized the unit cell of MoSSe. And the lattice parameters are a = 3.286 Å, b = 3.298 Å, which are similar with the previous work (a = 3.198 Å, b = 3.328 Å)[24]. In the consideration of the isolated lithium atom, we constructed a 4 × 4 × 1 MoSSe monolayer, and the 30 Å vacuum region was chosen to avoid the interactions between the interlayers. Meanwhile, the bond lengths of Mo-Se and Mo-S in the MoSSe-1 are 2.563 Å and 2.444 Å, and in MoSSe-2 the values are 2.561 Å and 4

2.446 Å, the two structures were all shown in the Fig. S1. the band gaps of them are 1.485 eV and 1.566 eV, respectively, both coincided well with the previous report[25, 26]. Afterwards, we built the heterostructures of MoSSe/graphene with a 5 × 5 × 1 graphene monolayer. Due to the different surface of MoSSe-2, we found three possible structures SeSC, SC, SeC. To investigate the stability of the structures, we calculated the stacking energy (Estack) of various configrations, and the Estack and the distance of layers were shown in Fig1. For comparison, we also considered the stacking energy of MoS2_Graphene and the result (23 meV per atom) was in good agreement with the literature (30 meV per atom)[44]. We found that the stacking energy of all heterostructures are higher than that of the MoSSe_Graphene; and Estack of SeC is the highest among three structures suggesting the most stable configuration. We also performed the phonon dispersion calculations of MoSSe and the MoSSe_Graphene. The results are shown in the Fig. S3-S4. The stable phono modes for the MoSSe_Graphene show that the monolayer MoSSe-1 was stablized by combined with the graphene. The monolayer MoSSe-2 and its heterostructures are relatively stable.

Fig. 1 Top and side view of (a)SeSC,(b)SC,(C)SeC. Values given with the top views show the stacking energy in eV per C atom; values given with the side views show the vertical distance between Mo and C in Angstroms.

3.2 Adsorption and Diffusion of Li ions on the two possible MoSSe Monolayers We investigated all the possible adsorption sites and got 4 stable sites both in the Janus structure and the MoSSe-1, the top site (A and B) above the Mo atom, the hollow site (C and D) above the center of hexagon, while in the MoSSe-1, the adsorption sites deviate slightly and are relatively close to the nearby S atoms and far from Se atoms, as depicted in the Fig. S2, which is corresponding with the previous research.[24] The adsorption information of all the possible adsorption sites are summarized in the Tab. 1. According to the results, we found that the adsorption site 5

A is much more stable than other sites in both 2 structures. As a comparison with the similar sites of MoSSe-1 and MoSSe-2, the site A on the surface of sulfur in the MoSSe-2 has the higher binding energy. The Mülliken charge population analysis and the distance from the nearest Mo atom of a Li atom at the adsorption sites of monolayer MoSSe were also depicted in the Tab. 1. We speculated that the distant is shorter, the adsorption energy is higher. And the Mülliken charge population analysis indicates that Li atom possesses the positive charge of 0.383 in MoSSe-1 and 0.404 in MoSSe-2, which means the adsorption of Lithium atoms and the MoSSe exists favorable ionic characteristics. Tab. 1 Calculated binding energy (Eb), charge transferred (q), and the distance from nearest S, Se and Mo atom of a Li atom (dLi−S, dLi−Se, dLi−Mo) at the energetically favorable adsorption sites of the monolayer MoSSe-1 and MoSSe-2. system

MoSSe-1

MoSSe-2

sites

Eb (eV)

q (|e|)

dLi-S (Å)

dLi-Se (Å)

dLi-Mo (Å)

A

1.00

0.383

2.378

2.566

3.050

B

0.90

0.376

2.356

2.544

3.142

C

0.88

0.441

2.387

2.65

3.743

D

0.77

0.436

2.376

2.593

3.942

A

0.87

0.404

2.409

/

3.295

B

0.24

0.367

/

2.526

2.948

C

0.75

0.451

2.417

/

3.943

D

0.12

0.433

/

2.587

3.542

Though we have investigated the Li atoms’ adsorption information of MoSSe, the diffusion of Li ion on the MoSSe should also be probed in detail. The LST/QST method was used to find the minimum energy path (MEP) for the diffusion of Li ions. We set the stable site as the initial site and design 3 different diffusion paths on MoSSe-1: A1-C-B1, A1-D-A2, A1-D-B2, and 2 different diffusion paths on the Janus structure: A1-C-A2, B1-D-B2. We found that there exists same two energy barriers in each diffusion pathways. The energy barriers of MoSSe-1 is 0.41 eV (A1-C-B1), 0.37 eV (A1-D-A2), 0.64 eV (A1-D-B2), and in the MoSSe-2 is 0.28 eV (A1-C-A2), 0.25 eV (B1-D-B2). The best way for Li ion diffusion is A1-D-A2 in MoSSe-1 and B1-D-B2 in the MoSSe-2, both are shown in Fig. 2, the results are similar with the previous researches[24]. The rest of other diffusion pathways of 2 structures are all shown in the Fig. S6-S7.

6

Fig. 2 The best diffusion pathway and the energy barrier of the Li atom diffusing on (a)MoSSe-1,(b)MoSSe-2.

3.3 Adsorption and Diffusion of Li ions on 3 possible MoSSe/graphene heterostructures In the same way, we investigated the 3 possible structures of MoSSe/graphene: SeSC, SC, SeC, as shown in the Fig. 1. The binding energy and the Mülliken charge transferred of each structure are all shown in the Tab. 2, these sorption sites are shown in the Fig. 3. We found that the sequence of the binding energy was same in the same sites of the heterostructure and the MoSSe. So that we found that the combination of the graphene doesn’t affect the stability of the sorption sites. And the adsorption energy of heterostructure is higher than the MoSSe monolayer in the same site, which indicates the combination with the graphene can stabilize the adsorption of Li atom.

Fig. 3 Top and side view of Li atom adsorption sites of(a)SeSC,(b)SC,(c)SeC.

Electronic conductivity is vital to deciding the electrochemical performance of an anode. Therefore, we calculated the electron density of state (DOS) to show the change of the electronic properties of MoSSe/graphene upon the lithium intercalation. As shown in the Fig. 4 total density of states (TDOS) and partial density of state 7

(PDOS) were compared and we can see the differences after lithium intercalation. The band structures of MoSSe-1 and SeC show that the pristine MoSSe-2 is a semiconductor, which limits the electronic conductivity, with the combination of graphene, the good conductivity of graphene is well exerted, and the electron gas around the C atom fills the crystal lattice, conduction band region of SeSC pushes the Fermi level upward. With the intercalation of a lithium atom, the contribution of the Li 2s orbital greatly changed the electronic states near the Fermi level, the conductivity of the system is greatly enhanced. After reaching the maximum capacity, the electron cloud density near the Fermi level is greatly increased, and the system exhibits a very strong metallic property.

Fig. 4 TDOS and PDOS of (a) MoSSe-2, (b) SeC, (c) SeC+Li, and (d)SeC+57Li. The Fermi level is set to zero.

We employed the LST/QST method to study the diffusion of Lithium ion on the MoSSe surface, graphene surface and the interface of MoSSe/graphene. As shown in the tab. 2, in the SeSC, A is the most stable adsorption site and it is suitable to be the initial site of the diffusion pathway on the interface, so we found 3 possible pathways A1-C-B1, A1-D-A2, A1-D-B2. K is the stable adsorption site on the graphene so we set the pathway K2-K3 as the possible diffusion way. G as the stable site on the MoSSe surface and the possible ways are created: E1-G-F1, E1-H-E2, E1-H-F2. We can find that the energy barriers of diffusion pathways on the interlayer are A1-C-B1 (0.65 eV), A1-D-A2 (0.67 eV), A1-D-B2 (0.67 eV), the energy barrier of the graphene surface is K2-K3 (0.5 eV), however, the diffusion energy barriers on the MoSSe surface are E1-G-F1(0.36 eV), E1-H-E2 (0.34 eV), E1-H-F2 (0.62 eV). The results were similar with the previous researches[24]. Tab. 2 Calculated binding energy (Eb), charge transferred (q), and the distance from nearest S, Se, 8

Mo and C atom of a Li atom (dLi−S, dLi−Se, dLi−Mo, dLi-C) at the energetically favorable adsorption sites of the heterostructures SeSC, SC and SeC。 system

SeSC

SC

SeC

sites

Eb (eV)

q (|e|)

dLi-S (Å)

dLi-Se (Å)

dLi-Mo(Å)

dLi-C (Å)

A

1.32

0.283

2.449

2.655

3.299

2.389

B

1.20

0.255

2.398

2.626

3.363

2.363

C

1.24

0.313

2.436

2.792

3.605

2.37

D

1.06

0.25

2.492

2.537

3.78

2.364

E

0.55

0.384

2.369

2.562

3.146

/

F

0.44

0.389

2.361

2.552

3.267

/

G

0.43

0.446

2.39

2.672

3.547

/

H

0.32

0.452

2.398

2.619

3.808

/

K1

0.70

0.28

2.386

2.661

3.776

2.341

K2

0.38

0.624

/

/

6.878

2.337

A

1.42

0.283

2.438

/

3.14

2.391

B

0.37

0.38

/

2.536

3.439

/

C

1.32

0.318

2.465

/

3.676

2.368

D

0.38

0.437

/

2.592

4.015

/

K1

1.31

0.322

2.436

/

3.673

2.375

K2

0.1

0.618

/

2.325

6.735

6.735

A

1.03

0.212

/

2.537

3.473

2.351

B

0.88

0.404

2.395

/

3.037

/

C

0.95

0.241

/

2.615

4.035

2.341

D

0.75

0.468

2.435

/

3.635

/

K1

0.95

0.238

/

2.585

3.979

2.326

K2

0.36

0.634

/

/

7.013

2.336

While in the system of SC and SeC, we found the most stable adsorption site in the interlayer is site A and we designed the diffusion pathway A1-C-A2. In the same way, the diffusion paths on the MoSSe surface and the graphene surface are B1-D-B2 and K2-K3, respectively. The Fig. 5 shows the energy barriers of SC and SeC, we can easily find that the diffusion barriers of the interlayer in the SC are slightly lower than the interlayer in the SeC. The energy barriers of SC and SeC are very close to the diffusion energy barriers of the MoS2[35] and are lower than the pristine graphene and silicene[45, 46]. We can find that the diffusion energy barriers on the interface are higher than the MoSSe surface, this is corresponding with the previous reports[36, 47, 48]. The rest of other diffusion pathways of 3 structures are all shown in the Fig. S8-S10.

9

Fig. 5 The best diffusion pathway and the energy barrier of the Li atom diffusing on (a) SeSC, (b) SC, (c) SeC.

3.4 Voltage profile and theoretical maximum lithium storage capacity It’s vital for us to explore the theoretical maximum lithium storage capacity because the maximum capacity determine whether it can be applied in the LIB or not. The theoretical capacity (C) of lithium atoms are calculated by the following equation: 𝐶=

𝑥𝐹 𝑀substrate

(1)

where x is the number of Li atoms adsorbed in per MoSSe unit, F is the Faraday constant (26800 mAh/mol), and the Msubstrate is the atomic mass of substrate, we found the theoretical maximum lithium storage capacity is determined by the x, so we calculated the intercalation of different number of Lithium atoms in all the 4 × 4 × 1 supercell MoSSe and MoSSe/graphene structures. The open-circuit voltage (OCV) is calculated to investigate the changes in the process of Lithium intercalation. It can be defined as: Ex2 – Ex1 – (x2 - x1)ELi (2) 𝑂𝐶𝑉 = ― (x2 - x1) Where Ex is the total energy of substrate with adsorbed Li atoms, ELi is the energy of per atom of bulk Li, and x represents the number of AM atoms adsorbed in per MoSSe unit. Following the definition, the OCV represent the total energy change during the whole lithium insertion process. We determined the filling sequence by the energy of adsorption sites. The higher energy means the more stable site, so according to the tab. 1, in the MoSSe-1 we set the order of filling sequence as A,C,B,D and in the MoSSe-2 is the A,B,C,D. In the MoSSe/graphene, according to the tab. 2, the filling sequence doesn’t change in SeC and SC, but also in the SeSC, the filling sequence is A,C,B,D,K1,E,F, in the SC is A,C,K1,B, and in the SeC is A,C,K1,B. Fig. 6 illustrated the OCV of 5 structures, we found that MoSSe-1 and MoSSe-2 both have a rather low capacity (259 mAh/g), and in the SeSC, SC, SeC, we can see that they both have a relatively higher capacity (390 mAh/g) in comparison with other structures like MoS2 (335 mAh/g)[47] and graphene (372 mAh/g)[49]. Meanwhile, in Fig. 7 , all the structures are stable after the heavy Li intercalation , and we performed the phonon dispersion calculations of SeC. The result is shown in the Fig. S11, the 10

stable phono modes for the heavy Li intercalation show that the Se was stablized by combined with the graphene. We also found that the distance between layers of SeC were increased from 5.21 Å to 8.10 Å when introduced the lithium atoms, which is correspond with the previous research[36].

Fig. 6 The voltage profile for Li ion intercalation on (a) MoSSe-1, (b) MoSSe-2, (c) SeSC, (d) SC, (e) SeC.

11

Fig. 7 Top and side views of theoretical maximum Li storage in optimized (a) MoSSe-1, (b) MoSSe-2, (c) SeSC, (d) SC, (e) SeC.

4

CONCLUSION

In this article, we investigated the adsorption and diffusion of lithium atoms in the MoSSe and MoSSe_graphene heterostructures by using the first principles method based on vdW corrected density functional theory, and we found that the combination with graphene makes the lithium atoms’ adsorption more stable, meanwhile, the diffusion barriers on the surface are lower than that at the interface, which are comparable to the barriers on the corresponding monolayers. The maximum lithium storage capacity of the heterostructure can be enhanced to 390 mAh/g, and the range of open circuit voltage (OCV) is suitable to be applied as anode materials. Our work made a comparison with all the typical structures of MoSSe and MoSSe_graphene, found that the electronic properities of MoSSe are enhanced after combined with graphene as heterostructures. When we made a comparison with the 3 heterostruecures, the SeSC has a higher energy barrier than the others, so we suggests that the S side of MoSSe-2 combined with graphene (SC) and the Se side of MoSSe-2 combined with graphene (SeC) may be good candidates for Li-ion batteries for their higher lithium capacity and charge/discharge rates. ACKNOWLEDGEMENTS This research was supported by the National Natural Science Foundation of China under No. 51574090. CREDIT AUTHOR STATEMENT 12

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Highlights  We model two different structures come from MoS2(1-x)Se2x which are very popular these years.  The adsorption sites are more satble after forming a heterojunction with graphene.  Energy barriers are declined obviously after forming a heterojunction with graphene.  MoSSe_Graphene heterostructures show good theoretical storage and excellent stability.

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