Electrochemical Lithiation Mechanism of Two-Dimensional Transition

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C: Energy Conversion and Storage; Energy and Charge Transport

Electrochemical Lithiation Mechanism of Two-Dimensional Transition-Metal Dichalcogenide Anode Materials: Intercalation versus Conversion Reactions Tianfeng Zhao, Haibo Shu, Zihong Shen, Huimin Hu, Jun Wang, and Xiaoshuang Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11503 • Publication Date (Web): 10 Jan 2019 Downloaded from http://pubs.acs.org on January 11, 2019

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The Journal of Physical Chemistry

Electrochemical Lithiation Mechanism of Two-Dimensional Transition-Metal Dichalcogenide Anode Materials: Intercalation versus Conversion Reactions Tianfeng Zhao,† Haibo Shu †,‡,* Zihong Shen,† Huimin Hu,† Jun Wang,† and Xiaoshuang Chen‡ † ‡

College of Optical and Electronic Technology, China Jiliang University, 310018 Hangzhou, China,

National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Science, 200083 Shanghai, China

ABSTRACT: The fundamental understanding of electrochemical lithiation mechanism in twodimensional layered transition metal dichalcogenides (TMDs) is essential for the development of highperformance TMD-based anodes for lithium ion batteries (LIBs). Here, we perform systematic densityfunctional theory calculations to reveal the thermodynamic stability and lithiation dynamics of TMD electrode materials. The calculated results show that there exist two different lithiation mechanisms: one is the reversible intercalation reaction mechanism in which TMD electrodes represented by NbS2 and ZrS2 can maintain their layered structures without significant structural distortions in the lithiation process. The other is the irreversible conversion reaction mechanism where the Li intercalation induces a layer-by-layer structural dissociation of TMD electrodes represented by MoS2 and SnS2 into Li2S and metal nanoparticles. Two contrasting lithiation mechanisms are attributed to a delicate competition between the Li-TMD interaction and metal-chalcogen bonding interaction. Furthermore, we develop a general guiding principle to predict the Li-intercalation mechanism of TMD anodes for LIBs.

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1. INTRODUCTION Rechargeable lithium-ion batteries (LIBs) have drawn much attention in the past decades due to their large potential as the most promising energy storage system for a variety of applications, such as electric vehicles and portable electronic devices1-3. The ever-growing demand for LIBs with long lifetime and high energy/power density greatly depends on the improvement of electrochemical performance of electrode materials. However, the specific capacities of conventional electrode materials for LIBs are approaching their theoretical limits.4,5 Therefore, the development of new high energy-density electrode materials is of particular importance. Two-dimensional (2D) transition metal dichalcogenides with the chemical formula MX2 (M = Mo, W, Sn, Ti, V, Pt, etc.; X = S, Se) have emerged as one of the most promising candidate materials for next-generation high-performance LIBs due to their unique structural and electronic properties as well as high theoretical specific capacities6-8. For example, MX2 materials with the layered structure possess relatively large interlayer distance (~6.5 Å) and weak van der Waals (vdWs) interactions among neighboring layers, which allows a fast diffusion of lithium (Li) ions without causing a significant volume expansion9,10. Most of MX2-based electrodes, such as MoS2, MoSe2, WS2, VS2, and SnS2, deliver higher reversible capacities than the theoretical capacity (372 mA hg-1) of commercial graphite anode11-14. Motivated by the huge potential of MX2 materials in LIBs, extensive efforts have been devoted to gaining a deep understanding of electrochemical reaction mechanisms of MX2-based electrodes for the further improvement of their reversible capacities and cycling stability15-18. Unlike the graphite anode with a intercalation-deintercalation mechanism, the lithiation mechanisms of MX2-based anodes in LIBs were generally considered by the following reactions,19,20 MX2 + xLi+ + xe-  LixMX2

(1)

+

(2)

-

LixMX2 + (4−x)Li + (4−x)e  Li2S + M

where x is the composition of Li ions in the lithiated MX2 with the range of 0 ≤ x ≤1. Here eq. 1 describes the intercalation reaction, and eq. 2 shows the conversion reaction. The early studies considered that the intercalation and conversion reactions of MX2 electrodes are reversible,21-23 but more recent reports suggested that the discharge product cannot be converted to the layered MX2 structures in the subsequent charge process, and the most typical examples like MoS2 and SnS224-26. For example, the recent studies26,27 have found that the structures of MoS2- and SnS2-based electrodes are unstable in the lithiation process, and they tend to convert into lithium polysulfides and Li2S. The formed Li2S can be oxidized to element sulfur at a higher charge potential, and consequently Li2S/sulfur becomes the sole redox couple in the subsequent cycling process (i.e., S + 2Li+ + 2e- ↔ Li2S)26. However, why these MX2-electrode materials do not still follow a reversible lithiation-delithiation mechanism but prefer to an irreversible conversion reaction mechanism in the charge-discharge process, which remains unknown. 2 Environment ACS Paragon Plus

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Different from the lithiation mechanism of MX2 (M= Mo, W, Sn; X = S, Se) electrodes, other some TMD materials, such as NbS2 and TiS2, were found to possess ultrahigh stability in the lithiation process28,29. Moreover, the intercalation of Li (or Na) ions into these MX2 electrodes can induce the formation of ternary LixMX2 (or NaxMX2) phase (x ≤1),28-31 which ensures that these MX2 electrode materials can maintain their layered structures after multiple cycling charge-discharge process. It means that the electrochemical reaction mechanism of these MX2 materials is very similar to that of graphite (i.e., MX2 + xLi+ + xe- ↔ LixMX2). Owing to the difference of electrochemical Li storage mechanisms, the reversible capacity of the intercalation-type MX2 electrodes (e.g., ~170 mA hg-1 for NbS2) 28 is far less than that that of conversion-type MX2 electrodes (e.g., ~1100 mA hg-1 for MoS2)32. However, what factor determines the difference of electrochemical reaction mechanisms among the MX2-based electrodes in LIBs? Although the electrochemical properties of various MX2 electrode materials have been widely studied, there is still no a systematic work to clarify this issue. Considering that the key role of lithiation mechanisms in the design of high performance MX2-based LIBs, a comprehensive understanding of inherent factors for determining electrochemical reactions in MX2 is strongly desired. In this work, we address this critical requirement by employing first-principles calculations to reveal underlying inherent factors that determine the Li intercalation chemistry of MX2-based electrodes. We start from the phase stability and structural evolution of Li-intercalated MoS2 and NbS2 to understand the difference of lithiation mechanism in the MX2 electrodes. Then, we reveal that the M-X and Li-MX2 interactions are responsible for the lithiation mechanism and electrochemical performance of MX2 electrodes. Finally, a simple and general guiding principle is proposed to evaluate the lithiation mechanism of MX2 electrodes and its feasibility is further validated. This work not only illuminates the lithiation mechanism of MX2-based anode materials for LIBs, but also provides a reference for the understanding of intercalation chemistry of MX2 materials with other types of alkali ions. 2. COMPUTATIONAL DETAILS All density-functional theory (DFT) calculations were performed using projector augmented wave (PAW) method33 as implemented in the Vienna ab initio Simulation Package (VASP)

34,35

. The

electronic exchange-correlation energy was treated by the generalized-gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE) 36, and the DFT-D2 method of Grime37 was adopted to describe the vdW interactions between the layers. To avoid the interlayer interactions between the slabs, they were separated by a vacuum layer of ~15 Å. The energy cutoff for the plane-wave expansion was set to 450 eV. The k-point sampling in the Brillouin zone was implemented by the Monkhorst-Pack scheme with the grids of 16×16×1, 8×8×1, 4×4×1 and 14×14×6 for the (1×1), (2×2), (4×4) 2D supercells and 3D bulks, respectively. All MX2 structures were relaxed until the total energy and forces acting on each atom were less than 10-5 eV and 10-2 eV/Å, respectively, and the optimized lattice constants were listed 3 Environment ACS Paragon Plus


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in Table S1 of Supporting Information. The climbing-image nudged elastic band (CINEB) method38 was used to determine the phase-transition barriers of lithiated MX2 structures. Bader charge analysis39,40 was carried out to examine the charge transfers among atoms in lithiated MX2 systems. Ab initio molecular dynamics (AIMD) simulations were used to investigate the dynamic phase stability and conversion reactions of Li intercalated MX2 nanosheets, and they were performed in the canonical (NVT) using a running time of 10 ps with a 1.0 fs time step at 300 K. In the calculations, the lithiated MX2 nanosheets were expanded into (4×4) supercells. Owing to the hugely time consuming to run an AIMD simulation for a large lithiated MX2 system (144 atoms), a 2×2×1 k-point mesh was used for the Brillouin-zone integration. The thermodynamic stability of lithiated MX2 nanosheets was studied by the thermodynamic phase diagram. In the calculations, we first assume that LixMX2 phase can be stabilized in the lithiation process, which requires that the chemical potentials of Li, M, and X atoms satisfy the following relation in a thermodynamic equilibrium condition, xLi  M  2 X  H f ( Lix MX 2 )

(3)

where ΔHf(LixMX2) is the formation entropy of LixMX2. Here the x value depends on the layer number of nanosheets. For instance, x is 2 for a fully lithiated MX2 monolayer and 1 for a fully lithiated MX2 bulk, respectively. To avoid the formation of secondary Li2X and MX2 phases as well as the formation of Li, M, and X crystals, the chemical potentials also satisfy the following constraints, 2Li   X  H ( Li2 X )

(4)

M  2 X  H (MX 2 )

(5)

Li  0, M  0,  X  0

(6)

where ΔHf(Li2X) and ΔHf(MX2) are the formation entropy of Li2X and MX2, respectively. The details of thermodynamic phase-diagram calculations can be found in S2 of the Supporting Information. The interactions between Li and MX2 nanosheets were evaluating by calculating Li binding energies (Eb) as follows, Eb  ( ELiMX 2  EMX 2  nELi ) / n

(7)

where ELi-MX2, EMX2, and ELi are the total energy of lithiated MX2 nanosheet, pristine MX2 nanosheet, and Li atom, respectively. 3. RESULTS AND DISCUSSION The formation of stable LixMX2 (x≤2) ternary phase is a key issue to evaluate the lithiation mechanism of MX2 electrodes. For example, the intercalation of Li ions into the conversion-type MX2 materials (e.g., MoS2) cannot induce the formation of highly stable LixMX2 phase. Meanwhile, the intercalation of 4 Environment ACS Paragon Plus

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Li ions also can cause the structural change of some MX2 materials. A typical example is that a 2H-to1T phase transition in MoS2 can be driven by the intercalation of Li ions17,18. Therefore, we first address the phase stability of MX2 electrode materials before and after the intercalation of Li ions by using MoS2 and NbS2 as the prototype examples. Figure 1a-c shows atomic structures of three different crystal phases of MoS2 and NbS2. It can be found that 2H phase possesses trigonal prismatic coordination for metal atoms with an AB stacking sequence (Figure 1a), and 1T phase has octahedral coordination for metal atoms with one layer per repeated unit (Figure 1b). For the 3R phase, it is a non-centrosymmetric structure in bulk with an ABC stacking sequence (Figure 1c). The calculated formation entropies (ΔHf) of MoS2 and NbS2 bulk with three different crystal phases are displayed in Figure 1d. The 2H phase of both MoS2 and NbS2 presents the largest ΔHf, which means that 2H phase of two MX2 materials is more stable than their 1T and 3R phases. Nevertheless, the energy difference between 2H and 3R phases in two materials is very small. This may be why the 3R phase can be also synthesized in experiment41. (a)

2H Phase

(b)

1T Phase

(c)

3R Phase

(d)

4.5

NbS2

Hf (eV)

4.0

MoS2

3.5 3.0 2.5 2.0

(e)

1T

2H

3R

1T

2H

3R

7

LiNbS2 LiMoS2

6

Hf (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


5

4

3

1T

2H

3R

1T

2H

3R

Figure 1 Side and top views of atomic structures of MX2 with (a) 2H phase, (b) 1T phase, and (c) 3R phase. Blue and yellow spheres correspond to transition metal M and chalcogen atom X, respectively. The dash lines indicate the range of unit cells. (d) The formation entropies (ΔHf) of pristine NbS2 and MoS2 bulk with three different crystal phases (i.e., 1T, 2H, and 3R). (e) The formation entropies of fully lithiated NbS2 and MoS2 (i.e., LiNbS2 and LiMoS2) with three different crystal phases.

The intercalation of Li ions leads to the structural transition of MX2 nanosheets into lithiated ones. The transition-state (TS) calculations indicate that the diffusion barrier of Li ion on the surface of MoS2 and NbS2 is 0.26 eV and 0.29 eV (Figure S1a), and the energy barrier of Li diffusion into the interlayer of MoS2 and NbS2 is 0.24 eV and 0.38 eV (Figure S1b), respectively. The low diffusion barriers imply that the lithiated MoS2 and NbS2 structures can be achieved by the intercalation of Li ions. We find that the phase stability of fully lithiated MoS2 and NbS2 structures (i.e., LiMoS2 and LiNbS2) is different from that of pristine ones. The intercalation of Li ions makes that the 3R-phase NbS2 and 1T-phase 5 Environment ACS Paragon Plus


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MoS2 become the most stable structure (Figure 1e). Moreover, the TS calculations show that the activation barrier for driving 2H-to-1T phase transition in MoS2 and 2H-to-3R phase transition in NbS2 is 0.18 eV and 0.12 eV, respectively. Therefore, the intercalation of Li ions can induce the formation of 1T-MoS2 and 3R-NbS2, which agrees well with a large number of experimental observations17,28,42. Furthermore, ΔHf of LiNbS2 is about 2 eV larger than that of NbS2. It means that the formation of LiNbS2 phase is an extremely exothermic process, suggesting that the intercalation of Li ions contributes to the stability of LixNbS2 phase. In contrast, there is small energy reduction (~0.7 eV) for the Li-insertion into MoS2 nanosheets, resulting in the formed 2D LixMoS2 phases with the smaller ΔHf (< 4.29 eV) (see Table S2). Furthermore, the energy of 2D LixMoS2 phases is correspondingly higher than the sum of the energy of Li2S and Mo. Therefore, the intercalation of Li ions may be insufficient to stabilize LixMoS2 phase in the lithiation process.

Figure 2 Thermodynamic phase diagrams of (a)-(d) lithiated MoS2 and (e)-(h) lithiated NbS2 structures. Both lithiated MoS2 and NbS2 structures include monolayer, bilayer, tetralayer, and bulk, respectively.

Although the lithiated 1T-MoS2 and 3R-NbS2 structures are energetically favorable, this does not mean that the ternary phases can be stabilized in the discharge process. To identify the phase stability of lithiated MoS2 and NbS2 nanosheets, thermodynamic phase diagrams have been calculated by eq. (3)(6). The details of phase-diagram calculations can be found in S2 of the Supporting Information. Figure 2 displays thermodynamic phase diagrams of lithiated MoS2 and NbS2. For lithiated MoS2 nanosheets (Figure 2a-c), there is no stable phase region for the formation of LixMoS2 in the allowed range of chemical potentials. Largely, varying the chemical potential from the Li-rich to S-rich condition leads to 6 Environment ACS Paragon Plus

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the phase transition from Li2S, Li2S+MoS2 mixed phase, to MoS2 phase. The result suggests that MoS2 nanosheets are unstable in the lithiation process and they prefers to the structural conversion into Li 2S and Mo nanoparticles, especially for the Li-rich condition. Meanwhile, we find that the mixed phase region shrinks with the increase of layer number. When the MoS2 structures change from 2D nanosheets to 3D bulk, a stable LiMoS2 phase appears (Figure 2d). Owing to the lack of surface layers in MoS2 bulk, Li-S species can be suppressed in the lithiation process, which is responsible for the formation of stable LiMoS2 phase. The corresponding mechanism will be discussed later. Different from the case of MoS2, the intercalation of Li ions into NbS2 nanosheets can lead to the formation of stable LixNbS2 ternary phase (Figure 2e-h). Moreover, the ternary phase region gradually expands with increasing layer number, suggesting the layer-dependent ternary phase region. Therefore, the LixNbS2 phase is more facile to be obtained in the thicker NbS2 nanosheets in the discharge process of LIBs. The result originates from two aspects: (i) the increase of layer number can reduce the ratio of Li ions in the lithiated nanosheets. (ii) The increase of layer number decreases the surface-to-volume ratio, which contributes to the suppression of the Li2S formation. From the thermodynamic viewpoint, there is a large possibility to induce the formation of stable LixNbS2 phase in the lithiation process, but the Li intercalation may cause the structural dissociation of MoS2 nanosheets.

0 ps

3 ps

6 ps LixSy

(a)

(b)

Li

S

S

Mo

Nb

Figure 3 Snapshots of trajectories for lithiated (a) MoS2 and (b) NbS2 nanosheets following 6ps AIMD simulation at 300 K. The circle regions show the formation of LixSy species in the lithiation process.

In addition to the thermodynamic limit, the lithium dynamics of MX2 electrode materials should be also considered. The reason is that the conversion reactions usually involve the release of S atoms from MX2 nanosheets and the formation of Li-S bonds. The process needs to overcome a series of reaction barriers. If these barriers are very high, the conversion reactions will be difficult to process even if the formation of Li2S phase is thermodynamically favorable. In order to explore the lithium dynamics of MoS2 and NbS2 nanosheets, AIMD simulations were performed at 300 K within 10 ps. Figure 3a shows 7 Environment ACS Paragon Plus


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a few snapshots of trajectories for a fully lithiated MoS2 bilayer nanosheet. The continuous precipitation of S atoms from the nanosheet surfaces leads to the formation of LixSy species within 6 ps. Moreover, all precipitated S atoms initiate from the surface layers of MoS2 nanosheet, and there is no S atoms released from the MoS2 interlayer. Therefore, the Li intercalation triggers a layer-by-layer dissociation of MoS2 nanosheets, which is supported by the recent experimental observation43. To facilitate our understanding, a real-time movie for the lithiated MoS2 nanosheet is presented in Movie S1 of Supporting Information. Owing to high chemical activity, the precipitated S atoms can trap more Li ions relative to the pristine MoS2 nanosheets, which causes the conversion of MoS2 into lithium polysulfides and Li2S (i.e. MoS2 + 4Li+ + 4e-  Mo +Li2S). Such a process is responsible for high first discharge capacities (~1100 mA hg1

) of MoS2-based electrode materials from previous reports22,32. In contrast, there is no any structural

destruction and the formation of Li-S species in the lithiated NbS2 nanosheet (see Figure 3b and Movie S2). Interestingly, the intercalation of Li ions leads to a relative sliding of NbS2 layers (Figure S2), which drives a phase transition of NbS2 from the 2H to 3R phase. The result agrees well with the phasestability analysis of LiNbS2 mentioned above. The high stability of NbS2 nanosheets also makes NbS2based electrodes with lower reversible capacities, which originates from that the number of intercalated Li ions completely depends on the interlayer adsorption sites and strength of Li-NbS2 interactions. For example, a fully lithiated NbS2 bulk (i.e., LiNbS2) only possess a theoretical capacity of ~174 mA hg-1. The above results suggest that the lithiation mechanism of MoS2 and NbS2 is totally different. The continuous intercalation of Li ions causes the structural instability of MoS2 nanosheets that induces a layer-by-layer structural dissociation into Li2S and Mo, so the electrochemical lithiation of MoS2 is irreversible. The Li intercalation into NbS2 nanosheets can lead to the formation of stable LixNbS2 phase, which ensures that the intercalation reaction is reversible by the delithiation of Li xNbS2 into NbS2. However, what determines the difference of lithiation mechanism between MoS 2 and NbS2 nanosheets? To answer this question, we firstly calculate the electronic structures of two MX2 systems. Figure 4 presents the band structures of 2H-phase MoS2 and NbS2. No matter MoS2 nanosheets or bulk are semiconductors, and the band gap of MoS2 gradually decreases with the structural change from the monolayer to bulk (Figure 4a-d). Moreover, there is an indirect-to-direct band gap transition with the thickness of MoS2 down to the monolayer, which is in good agreement with previous theoretical and experimental investigations

44,45

. On the contrary, all four NbS2 structures exhibit metallic character

(Figure 4e-h). Considering that the 3R-phase NbS2 can be induced by the intercalation of Li ions, the band structures of 3R-phase NbS2 have been also computed (Figure S3). Similar to the case of 2H phase, all 3R-NbS2 structures are also metallic. The metallic NbS2 nanosheets display larger charge transfer from metal atoms to S atoms than semiconducting MoS2 ones, resulting in the larger electronegativity of S element in NbS2 nanosheets. For example, the calculated Bader charges of Li atoms (ΔQLi) in four 8 Environment ACS Paragon Plus

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lithiated NbS2 structures are correspondingly larger than that of the lithiated MoS2 ones by 0.1 ~0.3 e (Figure 5a). The increase of electronegativity of S element can lead to the stronger Li-S interaction with the intercalation of Li ions into the NbS2 nanosheets, which can be verified by Li binding energies on two MX2 systems (Figure S4). The calculated Li binding energies (Eb) of four lithiated NbS2 structures are about 0.5~1.5 eV larger than those of lithiated MoS2 ones. Therefore, the strong Li-MX2 interaction is essential for the formation of stable LixMX2 phase.

Figure 4 Band structures of MoS2 and NbS2 nanosheets and bulk. (a)-(d) The band structures of 2H-MoS2 monolayer, bilayer, tetralayer, and bulk. The red and green squares represent the position of conduction band minimum (CBM) and valence band maximum (VBM), respectively. (e)-(h) The band structures of 2H-NbS2 monolayer, bilayer, tetralayer, and bulk. The metallic bands are labelled in red. The dash lines denote the position of Fermi level.

To maintain the intercalation reaction of MX2 structures in the discharge process, only the formation of strong Li-MX2 interaction is not enough. This is because the strong Li-X interaction may cause the precipitation of chalcogen atoms from MX2 surfaces if the M-X bonding interaction is not very strong, consequently resulting in the structural dissociation of MX2 into Li2X and metal nanoparticles. Therefore, the strong M-X bonding interaction is also important for preventing from the precipitation of chalcogen atoms. It is seen from Figure 5b and 5c that ΔQNb and ΔQS of lithiated NbS2 structures are correspondingly larger than ΔQMo and ΔQS of lithiated MoS2 ones. This means that the strength of Nb-S bonds is stronger than that of Mo-S bonds, which can be supported by the result of formation entropies (Figure S5). With the structural change from the monolayer to bulk, the formation entropies (ΔHf) locate at the range of 3.10~3.85 eV for the NbS2 structures and 1.62~3.33 eV for the MoS2 structures, respectively. The large Li binding energies is responsible for the strong Nb-S bonding interaction in NbS2, which ensures that the high stability of NbS2 nanosheets in the lithiation proccess. 9 Environment ACS Paragon Plus


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In addition, we observe that Eb and ΔHf values of both NbS2 and MoS2 structures decrease with the reduction of nanosheet thickness (Figure S4 and Figure S5). The result suggests that the decrease of layer number can weaken both the Li-MX2 and M-X interactions. This is why the stable LixMX2 phase region gradually shrinks with the MX2 structures from bulk to monolayer (see Figure 2).

Figure 5 Bader charges of (a) Li (ΔQLi), (b) metal Mo and Nb (ΔQMetal), and (c) S atoms (ΔQS) in lithiated MoS2 and NbS2 with the structure from monolayer to bulk. The circles and squares denote the data of LixMoS2 and LixNbS2, respectively.

Based on above analysis, a large formation entropy ΔHf means that a MX2 structure has a strong M-X bonding interaction that can prevent from the structural dissociation of MX2 nanosheets in the lithiation process, and a large Li binding energy Eb implies a strong Li-MX2 interaction that contributes to the stability of lithiated MX2 system. Hence, the lithiation mechanism of MX2 electrode materials actually depends on the competition between Li-MX2 interaction and M-X bonding interaction, and we can use ΔHf and Eb as the descriptor to predict the lithiation mechanism. Figure 6 shows formation entropies and Li binding energies of 20 MX2 structures including 10 bulks and 10 bilayers. For the conventional TMDs, such as MoS2, MoSe2, WS2, and WSe2, they have the medium ΔHf and smaller Eb. Therefore, the intercalation of Li ions into these electrode materials is difficult to induce the formation of stable LixMX2 phase. Although some in situ experiments have observed the formation of LixMX2 phases (e.g., LixMoS2),17,42,46 they are only the metastable states that cannot be stabilized after multiply charging/discharging cycles. For some 1T-TMDs such as VS2, PtS2, and SnS2, they present the larger Eb 10 Environment ACS Paragon Plus

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but smaller ΔHf, so their layered structures are more difficult to be maintained in the lithiation process and they prefer to the structural conversion into Li2S and metal nanoparticles. Therefore, the electrochemical lithiation reaction is also irreversible for the type of MX2 electrodes. The irreversible conversion reaction makes that the TMD-based electrode materials possess high Li storage capacities but suffer from weak cycling stability. To improve the cycling stability of these TMD-based electrodes, the key is to suppress the rapid surface dissociation. This scheme can be implemented by the creation of TMD-based heterointerfaces, which has been widely demonstrated in previous studies23,32,43. In contrast, the TMDs represented by ZrS2, TiS2, and NbS2 possess larger Eb and ΔHf that are responsible for the formation of stable LixMX2 phase in the lithiation process. Hence, the reversible intercalation reaction mechanism is preferred for the type of MX2 electrodes. It is noteworthy that there is no MX2 material with the large ΔHf but small Eb (yellow shadow region in Figure 6). In principle, the type of MX2 materials also prefer to the reversible intercalation mechanism. Owing to the weak interaction between Li ions and MX2 sheets, the Li intercalation into these MX2 structures should be difficult to form highly stable LixMX2 phase, resulting in lower Li storage capacities. Therefore, they are not ideal electrode materials for LIBs. On the other hand, we find that Eb and ΔHf values of all MX2 materials decrease with their structures from bulk to bilayer. The result suggests that the reduction of layer number will weaken the structural stability of MX2 nanosheets in the lithiation process.

Strong

Reversible intercalation reaction ZrS2

5 TiS2

Bulk Bilayer

ZrS2

NbS2

VS2

3

Strong

TiS2

NbS2

4

VS2

PtS2

2

Weak

Li-MX2 interaction

SnS2

MoS2 MoSe2

PtS2

M-X bonding

6

Hf (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


WS2

WS2 MoS2 WSe2 WSe2 MoSe2

SnS2

Irreversible conversion reaction 1

Weak -4.0

-3.5

-3.0

-2.5

-2.0

-1.5

Binding energy per Li atom Eb (eV) Figure 6 Formation entropies and Li binding energies as the descriptor for evaluating the Li-intercalation mechanism of MX2 electrode materials for LIBs. The spheres and squares denote the data of MX2 bulk and bilayers, respectively. The shadow and unfilled regions represent the lithiation of MX2 structures follow the reversible intercalation reaction mechanism and irreversible conversion reaction mechanism, respectively.



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In order to examine the feasibility for the use of descriptors Eb and ΔHf to predict the lithiation mechanism of MX2 electrodes, the lithiation dynamics of SnS2 and ZrS2 nanosheets are investigated by AIMD simulations. Figure 7 presents a few snapshots of trajectories for lithiated SnS2 and ZrS2 bilayer nanosheets at 300 K. It is found that the intercalation of Li ions induces a rapid dissociation of SnS2 nanosheet after 2 ps (see Figure 7a and Movie S3). This process is accompanied by the continuous precipitation of S atoms and the formation of lithium polysulfides, which can be largely described by xSnS2 + 4Li+ + 4e-  xSn + 2Li2Sx. Similar to the case of MoS2, the lithiation of SnS2 nanosheet also obeys a layer-by-layer dissociation mechanism. The ordered layered structure of lithiated SnS 2 gradually is converted into a disorder phase after the simulation time of 6 ps. The result suggests that the electrochemical lithium intercalation of SnS2 belongs to the irreversible conversion reaction mechanism, which is in good agreement with the prediction mentioned above. In contrast, the lithiated ZrS 2 nanosheet can still maintain its 1T-phase structure except for the slight structural distortion within a reaction time of 6 ps (see Figure 7b and Movie S4). Even if the reaction time reaches 10 ps, the 1Tphase structure of ZrS2 can be also maintained well. The large ΔHf and strong Li binding with ZrS2 layers are responsible for the lithiation of ZrS2 into the stable LixZrS2 phase and delithiation of LixZrS2 into ZrS2 in the discharge/charge process. Hence, the electrochemical lithiation of ZrS2 materials prefers to the reversible intercalation reaction mechanism.

0 ps

2 ps

6 ps

(a)

(b)

Li

S

Sn

Zr

Figure 7 Snapshots of trajectories for lithiated (a) SnS2 and (b) ZrS2 nanosheets following 6ps AIMD simulation at 300 K. The purple, yellow, green, and brick red spheres denote Li, S, Sn, and Zr atoms, respectively.

4. CONCLUSIONS In summary, we have performed systematic DFT calculations and AIMD simulations to gain deep insight into the electrochemical Li-intercalation mechanism of a series of 2D MX2 electrode materials for LIBs. Our results demonstrate that the MX2 electrode materials represented by MoS2 and SnS2 are 12 Environment ACS Paragon Plus

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unstable and prefer to a layer-by-layer structural dissociation into lithium polysulfide and Li2S in the lithiation process, while the MX2 electrode materials represented by NbS2 and ZrS2 are highly stable and they can maintain their layered structures without significant structural distortions with the intercalation of Li ions. Moreover, the phase stability of all lithiated MX2 structures strongly depends on the layer number, which results in their structural instability with the reduction of nanosheet thickness. Two contrasting lithiation mechanisms and layer-dependent phase stability are attributed to the competition between the M-X bonding interaction and Li-MX2 interaction, which is developed as a general guiding principle to predict the electrochemical lithiation mechanism of MX2 electrode materials. In addition, the present work can be also expanded to understand the intercalation chemistry of MX2 materials with other types of alkali ions.  ASSOCIATED CONTENT  Supporting Information Structural and energetic parameters of MX2 materials, details of thermodynamic phase diagram calculations, diffusion pathways and barriers of Li ions on MoS2 and NbS2 nanohseets, structural evolution of lithiated NbS2 nanosheet, band structures of 3R-NbS2 nanosheets and bulk, Eb values of LixMoS2 and LixNbS2 structures, ΔHf of MoS2 and NbS2 nanosheets and bulk, and movies for recoding AIMD simulation results. This material is available free of charge via the internet at http://pubs.acs.org.  AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Haibo Shu), Phone: 86-0571-86875622 Notes The authors declare no competing financial interest.  ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (Grant no. 61775201 and 11404309), the Fund of Shanghai Science and Technology Foundation (Grant no. 13JC1408800), and Program of Xinmiao Talents in Zhejiang Province (Grant no. 2017R409011). Computational resources from Shanghai Supercomputer Center are acknowledged. REFERENCES AND NOTES 13 Environment ACS Paragon Plus


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TOC Graphic Intercalation reaction

Conversion reaction

versus

Li2S Metal

Reversible

Irreversible

Li ions

2D Transition Metal Dichalcogenides


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Electrochemical Lithiation Mechanism of Two-Dimensional Transition

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