MS2 (M = Nb, Ta) Heterostructures As Promising

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Blue phosphorene/MS2 (M=Nb, Ta) heterostructures as promising flexible anodes for lithium-ion batteries Qiong Peng, Zhenyu Wang, Baisheng Sa, Bo Wu, and Zhimei Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03368 • Publication Date (Web): 11 May 2016 Downloaded from http://pubs.acs.org on May 16, 2016

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ACS Applied Materials & Interfaces

Blue phosphorene/MS2 (M=Nb, Ta) heterostructures as promising flexible anodes for lithium-ion batteries Qiong Peng1, Zhenyu Wang1, Baisheng Sa1,*, Bo Wu1,**, Zhimei Sun2 1

Multiscale Computational Materials Facility, College of Materials Science and Engineering, Fuzhou University, and Key Laboratory of Eco-materials Advanced Technology (Fuzhou University), Fujian Province University, Fuzhou 350100, P. R. China

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School of Materials Science and Engineering, and Center for Integrated Computational Materials Engineering,

International Research Institute for Multidisciplinary Science, Beihang University, Beijing 100191, P. R. China

Abstract The idea of forming van der Waals (vdW) heterostructures by integrating various two-dimensional materials breaks the limitation of the restricted properties of single material systems. In this work, the electronic structure modulation, stability, entire stress response and the Li adsorption properties of heterostructures by combining blue phosphorene (BlueP) and MS2 (M=Nb, Ta) together were systematically investigated using first-principles calculations based on vdW corrected density functional theory. We revealed that BlueP/MS2 vdW heterostructures possess good structural stability with negative formation energy, enhanced electrical conductivity, improved mechanical flexibility (ultimate strain >17%) and high-capacity (528.257 mAhg-1 for BlueP/NbS2). The results suggest that BlueP/NbS2 and BlueP/TaS2 heterostructures are ideal candidates used as promising flexible electrode for high recycling rate and portable lithium-ion batteries, which satisfy the requirement of next-generation flexible energy storage and conversion devices. Keywords: Van der Waals heterostructures; Electronic structures; Li adsorption properties; Flexible anodes; First-principles calculations *Corresponding author: [email protected] **Corresponding author: [email protected]

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1 Introduction Flexible electronic systems are physically and mechanically compatible with applications requiring folding (such as foldable displays) or form factors that require bending and, in some cases, stretching.1 The burgeoning field also includes hybrid flexible systems that comprise specially thinned silicon chips (which are flexible) together with other components made with inherently bendable/stretchable materials.2 Flexible energy conversion and storage devices possess great potential in practical applications of portable and wearable electronic devices, such as roll-up displays, electronic paper, mobile phones, and computers.1,3 On the other hand, lithium-ion batteries (LIBs) have extensive applications due to the high reversible capacity, high energy density, and good cycle life.4 In the past decades, graphite was used as anode material for LIBs widely, due to its high energy stability, cycling stability, and low cost.5 However, the relatively low capacity (372 mAhg−1) and weak Li adsorption strength restrict its further applications.6 To date, the rapid development of the electronic market reveals a breakthrough in terms of electrode materials.7 Drawing inspiration from graphene which become experimentally accessible,8,9 other graphene-like 2D materials such as transition metal dichalcogenides (TMDs)10 and transition metal carbides (MXenes)11 have also been widely studied, and some 2D materials exhibit high specific capacity and superior rate capability as anode materials for LIBs.1 Nevertheless, semiconducting TMDs, such as MoS2 and WS2 , suffer from poor rate behavior and rapid capacity fading resulting from its low electrical conductivity and substantial volume change during charge/discharge.12,13 As a typical member of layered metallic TMDs, 2H niobium disulfide (2H-NbS2) was confirmed to be the most stable ones in contrast to the 1T and 3R.14,15 Recently, a series of Li intercalated compounds LixNbS2 have been successfully synthesized using alkali metal naphthalide

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solution at room temperature.15 More importantly, NbS2 remains the same structure before and after the alkali metal adsorption, which is beneficial for long cycle lives in secondary high-power energy storage applications as promising anode material.14-16 In terms of cell voltage and capacity, however, NbS2 anode in sodium ion batteries (SIBs) has only a moderate theoretical voltage (0.95 V) and relatively low gravimetric capacity (264 mAg-1).16 In this regard, in order to improve the structural stability, mechanical flexibility, electrochemical properties and high capacity of electrode materials,1 nanotechnology has made progress in designing novel materials. The heterostructure between the layered structures is an effective way to construct devices that may enhance the properties of their isolated components.17 Despite a lot of work has mainly focused on graphene-based, BN-based and MoS2-based heterostructures as anode materials for LIBs,18-22 many other candidates are still rarely touched which may have good flexibility, electrical conductivity, high specific capacity, and excellent structural stability, such as NbS2 and TaS2, the rest of the same group of MoS2 transition metal dichalcogenides. Recently, phosphorus (P) was found to be electrochemically active and could be used as a LIBs anode.23 Soon after the experimental discovery of 2D black phosphorene, Zhu et al.24 proposed another polymorphous of phosphorus with a thickness of two atoms theoretically, namely, blue phosphorene (BlueP). The 2D semiconducting BlueP is nearly as thermally stable as monolayer black phosphorene with distinct properties.24,25 The single-layer structures of BlueP and 2H-MS2 (M=Nb, Ta) sharing hexagonal crystal structure with only around 2% lattice mismatch, are good counterparts for establishing high-quality heterostructures.14,24 Both single-layers BlueP and MS2 have a large surface-volume ratio; moreover, they form a puckered surface, which provides more space for lithium storage. Recent mechanical experiments show that some 2D TMDs monolayers can

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sustain as large as ~10% elastic strain resisting to inelastic relaxation and fracture.26,27 Therefore, in this work, BlueP/MS2 vdW heterostructures were explored in order to find their potential application as possible flexible anode materials for LIBs. The electronic structure modulation, stability, entire stress response and the Li adsorption properties of BlueP/MS2 vdW heterostructures were investigated systematically using first-principles calculations based on vdW corrected density functional theory. 2 Computational methods Our calculations were performed based on the density functional theory (DFT) in conjunction with the projector-augmented-wave (PAW) potential, as implemented in the Vienna ab initio Simulation Package (VASP).28,29 For the exchange-correlation energy, the Perdue-Burke-Ernzerhof (PBE) version of the generalized gradient approximation (GGA) was used.30 The van der Waals density functional (vdW-DF) of optB86b were considered for all simulations.31,32 The valence electron configurations for Li, P, S, Nb and Ta were 2s1, 3s23p3, 3s23p4, 4d45s1 and 5d36s2, respectively. A plane wave cutoff energy of 600 eV was used for the plane-wave expansion of the wave function. The atomic positions were fully relaxed until satisfying an energy convergence of 10-5 eV and force convergence of 10-2 eV/Å. A vacuum of 20 Å between the layers was considered to safely avoid the interaction between the periodically repeated structures. The charge distribution and transfer were studied quantitatively by Bader charge analysis.33 Similar to the 2D hexagonal lattice of graphene,34 two high-symmetry directions were identified in the crystal structures, namely, zigzag and armchair directions. Since the mechanical behavior of a defect-free material is ultimately controlled by the strength of its chemical bonds, the stress response of the 2D materials along the zigzag and armchair directions is different. Therefore, we chose an

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orthogonal supercell to calculate the stress-strain curves along the zigzag and armchair directions, which correspond to x and y directions, respectively. Starting from the relaxed structures without external pressure, applying a series of strain, a stress-strain curve was obtained, where the tensile strain up to 25% was applied in either the x (zigzag) or y (armchair) direction to explore their ultimate strength (the highest achievable strength of a defect-free crystal at 0 K)34 and ultimate strain (the strain at which ultimate strength reaches). The tensile strain is defined as27

ε =(a−a0)/ a0

(1)

where a and a0 are the lattice constants of the strained and relaxed structures, respectively. With each uniaxial strain applied, the lattice constant in the transverse direction was fully relaxed through the technique of energy minimization to ensure the force in the transverse direction is a minimum. To avoid the force being averaged over the entire simulation cell, including the vacuum space, we rescaled the stress by Z/d0 to obtain the equivalent stress, where Z is the cell length in the z direction and d0 is the effective thickness of the system.35,36 For example, d0 takes the interlayer spacing 5.975 Å for the monolayer NbS2, i.e., one half of the out-of-plane lattice constant of NbS2.37 Such computational methods have been applied successfully to investigate the mechanical properties of a range of 2D materials.27,35,36 Before exploring the diverse properties of the BlueP/MS2 vdW heterostructures, we explored firstly the most stable stacking pattern for BlueP on MS2. Six typical stack configurations were considered here for BlueP/MS2 vdW heterostructures, as illustrated in Figure S1, where the rotation angles of BlueP with respect to MS2 are 0°, 60°, 120°, 180°, 240°, and 300°, respectively. All systems are geometrically optimized to get the stable atomic configuration. In order to investigate the thermodynamic stability of the most stable configuration for each heterostructure, the interface

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adhesion formation energy was defined as the following equation:38 total total total Eform = EBlueP/ MS2 − EBlueP − EMS2

(2)

total total where EBlueP/ is the total energy of the BlueP/MS2 vdW heterostructures. EBlueP and EMtotal S2 MS 2

represent the total energies of pristine BlueP and MS2 monolayers, respectively. In addition, we introduced the heterostructure binding energy to evaluate the strength of the vdW force in the BlueP/MS2 vdW heterostructures according to the following equation: total total Ebinding = EBlueP/ MS2 − EMS2 + BlueP

(3)

where EMtotal S2 + BlueP is the sum of the total energy of mutually independent single-layered BlueP and MS2 fixed in the corresponding heterostructure lattice. For Li intercalation into BlueP/MS2 vdW heterostructures, the theoretical gravimetric capacity,

C , was determined from39 C=

nF M BlueP/ MS2 + nM Li

(4)

where n is the number of adsorbed Li adatoms, F is the Faraday constant (26801 mAh/mol),

M BlueP/ MS2 and M Li is the mole weight of BlueP/MS2 vdW heterostructures and Li adatom, respectively. The cell voltage was computed using a well-established approach36 according to the following formula, where a positive voltage indicates energetically favorable intercalation V = −( EBlueP/ MS2 +Li n − EBlueP / MS2 − n µ Li ) / n

(5)

where EBlueP/ MS2 +Li n is the total energy of BlueP/MS2 vdW heterostructures with n Li atoms intercalation, EBlueP/ MS2 denotes the total energy of pristine BlueP/MS2 vdW heterostructures, and

µLi is the chemical potential of the intercalating species. The adsorption energies of Li intercalation into BlueP/MS2 vdW heterostructures was calculated with39 Ead = ( EBlueP / MS2 +Lin − EBlueP/ MS2 − nELi ) / n 6


(6)

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where ELi is the energy of an isolated Li atom. According the present definition, a higher value of

Ead means stronger binding of Li to the adjacent layer. 3 Results and discussions 3.1. Geometry and stability The optimized lattice constants of BlueP, NbS2 and TaS2 are 3.268, 3.334, and 3.320 Å, respectively, which are in excellent agreement with the available literatures,14,24,40,41 as summarized in Table S1. The lattice mismatch between BlueP and NbS2 or TaS2 monolayers is only 2.02% and 1.59%, respectively, which is compatible for constructing BlueP/MS2 heterostructures. Six selective stacking patterns with different rotation angles between the adjacent sheets for BlueP/MS2 heterostructures are illustrated in Figure S1. All configurations were subjected to structure relax with convergence criteria in terms of both energy and force. Table S2 lists the calculated energy difference between the total energy of various stacking configurations and the most stable one, the interlayer distance between BlueP and MS2 layers, as well as the M-S and P-P bond lengths in the BlueP/MS2 heterostructures. The energy difference ∆E is evaluated by the following equation: ∆Ei = Ei − E0

(7)

where E0 is the total energy of the most stable configuration (see Figure 1), which corresponds to configuration b for BlueP/NbS2 and BlueP/TaS2 heterostructures in present study, and Ei is the total energy of each configuration. As seen in Table S1 and S2, the bond lengths change less, showing the very small atom rearrangements in the heterostructures. The calculated interface formation energies for BlueP/NbS2 and BlueP/TaS2 heterostructures are -277.86 and -260.48 meV, respectively. The negative formation energies and excellent lattice match indicate that these heterostructures are stable and preparable. In addition, the calculated binding energy between BlueP and MS2 monolayers is

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calculated to be 25.35 and 24.06 meV/Å2 for BlueP/NbS2 and BlueP/TaS2 heterostructures, respectively, which are close to the typical vdW binding energy of around 20 meV/Å2 obtained by the advanced DFT calculations.38,42 Thus, the BlueP/MS2 heterostructures belong to vdW heterostructures. 3.2. Electronic properties

The corrected optB86b-vdW DFT calculations were performed to calculate the electronic band structures for monolayers and heterostructures, and the corresponding results are illustrated in Figure S2 and Figure 2. The single-layer of BlueP is an indirect band gap semiconductor with the CBM located between the Γ and Μ points while the VBM located between the Γ and Κ points, as presented in Figure S2b. The magnitude of the band gap of BlueP is 2.02 eV using optB86b-vdW, which is in an excellent agreement with literature values using GGA and DFT-D2 methods with 1.94 and 2.00 eV, respectively.24,40 The band structures of NbS2 and TaS2 monolayers are shown in Figure S2c-d, where one band across the Fermi level, revealing their metallic nature, which are consistent with previous experimental and theoretic results.14,43 And their Fermi levels originate mainly from the d orbitals of transition-metal atoms. Further analysis the electronic band structures of hybrid system, as can be seen in Figure 2a-b, the electronic structures of BlueP/MS2 vdW heterostructures mimic both the BlueP and the MS2. The hybrid BlueP/NbS2 and BlueP/TaS2 heterostructures are identified as metallic character and their Fermi levels are also mainly contributed by the d orbitals of transition-metal atoms. Interestingly, due to the influence of BlueP in MS2 layer, the number of electronic bands (blue circles) near Fermi level increase significantly. The metallicity of BlueP/MS2 vdW heterostructures provides an intrinsic advantage in electrical conductivity as compared to semiconducting or insulating transition-metal oxides and TMDs.

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The total density of state (DOS) and the orbital-resolved project DOS of the MS2 monolayers and BlueP/MS2 vdW heterostructures are depicted in Figure 3a-d. Their Fermi levels locate at the shoulders of the peaks of total DOS, suggesting that all of them are stable,44 which are consistent with the previous discussion on their stability. In the range from -4 to -1 eV, the valence bands of both single layer NbS2 and TaS2 are mainly occupied by S-3p orbitals. Above the Fermi level, the conduction bands of NbS2 are composed of Nb-4d orbitals, and the conduction bands of TaS2 are dominated by Ta-5d orbitals. But there is no electronic states exhibit in the range from -1 to -0.6 eV and from 1 to 2.2 eV. For the BlueP/MS2 vdW heterostructures, the valence bands are also mainly occupied by S-2p orbital, while the conduction bands are dominated by d orbitals of transition-metals, which coincide with the analysis of band structures. Notably, the heterostructures display obviously increased densities of states at the edge of valence band than corresponding single-layer NbS2 and TaS2; more importantly, empty band disappears and new electronic state is generated owing to extra new bands formed, especially in the range from 1.0 to 2.2 eV. The enhanced DOS and new formed bands in BlueP/NbS2 and BlueP/TaS2 heterostructures could contribute to their excellent electronic conductivity. The metallic characteristics of BlueP/MS2 vdW heterostructures are considerably strengthened with higher carrier mobility than the monolayers, which is of great advantage for realizing enhanced conductivity of flexible electrodes in LIBs. To better understand the nature of the bonding mechanism and to explore the charge transfer between adjacent layers in BlueP/MS2 vdW heterostructures, plane-averaged electron density difference ∆ρ along the perpendicular direction to the interface was calculated based on the following equation:7

∆ρ = ρ BlueP/MS2 − ρ BlueP − ρ MS2

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(8)


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where the ρ BlueP/MS 2 , ρ BlueP and ρ MS2 are the charge density of BlueP/MS2 vdW heterostructures, BlueP and MS2 systems, respectively. The calculated results are shown in Figure 4, where the positive value (magenta regions) indicates electron accumulation while negative value (cyan regions) denotes depletion in the combined system relative to the two isolated components. Moreover, as a result of the interlayer coupling effect, significant charge redistribution exists in the BlueP/MS2 heterointerfaces. The change at interfaces indicates that electrons transfer from the BlueP side to the MS2 side across the interface, whereas the holes remained in the BlueP side. To quantify the change

of charge density, we utilized Bader charge analysis for the charge densities. The results show that 0.045 and 0.036 electrons transferred from BlueP to MS2 slabs in the BlueP/NbS2 and BlueP/TaS2 heterostructures, respectively. Net charge accumulation lead to forming a built-in electric field at the interface, which is helpful to the separation of electrons and holes.45 The electron localization functions (ELF) visualizes the chemical bonding changes of 2D NbS2, TaS2 monolayers, as well as BlueP/NbS2 and BlueP/TaS2 heterostructures. The ELF contour plots projected in the (110) plane are displayed Figure 5, where ELF=1 corresponds to perfect localization and ELF=0.5 for uniform electron gas.46 The marked bond point of the vdW gap in the heterostructure can be defined as the saddle point with two negative eigenvalues and one positive eigenvalue of the Hessian matrix of the ELF.47 It is clearly seen that the value of ELF at the S and P sites is large, which indicates a strong local character. The ELF between interlayer is negligibly small (the bond points of the vdW gap are 0.093 and 0.083 for BlueP/NbS2 and BlueP/TaS2 heterostructures, respectively), which could interpret the weak vdW interaction between BlueP and MS2 in BlueP/MS2 vdW heterostructures. 3.3. Mechanical properties

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The calculated stress-strain relations of monolayers and heterostructures along zigzag (x) and armchair (y) directions are illustrated in Figure 6, where two kinds of uniaxial load conditions are considered and the corresponding values are shown in Figure 7. As applied strain increases (ε>4%) resulting in destruction of the hexagonal structural symmetry, the stress-strain behaviors become nonlinear. Upon straining further, the stress continues to increase till it reaches a maximum, termed as the ultimate strength σ*, and the corresponding ultimate strain termed as ε* for all compositions.48 The stresses for all system exhibit linear dependence on the applied strains for both loading directions within small strain. Following the continuum mechanics, the Young’s modulus E is derived from the slope of stress-strain curves with strain up to 4%.27,48 It is worth noting that the Young’s modulus of BlueP/MS2 vdW heterostructures are smaller than the counterpart monolayers (see Figure 7a-b), and also considerably lower than MoS2 (270 GPa)26 and graphene (1000 GPa)49. For instance, the Young’s modulus of BlueP/NbS2 heterostructure is 102.65 and 110.14 along the zigzag and armchair directions, respectively, which are distinctly smaller than that of NbS2 (131.46 and 139.37 along the zigzag and armchair directions, respectively). So the formation of BlueP/MS2 vdW heterostructures improves the mechanical flexibility of the single 2D materials, which is beneficial for fabricating flexible devices. The ultimate strains of BlueP are 22% and 17% in the zigzag and armchair directions, respectively. And its corresponding ideal strengths are 29.03 GPa (zigzag) and 29.62 GPa (armchair), which is apparently larger than the concerned MS2 monolayers (Figure 7c-d). For MS2 monolayers, the stress develops upon loading in armchair direction (Figure 7d) is much larger than in the zigzag direction (Figure 7c) (22.80 vs. 13.41 and 25.77 vs. 16.20 for NbS2 and TaS2, respectively). For BlueP/MS2 vdW heterostructures, the stress response under large strains along zigzag and armchair

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directions are analyzed further. It is seen in Figure 7e-f that BlueP/NbS2 and BlueP/TaS2 heterostructures can sustain the tensions strain as large as 20% and 23%, respectively, along the zigzag direction. While both BlueP/NbS2 and BlueP/TaS2 heterostructures can sustain up to 17% tensions strain along the armchair direction. As seen, the BlueP/MS2 vdW heterostructures can sustain larger strains than 2D Ti2C36 (18% and 17% along zigzag and armchair, respectively) and graphene50 (15% biaxial). Briefly, compared with single-layer NbS2, TaS2 and some other single-layer compounds, the hybrid BlueP/MS2 vdW heterostructures can keep large fracture strain and possess lower Young’s modulus. They are able to accommodate their volume expansion/contraction upon lithiation/de-lithiation while retaining their function in addition to ensuring high electrical conductivity, which agree to flexible electrode applications in flexible and/or stretchable LIBs. In order to explore the deformation mechanism under uniaxial tensile strains of BlueP/MS2 vdW heterostructures, the M-S and P-P bond lengths along zigzag and armchair directions are analyzed and shown in Figure S3a-d to shed light on the impact of the uniaxial tensile strain on crystal structures of the adjacent BlueP and MS2 layers. The M-S lengths are elongated monotonically with the increase of applied strain from the initial state to the critical tensile strain, resulting in a continuously increase of stress. At the same time, the P-P bond lengths along zigzag increase monotonically with strain, but decrease slowly along armchair directions, which interprets the ultimate strains of BlueP/MS2 vdW heterostructures under uniaxial tensions along zigzag direction are larger than that along armchair. The interlayer distances under different applied strains are shown in Figure S3e-f to gain more insight into the effects of tensile strains on the flexible materials. It is seen that with the increase of applied strain, the interlayer distances tend to keep constant in the

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initial elastic stage, which are eminently contracted by 0.2 and 0.4 Å in zigzag and armchair directions, respectively. However, at the final stage of the applied tension close to ultimate strain, interlayer distance of each heterostructure tend to keep constant again, indicating the hexagonal symmetry is broken when tension stress reaches the maximum states. Therefore, the failure mechanisms of the heterostructures are due to the elastic disability and independence of the loading direction. 3.4. Li adsorption on BlueP/NbS2 and BlueP/TaS2 heterostructures

Next, we further investigate the Li adsorption properties. Herein, we started with the intercalation of one Li ion into 2×2×1 supercells in BlueP, MS2 monolayers and BlueP/MS2 vdW heterostructures. It was reported that the most stable adsorption site of alkali ions was above the transition metal atoms in MoS2 and NbS2 monolayers.51 Meanwhile, the most stable lithiated configurations for BlueP is directly above P atom.52 Hence, there are three typical adsorption places for BlueP/MS2 vdW heterostructures, as shown in Figure 8a: (1) Li ion adsorbed on the outside surface of MS2 with stoichiometry of NbS2/Li0.25 and TaS2/Li0.25 (Ht site), (2) Li embedded in the interlayer of BlueP/MS2 vdW heterostructures with stoichiometry of BlueP/Li0.25/NbS2 and BlueP/Li0.25/TaS2 (Hi site), and (3) Li ion adsorbed on the outside surface of phosphorene (Hp site), corresponding to stoichiometry of Li0.25/BlueP/NbS2 and Li0.25/BlueP/TaS2. The structures of Li incorporation into BlueP/MS2 vdW heterostructures under different adsorption sites of Li adatom were fully geometry optimized, and no noticeable structural change was found. The calculated stable adsorption sites of Li and the corresponding adsorption energies are listed Table 1. Three important results should be addressed. Firstly, the most stable adsorption sites of Li adsorption on BlueP/MS2/Li0.25 system are similar to pristine monolayer (MS2/Li0.25), where Li

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adsorption site is below the transition metal atoms (Ht site), and a negative value of adsorption energy implies that the lithiated compound is thermodynamically stable. Secondly, the BlueP/MS2/Li0.25 systems exhibit a remarkable feature that the Li atoms feel the presence of the BlueP (MS2) sheet. Both Li adsorption energy and equilibrium geometry greatly depend on the atomic structure around the Li atom on the outside surface of MS2 in BlueP/MS2 vdW heterostructures. For instance, the BlueP/NbS2/Li0.25 system with adsorption energy of -3.753 eV/atom is energetically more stable than BlueP/Li0.25/NbS2 (Li0.25/BlueP/NbS2) system and pristine BlueP/Li0.25 (NbS2/Li0.25) system by 0.234 eV (1.397 eV) and 2.018 eV (0.071 eV), respectively. These findings indicate that Li atoms are more likely to insert into the outside surface of MS2 rather than adsorption on the outside surface of phosphorene and the interlayer in BlueP/MS2 vdW heterostructures during the lithiating process, in other words, Li ions occupy the outside surface of MS2 firstly, and then take the other adsorption sites in BlueP/MS2 vdW heterostructures. Finally, the

corresponding open circuit voltages for BlueP/NbS2/Li0.25 and BlueP/TaS2/Li0.25 are 2.119 and 1.929 V, respectively, which is higher than that in pristine NbS2 (2.048 V) and TaS2 (1.862 V). Inserting Li cations into the BlueP/MS2 vdW heterostructures, the adsorption energy increases gradually, see Table S3. The increase in adsorption energy comes from two main factors. One is the weak electrostatic attraction between BlueP/MS2 host and Li cations, and the other is the enhanced Li-Li repulsion at relatively high Li concentrations. The calculated open circuit voltage of BlueP/MS2 vdW heterostructures is in the range of 1.508-0.097 V during the lithiating process, which enables a high energy density but prevents lithium plating. When the BlueP/MS2 vdW heterostructures accommodate up to 5 Li ions per formula unit, corresponding to a chemical ratio of Li2/BlueP/Li1/MS2/Li2 (see Figure 8b), nondistortion structure occurs, and the calculated Li

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adsorption energies are -2.110 and -2.068 eV for BlueP/NbS2 and BlueP/TaS2 heterostructures, respectively, corresponding to open circuit voltage of 0.476 and 0.434 V. Herein, we take BlueP/NbS2 heterostructure as an example, the delocalized electrons around the absorbed Li atoms in Figure 8c indicate the absorbed Li atoms are stable around the BlueP/NbS2 heterostructure. Ideally, hybrid BlueP/NbS2 and BlueP/TaS2 heterostructures electrodes can maintain high theoretical capacities of 528.257 and 392.154 mAhg-1, respectively, (see Figure 8d) which is greater than that of graphite anode (372 mAh/g) in rechargeable LIBs.53 In order to clarify the ion bond strength between the adsorbed Li and BlueP/MS2 vdW heterostructures further, we quantitatively estimated the amount of charge transfer in lithiated BlueP/MS2/Li0.25, BlueP/Li0.25/MS2 and Li0.25/BlueP/MS2 system through Bader charge analysis. The results are listed in Table 1. For simplicity, we only take BlueP/NbS2 heterostructure as an example. First, when one Li atom adsorption on the outside surface of NbS2 (BlueP/NbS2/Li0.25), the corresponding charge of Li is +0.991 |e|, while the calculated charges of BlueP and NbS2 are +0.189 |e| and -1.180 |e|, respectively (Ht site). This result reveals that the charge of Li atom is predominantly transferred to the adjacent NbS2 layer. Moreover, when one Li atom is adsorbed on the outside surface of BlueP (BlueP/Li0.25), the calculated charges of BlueP and NbS2 are -0.527 |e| and -0.467 |e|, respectively (Hp site). Similarly with BlueP/NbS2/Li0.25 system, the charge of adsorbed Li is transferred to adjacent BlueP layer, and the interaction between the adsorbed Li atom and P atoms are predominantly ionic. Besides, as Li atom is embedded in the interlayer of the BlueP/NbS2 (BlueP/Li0.25/NbS2), (Hi site), the charges of Li transfer to BlueP and NbS2 with -0.122 |e| and -0.870 |e|, respectively. Above all, all the Li incorporation configurations show obvious charge transfer from Li to BlueP/NbS2, which strongly supports the strong ionic interaction between Li and BlueP/NbS2.

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This phenomenon is also found in black phosphorene/graphene heterostructure after lithiation.7 To visualize the ionic bond of Li incorporation at the different sites, we calculated the charge density difference of lithiated BlueP/MS2/Li0.25, BlueP/Li0.25/MS2 and Li0.25/BlueP/MS2 systems (Figure 9). For the case of Li atom embedded in the interlayer of BlueP/NbS2 heterostructure, a net loss of electronic charge could be found above the Li and also a net gain of electronic charge is found on its adjacent BlueP and NbS2 layer. The large gains in charge demonstrate the significant electronic transfer from Li atom to the neighboring BlueP and NbS2 layer, which indicate the strong ionic bonding of the embedded Li atom. Besides, as the Li atom adsorption on the outside surface of BlueP and NbS2, see Figure 9a and 9c, most of the charge of Li is mainly transferred to the adjacent BlueP (Li0.25/BlueP/NbS2 system) or NbS2 (BlueP/NbS2/Li0.25 system). These findings are in consistent with the previous Bader analysis, which further verify the strong ionic interaction between Li and BlueP/NbS2. The charge density difference of lithiated BlueP/TaS2/Li0.25, BlueP/Li0.25/TaS2 and Li0.25/BlueP/TaS2 system (see Figure S4) resemble that of lithiated BlueP/NbS2 heterostructure. Overall, our findings show that the BlueP/MS2 vdW heterostructures exhibit excellent chemical stability, electrical conductivity, flexibility and high capacity, thus the BlueP/MS2 vdW heterostructures are good candidates for flexible anode materials in LIBs, especially for BlueP/NbS2 heterostructure with a high capacity of 528.257 mAhg-1. In all, present studies enhance the understanding of 2D TMDs-based heterostructures, which is important for the rational design of high-performance electrode materials for LIBs. 4 Conclusions The stacking configurations induced electronic structures, mechanical flexibility and Li adsorption properties of BlueP/MS2 (M=Nb, Ta) vdW heterostructures were studied systematically

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using first-principles calculations. Their negative formation energies as well as excellent lattice match guarantee structural stability. The enhanced electronic bands near the Fermi level and weak van der Waals interactions were found between the adjacent layers. The formation of heterostructures reduce the Young’s modulus to be 106.40 and 116.86 GPa for BlueP/NbS2 and BlueP/TaS2 heterostructures that are much better than the single 2D materials. Moreover, for BlueP/MS2 vdW heterostructures, it can sustain larger strains as large as 20% and 23% under uniaxial tensions along zigzag armchair direction, respectively, than 2D Ti2C (18% and 17% in zigzag and armchair, respectively). The BlueP/MS2 vdW heterostructures, especially for BlueP/NbS2, are able to provide much higher capacity of 528.257 mAhg-1, which is much better than black phosphorene/graphene heterostructure (485.31 mAhg-1) anode Material. The results reveal that BlueP/MS2 vdW heterostructures are ideal candidates as promising flexible electrodes for high performance and portable LIBs with good structural stability, enhanced electrical conductivity and improved mechanical flexibility. This study provides a clear understanding of structure-property relationship, hence contributes to guide the future design of high-efficiency TMDs-based flexible electrodes for LIBs, in particular for the ever growing market of portable and wearable energy conversion and storage devices. Acknowledgments This work is financially supported by the National Natural Science Foundation for Distinguished Young Scientists of China (Grant No. 51225205), the National Natural Science Foundation of China (Nos. 51171046, 61274005 and 61504028), the Research Fund for the Doctoral Program of Higher Education of China (Ph.D supervisor) (No. 20133514110006), the Natural Science Foundation of Fujian Province (Nos. 2014J01176 and 2016J01216) and the Science

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Foundation of Department of Education of Fujian Province under grant (No. JA15067). Supporting Information. The structural information of BlueP and MS2 (M=Nb, Ta) monolayers; energy and structural information of BlueP/MS2 vdW heterostructures; adsorption energy, cell voltage, theoretical gravimetric capacity and the charge transfer of Li adsorption on BlueP, MS2 monlayers and BlueP/MS2 vdW heterostructures; geometric structures of BlueP/MS2 vdW heterostructures with various stackings; band structures of BlueP and MS2 monolayers; uniaxial strain dependent M-S and P-P bond lengths, interlayer distances in BlueP/MS2 vdW heterostructures along zigzag and armchair directions; charge density difference of one Li adsorption on BlueP/TaS2 heterostructure.

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Table 1. The adsorption energy Ead (eV/atom), cell voltage (V), theoretical gravimetric capacity (mAhg-1) and the charge transfer of Li atom, MS2 and BlueP layer, ∆QLi, ∆QT and ∆QP (|e|), respectively, for one Li ion adsorption on 2×2×1 supercell of BlueP, MS2 and BlueP/MS2 (M=Nb, Ta). Ht means Li ion adsorbed on the outside surface of MS2; Hi indicates Li embedded in the interlayer of BlueP/MS2; and Hp denotes Li ion adsorbed on the outside surface of phosphorene. System

Li site

Ead

Voltage

Capacity

∆QLi

BlueP/Li0.25

Hp

-1.735

0.101

105.213

+0.994

NbS2/Li0.25

Ht

-3.682

2.048

42.203

+0.991

-0.991

TaS2/Li0.25

Ht

-3.496

1.862

27.148

+0.991

-0.991

BlueP/NbS2/Li0.25

Ht

-3.753

2.119

30.358

+0.991

-1.180

+0.189

BlueP/Li0.25/NbS2

Hi

-3.519

1.885

30.358

+0.991

-0.870

-0.122

Li0.25/BlueP/NbS2

Hp

-2.356

0.722

30.358

+0.993

-0.467

-0.527

BlueP/TaS2/Li0.25

Ht

-3.563

1.929

21.701

+0.991

-1.142

+0.151

BlueP/Li0.25/TaS2

Hi

-3.422

1.788

21.701

+0.992

-0.835

-0.156

Li0.25/BlueP/TaS2

Hp

-2.271

0.637

21.701

+0.993

-0.416

-0.577

22


∆QT

∆QP -0.994

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Figure 1. (a) Top and (b) side views of BlueP/MS2 (M=Nb, Ta) with the most stable configuration.

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Figure 2. Band structures of (a) BlueP/NbS2 and (b) BlueP/TaS2 heterostructures, where the energy is scaled with respect to the Fermi energy EF and the size of the red, green and blue circles illustrates the projected weight of M-d

(M=Nb, Ta), S-p and P-p electrons, respectively.

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Figure 3. The total density of states (DOS) and orbital-resolved partial DOS of (a) NbS2, (b) TaS2 monolayers, as well as (c) BlueP/NbS2 and (d) BlueP/TaS2 heterostructures. Inset red arrows denote the increased DOS in heterostructures.

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Figure 4. The plane-averaged electron density difference along the direction perpendicular to the interface of (a) BlueP/NbS2 and (b) BlueP/TaS2 heterostructures, where the positions of the M (M=Nb, Ta), S and P atoms are indicated by red, green and blue solid circles, respectively. And the magenta and cyan regions indicate electron accumulation and depletion, respectively.

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Figure 5. The ELF contour plots projected on the (110) plane of (a) NbS2, (b) TaS2 monolayers, as well as (c) BlueP/NbS2 and (d) BlueP/TaS2 heterostructures.

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Figure 6. Tensile stress σ as a function of uniaxial strain ε of (a)-(b) BlueP, (c)-(d) MS2 (M=Nb, Ta) monolayers and (e)-(f) BlueP/MS2 vdW heterostructures along zigzag and armchair directions, respectively.

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Figure 7. The comparison of (a)-(b) Young’s modulus E (GPa), (c)-(d) ultimate strength σ* (GPa), (e)-(f) ultimate strain ε* of BlueP, MS2 (M=Nb, Ta) monolayers and corresponding BlueP/MS2 vdW heterostructures along zigzag and armchair directions, respectively.

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Figure 8. (a) Li adsorption sites in BlueP/MS2 (M=Nb, Ta) vdW heterostructures (BlueP/MS2/Li, BlueP/Li/MS2 and Li/BlueP/MS2); (b) lithiated configuration of two-layer Li adsorption on BlueP/MS2 vdW heterostructures; (c) the ELF contour plot projected on the (110) plane of two-layer Li adsorption on BlueP/NbS2 heterostructure; (d) theoretical specific capacity of MS2 monolayers and BlueP/MS2 vdW heterostructures as anode materials for the LIBs.

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Figure 9. Top and side views of the charge density difference of one Li (a) adsorption on the out-surface of NbS2; (b) insert into interlayer of BlueP/NbS2; (c) adsorption on the out-surface of phosphorene. The loss of electrons is indicated in blue and gain of electrons is indicated in yellow.

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Graphic abstract

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MS2 (M = Nb, Ta) Heterostructures As Promising

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