Mechanical Properties, Electronic Structures, and Potential

Article pubs.acs.org/JPCC

Mechanical Properties, Electronic Structures, and Potential Applications in Lithium Ion Batteries: A First-Principles Study toward SnSe2 Nanotubes Chongyi Ling,†,‡ Yucheng Huang,*,†,‡ Hai Liu,†,‡ Sufan Wang,†,‡ Zhen Fang,†,‡ and Lixin Ning†,§ †

Center for Nano Science and Technology, ‡College of Chemistry and Material Science, The Key Laboratory of Functional Molecular Solids, Ministry of Education, and §Department of Physics, Anhui Normal University, Wuhu, 241000, Peoples’ Republic of China S Supporting Information *

ABSTRACT: First-principles calculations were carried out to investigate the mechanical and electronic properties as well as the potential application of SnSe2 nanotubes. It was found that the mechanical properties are closely dependent on diameter and chirality: the Young’s modulus (Y) increases with the enlargement of diameter and converges to the monolayer limit when the diameter reaches a certain degree; with a comparable diameter, the armchair nanotube has a larger Young’s modulus than the zigzag one. The significantly higher Young’s modulus of SnSe2 nanotubes with the larger diameter demonstrates that the deformation does not easily occur, which is beneficial to the application as anode materials in lithium ion batteries because a large volume expansion during charge−discharge cycling will result in serious pulverization of the electrodes and thus rapid capacity degradation. On the other hand, band structure calculations unveiled that SnSe2 nanotubes display a diversity of electronic properties, which are also diameter- and chirality-dependent: armchair nanotubes (ANTs) are indirect bandgap semiconductors, and the energy gaps increase monotonously with the increase of tube diameter, while zigzag nanotubes (ZNTs) are metals. The metallic SnSe2 ZNTs exhibit terrific performance for the adsorption and diffusion of Li atom, thus they are very promising as anode materials in the Li-ion batteries.

1. INTRODUCTION An entirely new field of one-dimensional (1D) materials of nanotube (NT) has been opened up since the first finding of carbon nanotubes (CNTs).1−4 Single-walled CNTs display various electronic properties, from semiconductor to metal, depending on their morphology, diameter, and chirality,5 leading to many exciting nanotechnology applications such as nanophotonic and nanoelectronic devices.6,7 Motivated by the versatile electronic structures and a wide range of applications of CNTs, recently, extensive investigations have been devoted to other inorganic nanotubes, including their synthesis, properties, as well as applications.8−27 For example, boron nitride nanotubes (BNNTs), which have been synthesized shortly after the finding of CNTs,8 are proved to have various potential applications in hydrogen storage, composite materials, as well as force sensors,9−11 etc. Over the last two decades, another form of NT based on metal dichalcogenides MX2 (X = S, Se, Te) has also gained extensive interest in both experimental and theoretical fields, such as MoS2,12−14 MoTe2,15−18 WS2,19−21 MoSe2, WSe2,22 SnS2,23,24 TaS2, NbS2,25 and so on. These NTs can be viewed as rolling up their corresponding 2D nanosheets but can display distinct electronic properties to the counterpart of nanosheets. For example, the MoS2 nanosheet is a semiconductor with an energy gap of 1.68 eV,28 while the tube structures have a much narrower band gap than that of the nanosheet.14 © XXXX American Chemical Society

As a typical kind of MX2, tin diselenide (SnSe2) is a IV−VI group semiconductor with a hexagonal crystal structure of the type CdI2.29 It has been exploited for the potential applications in the fields of lithium ion batteries,30 supercapacitors,31 and phase change memory,32 etc. Bulk SnSe2 displays a nonmagnetic semiconducting feature, and its derived 2D monolayer and 1D nanoribbons are also proved to be semiconductors without magnetism.33 Very recently, we systematically investigated the modulation of the electronic and magnetic properties of SnSe2 nanostructures by the strain. The nanotubes are predicted to be feasibly prepared in a laboratory, and the electronic properties strongly depend on the chirality.34 It is very interesting that over the years researchers attempted to use various NTs derived from the metal dichalcogenides as anodes for the applications in lithium ion batteries (LIBs). On one hand, like the current widely used anode material of graphite, the MX2 tube has a cavity which can provide ideal space for Li atom intercalation. On the other hand, due to the large electrolyte−electrode interface and reduced ion diffusion pathway, MX2-derived NTs have been reported with significant improvement in cycle life and rate performance.35−37 In addition, Sn-based materials gained Received: September 23, 2014 Revised: November 12, 2014

A

dx.doi.org/10.1021/jp509623k | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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analysis with only one negative frequency. As we know, carbon nanotubes are formed by rolling up a single layered graphene sheet. By contrast, SnSe2 nanotubes are more complicated due to the basic Se−Sn−Se triple layer as a structural unit, resulting in SnSe2 nanotubes easily containing several hundreds of atoms, far too large for typical ab initio calculations. Therefore, in this paper, we use a curved surface technique to reduce the computational burden. During the structure relaxation, the unit cell shape and volume were kept fixed, and only the positions of Sn and Se atoms near the Li atom were allowed to change. To evaluate the accuracy for the simplification, we compared the energy barrier and reaction heat of Li transfer on the full tube with those on the curved surface (Figure S1, Supporting Information). The results show that the relative error is around 4%; obviously, the small difference will not affect the conclusions.

growing research attention as anodes for LIBs due to their high theoretical capacity and low cost.30,38−44 Thus, we cannot help but asking, can SnSe2 NTs be applied in Li-ion battery? Obviously, uncovering the answer to this question can make us better understand the important and charming SnSe2 nanotubes, especially in directing the way to the potential applications. As one of the most important energy storage devices, LIBs have attracted intensive attention over the past years, especially the issue of searching for a better anode material with higher theoretical specific capacity.45,46 Generally, a good electrode material in Li-ion cells requires high electron conductivity and strong Li binding strength as well as an appropriate low diffusion barrier during the progress of Li-ion “intercalation” and “deintercalation”. In addition, it is known that many electrolyte materials suffer from severe volume variation during the charge−discharge cycling, such that the mechanical stress of the electrode is huge, which will greatly limit its lifetime. Therefore, material that does not readily deform will benefit the structural stability, thereby extending the working life. In this contribution, by mean of first-principles calculations, we first systematically investigate the mechanical and electronic properties of SnSe2 NTs. Then, the adsorption and diffusion of lithium atoms on the NTs are carefully examined. Our results show that both the mechanical and electronic properties of SnSe2 NTs are diameter- and chirality-dependent: (i) the Young’s modulus increases with the enlargement of diameter and finally converges to the monolayer limit; (ii) the armchair nanotube (ANT) has a larger Young’s modulus than the zigzag nanotube (ZNT) with a comparable diameter; and (iii) ANTs present indirect gap semiconducting properties and the energy gaps increase monotonously with increasing diameter, while the ZNTs display a metallic feature. On the basis of these results, we choose ZNTs with larger diameters as prototypes to evaluate the possibilities for the application in Li-ion batteries. In comparison to SnSe2 bulk and monolayer, SnSe2 ZNTs are proved to be more suitable and promising as anode materials.

3. RESULTS AND DISCUSSION We first investigate the structure and electronic properties of the SnSe2 2D monolayer. As shown in Figure 1a, the SnSe2

Figure 1. (a) Geometric structure and (b) electronic structure of the SnSe2 2D monolayer.

monolayer presents a triple-layer structure, which consists of a Sn layer sandwiched between two Se layers. The thickness of the triple-layer structure and the Sn−Se bond length are calculated to be 3.19 and 2.74 Å, respectively. It is shown that the 2D SnSe2 monolayer is an indirect bandgap semiconductor with an energy gap of 0.79 eV (Figure 1b). On the basis of optimized geometry of the SnSe2 sheet, two kinds of NTs were built, which can be classified into the categories of either “armchair” (n, n) or “zigzag” (n, 0), depending on the rolling direction as defined in CNTs.55 To investigate the effect of tube diameter, from (8, 8)/(11, 0) to (13, 13)/(16, 0) of SnSe2, ANTs/ZNTs were constructed. In the following we will present their corresponding geometrical configurations, electronic structures, mechanical properties, as well as the potential applications. 3.1. Geometric Structures. Figure 2a and 2b show the top and side view of the optimized structures of armchair and zigzag SnSe2 NTs, respectively (here (10, 10) and (14, 0) NTs were illustrated as representations). Table 1 presents some important structural parameters of these NTs with various diameters, including the inner radius (Ri), outer radius (Ro), radius (R), wall thickness (T), and Sn−Se bond lengths in the inner (Li) and outer (Lo) layer, where R = (Ri + Ro)/2 and T = Ro − Ri. It can be seen that the (n, n) NTs have a much wider radius than that of (n, 0) NTs when the value of n is equal. The wall thickness of all the NTs studied is in the range from 3.14 to 3.20 Å, which is similar to that of a triple layer in the 2D monolayer (∼3.19 Å). The variation of Sn−Se bond lengths in the inner shell (Li) is almost unchanged. In contrast, the Sn−Se

2. COMPUTATIONAL DETAILS Our first-principles DFT computations were performed by using the projector-augmented plane wave (PAW)47 method to model the ion−electron interaction as implemented in the Vienna ab initio simulation package (VASP).48,49 The generalized gradient approximation (GGA) with the PBE50,51 functional and a 350 eV cutoff for plane-wave basis set were adopted for the overall calculations. The convergence threshold was set to 10−5 eV for energy and 10−2 eV/Å for force, respectively. To avoid interaction with adjacent neighbors, the vacuum space between two periodical units is at least 12 Å. Only spin-unpolarized results are presented in this work as our earlier results showed that all of the SnSe2 NTs are nonmagnetic.34 For the geometric calculations, the Brillouin zone sampling was using six Monkhost−Pack52 k point grids. On the basis of the equilibrium structures, 21 k points were then used to compute the electronic band structures. To search for the migration mechanism of Li on the tubes, we first use the climbing nudged elastic band method (CNEB)53,54 to roughly locate the transition states (TSs), which starts by inserting a series of image structures between the initial and final states of the reaction. Then the TSs were further optimized to the saddle point with quasi-Newton algorithm until the residual forces are less than the standard we set (2 × 10−2 eV/Å). Each TS was confirmed by vibration B



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al. defined l as the wall thickness for CNT, which is 0.66 Å.58 To avoid this controversial issue, an area-based Young’s modulus was introduced: the volume V0 is replaced by the cross section of the tube (2πRc0), resulting in the Young’s modulus being independent of the l.59 Scrutinizing the published literature, the definition of l for the metal dichalcogenides is always believed as the interlayer distance.18,60,61 In the case of the SnSe2 NT, we hold the opinion that using wall thickness to calculate Y is more reasonable and can be extended to other single-walled NTs with a triple layer, as we simply deduced from a geometric approach in the Supporting Information (SI Figure S2). The calculated Young’s modulus as a function of tube diameter is plotted in Figure 3. Obviously, the Young’s modulus Figure 2. Top and side view of (a) armchair (10, 10) and (b) zigzag (14, 0) SnSe2 NTs.

Table 1. Inner Radius (Ri), Outer Radius (Ro), and Radius (R), As Well As the Wall Thickness (T) and Sn−Se Bond Lengths in the Inner (Li) and Outer (Lo) Layer of SnSe2 Zigzag and Armchair NTs NTs

Ri (Å)

Ro (Å)

R (Å)

T (Å)

(8, 8) (9, 9) (10, 10) (11, 11) (12, 12) (13, 13) (10, 0) (11, 0) (12, 0) (13, 0) (14, 0) (15, 0)

7.36 8.37 9.42 10.41 11.45 12.52 4.86 5.15 6.29 6.91 7.50 8.08

10.51 11.51 12.57 13.57 14.61 15.68 8.06 8.35 9.44 10.05 10.65 11.23

8.93 9.94 10.99 11.99 13.03 14.10 6.46 6.75 7.87 8.48 9.08 9.66

3.15 3.14 3.15 3.16 3.16 3.16 3.20 3.20 3.15 3.14 3.15 3.15

Li (Å) 2.70, 2.70, 2.70, 2.70, 2.70, 2.71, 2.73, 2.76, 2.69, 2.69, 2.69, 2.69,

2.71 2.71 2.71 2.71 2.71 2.71 2.76 2.82 2.72 2.71 2.71 2.71

Lo (Å) 2.75, 2.76, 2.76, 2.76, 2.76, 2.76, 2.75, 2.78, 2.71, 2.70, 2.71, 2.71,

3.02 2.96 2.92 2.88 2.86 2.85 2.88 2.80 2.91 2.90 2.89 2.88

Figure 3. Young’s modulus as a function of tube diameter for SnSe2 nanotubes.

is closely associated with the diameter and chirality. For both kinds of NTs, the Young’s modulus increases with the enlargement of diameter and converges to a constant value of approximate 188 or 181 GPa for ANT or ZNT, respectively. No experimental values are available for SnSe2 NTs, but the calculated asymptotic limit is close to the calculated Young’s modulus of the monolayer, i.e., 187 and 185 GPa along the armchair and zigzag direction, respectively. In addition, the calculated Young’s modulus of ANT is larger than that of the ZNT with a comparable diameter. The significantly higher Young’s modulus of SnSe2 nanotubes with the larger diameters demonstrates that they are not easily deformed in the elastic regime. This characteristic indicates that they may have an advantage as electrodes in the application of Li-ion batteries because a severe volume variation during the charge−discharge cycle will lead to serious pulverization of the electrodes and even rapid capacity shrinkage. It is to be noted that the mechanical properties of many metal dichalcogenides have been reported. For example, as a similar structural orientation to the SnSe2 nanotube, a recent study reported the Young’s modulus of the TiS2 (1T structure, where the metal atoms are coordinated octahedrally by the sulfur atom as in the case of SnSe2) nanotube reaches 140 GPa· nm.62 Our area-based Young’s moduli of SnSe2 ANT and ZNT are calculated to be 59 and 57 GPa·nm, respectively. The significant lower values indicate that the SnSe2 nanotube is softer than the TiS2 nanotube. This is expected as the Sn atom is from the main group element. After further comparison with other metal selenides (for example, the calculated asymptotic limit is approximately 116 GPa·nm for MoSe2 and 136 GPa·nm for WSe263), we found that the magnitudes of the Young’s

bond lengths in the outer layer (Lo) are somewhat enlarged compared to the ones in the SnSe 2 monolayer. The periodicities along the tube axis (c0) are 3.86 and 6.59 Å for armchair and zigzag NTs, respectively. 3.2. Mechanical Properties. To evaluate the mechanical properties of SnSe2 NTs, the Young’s modulus is calculated, which is defined as the second derivative of the strain energy with respect to the strain at equilibrium configuration. According to previous conventions,18,56 the Young’s modulus (Y) can be written as Y=

1 ⎛ ∂ 2E ⎞ ⎟ ⎜ V0 ⎝ ∂ε 2 ⎠

where V0 is the relaxed equilibrium volume; E is the strain energy per unit cell; and ε is the axial strain. For a unit cell of SnSe2 NTs, the V0 can be calculated by the following equation

V0 = 2πRlc0 Except for l, the definitions of these parameters have been described in section 3.1, and the corresponding values are tabulated in Table 1. It is to be noted that the definition of tube thickness (l) is not completely consistent in the previous publications. For example, the tube thickness for a single-walled CNT is defined by Lu57 as the interlayer distance between two neighboring layers of graphite (3.40 Å). However, Yakobson et C



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electronegativity of Se compared with S, which leads to more delocalization of the valence electrons and the stronger covalent bonding with Sn. Scrutinizing the band structures of these two tubes, one can find that the curves near the Fermi level of the SnS2 nanotubes are smoother, thus giving rise to a larger separation of the band gap.24 It can be expected, on going from S to Se to Te, that the energy gap of MX2 will decrease monotonously. This phenomenon has been confirmed in the MoX2 and WX2 (X = S, Se, Te) systems.65 Briefly, rolling up the SnSe2 monolayer into the SnSe2 nanotube can greatly reduce the energy gap and even can trigger the semiconductor−metal transition when the rolling direction is along the zigzag edge. The metallic property is a benefit to the conduction, which makes ZNTs have an advantage of being applied as an electrode in Li-ion batteries. To have a deeper insight into the diversity of electronic structure of SnSe2 NTs, we plotted the total density of states (TDOS) and partial density of states (PDOS) in Figure 6. Because the PDOS near the Fermi level from Se s orbital contribution is insignificant, we do not plot it for the sake of concision. At first glance from Figure 6 it is seen that the DOS for these two NTs presents a similar tendency: at the lowenergy regions (from −5.0 eV to Fermi level), the contribution to the TDOS primarily originates from the p orbitals of both Sn and Se atoms, indicative of the strong p−p bonding interaction; above the Fermi level but below 1.5 eV, the DOS consists of the coupling between Se 4p and Sn 5s orbitals; in the energy window from 3.0 to 4.0 eV, a dominant contribution from the Sn 5p orbital can be viewed. However, the big difference between the ANT and ZNT is that there are some states contributed by Se p orbitals across the Fermi level for ZNTs, while those of ANTs do not. Therefore, ZNTs present metallic properties, and ANTs display semiconducting features. 3.4. Potential Application in Lithium Ion Batteries. Through thorough investigations regarding the mechanical properties and electronic structures, above we show that SnSe2 ZNTs with larger diameter may have potential applications in the LIBs. Following, we will present the adsorption and diffusion of Li on SnSe2 ZNTs. Here SnSe2 ZNTs with diameter larger than (16, 0) are chosen as prototypes. On one hand, large diameter NTs are harder than small ones, and on the other hand, the metallic properties of SnSe2 ZNTs provide them an intrinsic advantage as they can improve electrochemical performances in LIBs.66 To avoid the interaction between two Li atoms, we double the size along the periodic direction in which the distance between two neighboring Li atoms is 13.18 Å. The curvature effects are also considered as we select different diameters for NTs, i.e., (16, 0), (18, 0), and (22, 0) tubes. As a comparison, the cases of SnSe2 bulk and the monolayer are also performed. We first examined the possible adsorption sites for Li atoms. Four representative sites are available: the top site (Ti and To, here the subscripts “i” and “o” denote “inside” and “outside” the tube, respectively) where Li is directly above one Sn atom, and the hollow site (Hi and Ho) where the Li locates on the center of the Sn3Se3 hexagon (Figure 7). To evaluate the stability of Li atom adsorption on SnSe2 NTs, we calculate the binding energies (Eb) of the Li atom. Here the Eb is defined as

modulus are closely associated with the hardness of the metal, i.e., the higher Moh’s hardness of the metal, the larger the value of the Young’s modulus. 3.3. Electronic Structures. Next, the band structures of SnSe2 NTs with different diameters are calculated, and the results are illustrated in Figure 4. Similar to the nanotubes such

Figure 4. Calculated band structures of SnSe2 (a) ANTs and (b) ZNTs with different diameters. The red dashed lines represent the Fermi level.

as CNT5 and MoTe2 NT,18 SnSe2 NTs have different band structures, which are dependent on the chirality. The SnSe2 ANTs are typical indirect bandgap semiconductors with a relatively narrow energy gap, while the ZNTs display metallic properties. For the semiconducting ANTs, the valence band maximum (VBM) and conduction band minimum (CBM) are located at ΓH and ZL points, respectively. With the increase of tube diameter, the energy gap increases monotonously but nonlinearly varies from 0.28 to 0.43 eV (Figure 5). Similar

Figure 5. Energy gaps of SnSe2 ANTs as a functional of diameter.

trends have also been found in MoS2,14 SnS2,24 and TiS264 NTs. It is interesting that SnSe2 ZNTs present a metallic property, regardless of their diameters. Analysis from density of states points out that states across the Fermi level are contributed by the Se p orbital (see below). This scenario is quite different from the result of the SnS2 nanotube, where a moderate band gap is presented.24 The reason may be derived from the less

E b = (ESnSe2 + E Li) − ESnSe2 − Li

where ESnSe2−Li and ESnSe2 are the total energy of SnSe2 with and without the adsorption of Li, respectively. ELi is the energy of D



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Figure 6. TDOS and PDOS of (a) (10, 10) ANT and (b) (14, 0) ZNT. The red dashed lines represent the Fermi level.

Figure 7. Adsorption sites and diffusion path of the Li atom on the (a) inner and (b) outer shell of SnSe2 ZNTs.

ZNT, respectively. For (18, 0) and (22, 0) ZNTs, the values of Ea on the outer shell are, respectively, 0.167 and 0.157 eV, while the corresponding ones on the inner shell are 0.210 and 0.207 eV. It is worth noting that, although the barrier difference on the inner shell is minute with the diameter variation, the regularity revealed here is reliable as more rigid test calculations have been carried out. It can be seen that the diffusion barriers of Li inside the tube are larger than the ones outside the tube, indicating that Li diffusion on the “convex” curved surface with a positive curvature is kinetically more favorable than the “concave” curved surface with a negative curvature. Plotting a parallel line through the point of monolayer in Figure 8, the scatter points of six diffusion barriers can be divided into two parts: one is the barrier inside the tube, locating above the parallel line, and the other is the barrier outside the tube, which is approximately considered as below the line (Figure 8). Obviously, with the tube diameter increasing, the diffusion barrier decreases on both inside and outside tubes. However, the barrier of the inside tube gets close to the one on the planar monolayer, but the barrier of the outside tube is away from it. This phenomenon demonstrates that the curvature of the surface plays a paramount role on the Li diffusion, in which the surface with the positive curvature is in favor of Li diffusion, while the surface with the negative curvature does not. It should be noted that, although different curvatures have a different effect on the Li diffusion barrier, these values are all around the value of the monolayer. The relative small values indicate that Li atoms can diffuse readily on both shells of SnSe2 ZNTs. We now compare the performance of various SnSe 2 nanostructures for the application in LIBs. Although SnSe2

the Li atom in its metal crystal (with body-centered cubic structure and a lattice constant of 3.491 Å). According to this definition, a more positive Eb indicates a more favorable bonding between SnSe2 and Li. As shown in Table 2, three Table 2. Binding Energies of Li Atom at the Examined Sites on SnSe2 ZNTs (in eV) NTs

Hi

Ti

Ho

To

(16, 0) (18, 0) (22, 0)

1.52 1.51 1.47

1.48 1.46 1.43

1.46 1.42 1.37

1.31 1.30 1.28

main conclusions can be drawn: (i) The binding strength increases with the decreasing tube diameter; (ii) at the same type of adsorption site on one certain tube, Li adsorption at the inner site has higher binding energy than the one at the outer site; and (iii) the H site is energetically more favorable than the corresponding T sites. Obviously, the observations of (i) and (ii) can be ascribed to the effect of curvature, indicating that a smaller diameter tube would have a stronger binding strength with Li, while the binding energy of a larger diameter tube would be close to the monolayer limit. A good anode material for LIBs requires a strong Li binding strength, but an appropriate low diffusion barrier on the process of Li “intercalation” and “deintercalation” is more important. Here the diffusion path is schemed between two neighboring H sites, passing through a T site (Figure 7). This process can be decomposed into two equivalent channels, i.e., from H to T and from T to H, experiencing a bridge site transition (Figure S3, Supporting Information). The computed energy barriers (Ea) are 0.188 and 0.213 eV on the outer and inner shell of (16, 0) E



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ASSOCIATED CONTENT

S Supporting Information *

The comparison between a full tube and a curved surface, the deduction of the equilibrium volume, the transition state structures along Li diffusion on ZNT, adsorption sites and binding energies of Li on SnSe2 bulk and monolayer, as well as the diffusion paths of Li on SnSe2 bulk. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Younger Natural Science Foundation of China No. 21203001, National Natural Science Foundation of China No. 21171007 to ZF, Natural Science Foundation of Anhui Province No. 1208085QB37, and Doctoral Scientific Research Funding of Anhui Normal University.

Figure 8. Adsorption and diffusion energies of Li atom on SnSe2 ZNTs, monolayer, and bulk.

bulk has a higher Eb than the others, the diffusion of Li needs to conquer a relatively high energy barrier of 0.73 eV (PBE+D2 method with the Grimme vdW correction,67 see Figures S4 and S5, Supporting Information). As SnSe2 bulk is exfoliated into a monolayer, the Ea will sharply decrease to 0.18 eV, but unfortunately, this decrease is accompanied by the reduction of binding energy (from 2.33 to 1.31 eV). As for SnSe2 ZNTs, the calculated Ea is more than 0.55 eV lower than the bulk, which will result in an increase of Li mobility by a factor of 108 according to the Arrhenius formula (D ∝ e(Ea/kT)). Although the binding energies of ZNTs are lower than bulk, these values are higher than that of the monolayer. The advantage of the tube is vividly seen in Figure 8, where the diffusion barrier as a function of the binding energy is plotted. Clearly, the closer to “the origin point”, the better the performance this material has. The area for the tubes is the nearest to “the origin point” (highlighted by yellow in Figure 8), indicating that SnSe2 ZNTs are very promising as anode materials of Li-ion batteries, especially considering that the SnSe2 nanosheet already exhibits terrific performance in Li-ion batteries.30

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REFERENCES

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CONCLUSION In summary, we systematically investigated the geometrical, mechanical, and electronic properties as well as the potential application in LIBs of SnSe2 NTs with different diameters and chiralities by means of first-principles calculations. Our results show that the mechanical and electronic structures of SnSe2 NTs are totally dependent on their diameter and chirality. The Young’s modulus of both kinds of NTs becomes large when the diameter increases. With a comparable diameter, ANT has a higher Young’s modulus than ZNT. Electronic calculations revealed that SnSe2 ZNTs display a metallic property, while ANTs are semiconductors with moderate band gaps. The energy gap increases monotonously with the enlargement of tube diameter. Comparing the Li binding strengths and diffusion barriers with those on the SnSe2 bulk and monolayer, the metallic SnSe2 ZNTs are believed to be a promising anode material in Li-ion batteries. In closing, we hope this theoretical investigation will provide useful information for further experimental and theoretical studies on the novel SnSe2 nanotubes, especially paving the way to the practical applications. F



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