Metallic BSi3 Silicene: A Promising High Capacity Anode Material for

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Metallic BSi3 Silicene: A Promising High Capacity Anode Material for Lithium-Ion Batteries Xin Tan, Carlos R. Cabrera, and Zhongfang Chen* Department of Chemistry, Institute for Functional Nanomaterials and NASA-URC Center of Advanced Nanoscale Materials, University of Puerto Rico, Rio Piedras Campus, San Juan 00931, Puerto Rico S Supporting Information *

ABSTRACT: Very recently, intrinsically metallic B-substituted silicenes, namely, H-BSi3 and R-BSi3 (H and R denote the hexagonal and rectangular symmetry), have been predicted as the global minimum structures of the BSi3 monolayer (J. Phys. Chem. C 2014, DOI: 10.1021/jp507011p). With unusual planar geometry and better electronic conductivity relative to the buckled and semimetallic pristine silicene sheet, the B-substituted silicenes are expected to have good applications in high capacity lithium-ion batteries (LIBs) anodes. By means of density functional theory (DFT) computations, we systematically investigated the adsorption and diffusion of Li on H-BSi3 and R-BSi3, in comparison with silicene and graphite. Their exceptional properties, including good electronic conductivity, very high theoretical charge capacity (1410 and 846 mA·h/g for single- and double-layer, respectively), fast Li diffusion, and low opencircuit voltage (OCV), suggest that the BSi3 silicene could serve as a promising high capacity and fast charge/discharge rate anode material for LIBs.

1. INTRODUCTION

Given that the B-substituted graphite/graphene matrix could better accommodate for extra electrons donated from the 2s electrons of Li, substitution doping with B has been proposed as an efficient approach to modify graphite/graphene to achieve better performance as a LIB anode. Especially, density functional theory (DFT) computations predicted the layered BC3 compounds as a promising high capacity carbon-based anode material for LIBs.23,24 The charge capacities of SL (714 mA·h/g) and stacked (857 mA·h/g) BC3 are about twice as large as graphite’s, while the mobility of Li on the surface of BC 3 is similar to graphite/graphene. Moreover, DFT computations also suggested that the BC3 nanotubes have enhanced capability for lithium storage over pristine carbon nanotubes.25 Very recently, two BSi3 silicene sheets, one with hexagonal symmetry (H-BSi3) and another with rectangular symmetry (RBSi3), have been predicted as the global minimum structures of B-substituted silicene with the stoichiometry B:Si = 1:3 and are rather feasible for experimental realization.26,27 Additionally, both H-BSi3 and R-BSi3 (named c-BSi3 and R-I in ref 26) have the planar geometry and are intrinsically metallic, which is in stark contrast to the low buckled and zero-band semimetallic pristine silicene sheet. In principle, the metallic feature and the better conductivity of the BSi3 silicene could facilitate its

Lithium-ion batteries (LIBs) are ubiquitously used in portable and telecommunication electronic devices and are also promising for electric vehicles and electric grid applications.1,2 The search for higher specific capacity electrodes is one of the central topics in LIBs,3−7 particularly in higher capacity alternatives for graphite anodes. The most commonly used anode in current commercial batteries is graphite, which has acceptable reversibility and charge capacity of ∼372 mA·h/g (Li atoms are stored as LiC6 and inserted between two graphene layers).8−10 Silicon is another promising anode material for high capacity LIBs because of its high theoretical specific capacity (4200 mA· h/g), which is ∼10 times larger than that of the conventional graphite anode.11−14 However, Li insertion causes irreversible structural changes and mechanical fracture in battery operation.14,15 Further studies have shown that many silicon nanostructures, such as nanoparticles,16 nanotubes,17,18 and nanowires,19−21 could overcome the mechanical instability issues and perform well as anodes. In particular, silicene, the silicon analogue of graphene, has been predicted theoretically to be the best silicon-based high capacity LIB anode material.22 The specific charge capacities of single-layer (SL) (954 mA·h/ g) and double-layer (DL) (715 mA·h/g) silicene are comparable to BC3 and significantly higher than that of graphite; meanwhile, the diffusion barriers of Li on the silicene surface are relatively low, typically less than 0.6 eV. © 2014 American Chemical Society

Received: April 12, 2014 Revised: August 4, 2014 Published: September 11, 2014 25836

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performance as LIB anodes.28,29 The B-substituted doping is an efficient approach to modify graphene as a better LIB anode, and then a natural question arises: could the intrinsically metallic BSi3 silicene serve as a better high capacity anode than pristine silicene for LIBs? Here we performed extensive DFT computations to explore the feasibility of using the intrinsically metallic BSi3 silicene (HBSi3 and R-BSi3) as a high capacity anode material for LIBs. Our results revealed that the capacity of the BSi3 silicene (1410 and 846 mA·h/g for SL and DL, respectively) is significantly higher, while the Li diffusion barriers on the BSi3 silicene (typically less than 0.4 and 0.6 eV for SL and DL, respectively) are comparable to those of silicene. Moreover, the average open-circuit voltage (OCV) of the BSi3 silicene is relatively low. All these characteristics, including good electronic conductivity, very high theoretical charge capacity, fast Li diffusion, and low OCV, indicate that the BSi3 silicene could serve as a promising high capacity and fast charge/discharge rate anode material for LIBs.

Figure 2. Top (upper) and side (lower) views of DL (a) H-BSi3 and (b) R-BSi3. The yellow and light magenta spheres denote the Si and B atoms, respectively. The unit cells of DL H-BSi3 and R-BSi3 are indicated by dashed lines, and the possible adsorption sites for Li on DL H-BSi3 are labeled in (a).

BSi3 silicene. In the following, we used the notation LixB0.25(1−x)Si0.75(1−x) to distinguish lithiated BSi3 compounds with different Li content x. To evaluate the deviation of adsorption energy with respect to Li content x, we calculated the average OCV for a given content x as follows

2. COMPUTATIONAL DETAILS Our DFT calculations employed the linear combination of atomic orbital and spin-unrestricted method implemented in the Dmol3 package.30 The generalized gradient approximation (GGA) in PW91 functional form31 together with an all-electron double numerical basis set with polarization function (DNP) were adopted. Since the standard PW91 function is incapable of giving an accurate description of weak interactions, we adopted a DFT+D (D stands for dispersion) approach with the Ortmann−Bechstedt−Schmidt (OBS) vdW correction in our computations.32 The real-space global cutoff radius was set to be 5.1 Å. For SL (Figure 1) and DL (Figure 2) H-BSi3 and R-BSi3, a 1 × 1 unit cell with periodic boundary conditions in the x−y plane was employed. The vacuum space was set to larger than 20 Å in the z direction to avoid interactions between periodic images. In geometry optimizations, all the atomic coordinates were fully relaxed up to the residual atomic forces smaller than 0.001 Ha/Å, and the total energy was converged to 10−5 Ha. The Brillouin zone integration was performed on a (8 × 8 × 1) Monkhorst−Pack k-point mesh.33 In order to investigate the lithiation of the BSi3 silicene, we defined the adsorption energy of Li on BSi3 silicene, Eads, as Eads = E(BSi3) + nE(Li_atom) − E(BSi3−nLi), where E(BSi3), E(Li_atom), and E(BSi3−nLi) are the total energies of bare BSi3 silicene, isolated Li atom, and lithiated BSi3 silicene with n Li atoms, respectively. According to this definition, a more positive adsorption energy indicates a stronger binding of Li to

OCV = [E(Li x1B0.25Si 0.75) − E(Li x2B0.25Si 0.75) + (x 2 − x1) ·E(Li_bulk)]/[(x 2 − x1) ·F ]

where E(Lix1B0.25Si0.75), E(Lix2B0.25Si0.75) are the total energies of lithiated BSi3 silicene compounds with Li content x1 and x2, respectively. F is the Faraday’s constant, and E(Li_bulk) is the total energy of bulk Li. According to our definition, a negative OCV indicates that the adsorption energy of Li atoms between x2 and x1 are smaller than the cohesive energy of bulk Li (1.71 eV), and the adsorbed Li atoms tend to form clusters on the BSi3 silicene surface. The energy barriers and transition states related to the Li diffusion on the BSi3 silicene surface were obtained using the synchronous method with conjugated gradient (CG) refinements.34 This method involves linear synchronous transit (LST) maximization, followed by repeated CG minimizations, and then quadratic synchronous transit (QST) maximizations and repeated CG minimizations until a transition state is located.

3. RESULTS AND DISCUSSION Here, we used H-BSi3 and R-BSi3, including SL (Figure 1) and DL (Figure 2), as the simulation models to assess the suitability of the BSi3 silicene as an anode material for LIBs. The SL HBSi3 is a strictly planar hexagonal honeycomb monolayer with lattice constant aSL‑H = 7.308 Å, and the SL R-BSi3 is a strictly planar rectangular monolayer with lattice constants aSL‑R = 6.307 Å and bSL‑R = 7.315 Å.26 Analyzing the electronic band structure and density of states (DOS) of SL H-BSi3 and R-BSi3 (see Figure S1 in Supporting Information) revealed that these planar nanostructures are metallic, and the states at the Fermi level originate mainly from Si-3p and B-2p, which are consistent with previous calculations.26,27 To investigate the structural stability of DL H-BSi3 and RBSi3, we considered both AA and AB arrangements (see Figures S2 and S3 in the Supporting Information). After full geometry optimization, the lowest-energy structures of DL H-BSi3 and RBSi3 (Figure 2) are quite similar. In contrast to planar SL H-

Figure 1. Top (upper) and side (lower) views of SL (a) H-BSi3 and (b) R-BSi3. The yellow and light magenta spheres denote the Si and B atoms, respectively. The unit cells of SL H-BSi3 and R-BSi3 are indicated by dashed lines, and the possible adsorption sites for Li on SL H-BSi3 and R-BSi3 are labeled in (a) and (b). 25837

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Figure 3. (a)−(e) Lowest-energy structures of lithiated SL H-BSi3, SL-H-LixB0.25(1‑x)Si0.75(1−x), with different Li content x. Top (upper) and side (lower) views are shown separately, and the unit cell is denoted in each top view. (f) Adsorption energy Eads of Li and (g) OCV as a function of Li content x for SL H-BSi3 (red), DL H-BSi3 (blue), and SL R-BSi3 (green).

BSi3 and R-BSi3, both the DL H-BSi3 and DL R-BSi3 are buckled. The lattice constants of DL H-BSi3 are aDL‑H = 7.326 Å and bDL‑H = 7.280 Å, and the average distance between two atomic layers is 2.30 Å. In comparison, the lattice constants of DL R-BSi3 are aDL‑R = 6.283 Å and bDL‑R = 7.387 Å, and the average distance between two atomic layers is 2.34 Å. The binding energies of DL H-BSi3 and R-BSi3 relative to two separated SL H-BSi3 and R-BSi3 are the same (0.30 eV/atom), which are much larger than DL silicene (0.19 eV/atom).22 Such a large binding energy confirms the high stability of the DL BSi3 silicene. The same as SL H-BSi3 and R-BSi3, the electronic band structure and DOS show that DL H-BSi3 and R-BSi3 are metallic, and the states at the Fermi level are mainly contributed by Si-3p and B-2p (see Figure S4 in the Supporting Information). We expect that such an intrinsically metallic feature of the BSi3 silicene (both SL and DL) leads to better conductivity than that of semimetallic pristine silicene, which could facilitate its performance as a LIB anode. 3.1. Li Adsorption and Diffusion on SL H-BSi3 Silicene. First, we discussed the adsorption of Li on SL H-BSi3. All the possible Li adsorption sites on SL H-BSi3, i.e., the hollow sites (H1 and H2), the top sites (TSi and TB), and the bridge sites, were considered. Different Li concentrations were examined. When the unit cell contains only a single Li atom (SL-HLi0.11B0.22Si0.67), the most stable adsorption site is H2 (Figure 3a), where the Li resides above the center of the hexagonal ring constituted with Si and B atoms. The corresponding adsorption energy (3.02 eV) is 0.16 and 0.38 eV higher than that at H1 and TB sites, respectively. Note that for other adsorption sites, i.e., TSi and bridge sites, the Li atom relaxed to a hollow site during the optimization. Our computations showed that each unit cell of SL H-BSi3 can favorably adsorb up to 10 Li atoms, and their lowest-energy configurations are given in Figure 3b−e. In the case of SL-HLi0.20B0.20Si0.60 (Figure 3b), two Li are adsorbed above and below a single H2 site with Eads = 2.94 eV and OCV = 1.15 V. At moderate Li content (SL-H-Li0.33B0.17Si0.50) (Figure 3c), four

Figure 4. In-plane diffusion of Li (red circle) on SL H-BSi3 at low Li content (SL-H-Li0.11B0.22Si0.67, (a) H1 → (b) H2 → (c) TB sites) and at high Li content (SL-H-Li0.38B0.15Si0.46, (e) H1 → (f) H2−1 → (g) H2−2 sites) and the corresponding diffusion pathways and barriers at (d) low and (h) high Li contents, respectively. H2−1 and H2−2 indicate two different configurations of Li at the H2 site at high Li content.

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Figure 5. (a)−(e) Lowest-energy structures of lithiated DL H-BSi3, DL-H-LixB0.25(1−x)Si0.75(1−x), with different Li contents (x = 0.06−0.43). Top (upper) and side (lower) views are shown separately, and the unit cell is denoted in each top view.

Figure 6. Li in-plane diffusion pathways on the top external surface of the DL H-BSi3 at (a) H1, (b) H2, (c) H3, and (d) H4 sites. The stars and arrows with different colors denote different adsorption sites and diffusion pathways (red, blue, black, and brown denoted H1, H2, H3, and H4, respectively).

Table 1. Energy Barriers Ediff of the Li in-Plane Diffusions on the Top External Surface of the DL H-BSi3a H1

Ediff (eV)

H2

Ediff (eV)

H3

Ediff (eV)

H4

Ediff (eV)

1 2 3 4 5 6

0.35 0.30 0.53 0.54 0.54 0.31

1 2 3 4 5 6 7 8

0.02 0.26 0.46 0.20 0.17 0.08 0.22 0.17

1 2 3 4 5 6 7 8

0.54 0.20 0.25 0.08 0.35 0.20 0.08 0.26

1 2 3 4 5 6 7 8

0.02 0.26 0.17 0.21 0.20 0.08 0.17 0.45

adsorption energy and OCV are 2.22 eV and 0.43 V, respectively. When further increasing the Li content (x > 0.56), the Li adsorption energy and OCV decrease significantly. For instance, at the Li content of x = 0.6, the adsorption energy and OCV are 2.08 eV and −0.33 V, respectively, indicating that the adsorption energy of Li between x = 0.56 and x = 0.6 is lower than the cohesive energy of bulk Li, and the clustering of adsorbed Li occurs. Therefore, we used SL-H-Li0.56B0.11Si0.33 to represent the fully lithiated structure of SL H-BSi3. The charge capacity of SL H-BSi3 is 1410 mA·h/g (see Figure S5 in Supporting Information), which is significantly higher than those of graphite (372 mA·h/g)8−10 and the best Si-based anode, SL silicene (954 mA·h/g).22 The average Li adsorption energy decreases with increasing lithiation ratio, from 3.02 for x = 0.11 to 2.22 eV for x = 0.56 (Figure 3f). Note that even at rather high Li content of x = 0.56 the Li adsorption energy (2.22 eV) is still higher than those of SL silicene (2.1− 2.3 eV),22 silicon nanowires (less than 1.6 eV),21 bulk crystalline silicon (1.65 eV),15 and graphene (1.7−2.1 eV)35 and is comparable to those of graphdiyne and graphyne (2.1− 3.1 eV).35,36 In addition, the OCV decreases from 1.31 to 0.43 V as the Li content increases (Figure 3g).

a The energy barriers of the diffusion pathways denoted with dashed arrows are in bold.

Li are adsorbed above and below two H2 sites with Eads = 2.44 eV and OCV = 0.23 V. For SL-H-Li0.50B0.13Si0.38 (Figure 3d), all the hollow sites are completely saturated with eight Li, with half of Li above and half of Li below the H-BSi3, and the corresponding adsorption energy and OCV are 2.24 eV and 0.47 V, respectively. At high Li content (SL-H-Li0.56B0.11Si0.33) (Figure 3e) where 10 Li atoms occupy a unit cell, eight Li are at the hollow sites, while two Li are adsorbed at TB sites with one Li above and one Li below the H-BSi3 monolayer; the 25839

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Figure 7. (a) Li out-of-plane diffusion pathways and (b) corresponding energy barriers between the external surfaces and interior of DL H-BSi3 at low Li content (DL-H-Li0.06B0.24Si0.71). The stars and arrows with different colors denote different adsorption sites and diffusion pathways (red, blue, black, and brown denoted H1, H2, H3, and H4), respectively. Note that the curves of H2 and H4 are overlapped in (b).

To summarize, both SL H-BSi3 and R-BSi3 have very high charge capacity (1410 mA·h/g) and small in-plane diffusion barriers (typically less than 0.4 eV). Thus, they could serve as promising high capacity anodes for LIBs. 3.3. Adsorption and Diffusion of Li on DL H-BSi3 Silicene. In order to examine the effect of the additional atomic layer on the BSi3 silicene anode for LIBs, we studied the adsorption and diffusion of Li on the DL structure of BSi3 silicene. Considering the similar lithiation and Li diffusion behavior of SL H-BSi3 and R-BSi3, we only discussed the adsorption and diffusion of Li on DL H-BSi3 in this paper, and we believe that all the trends for DL H-BSi3 are valid for DL RBSi3. Unlike the case of SL H-BSi3, many nonequivalent stable Li adsorption sites exist due to the symmetry loss in the buckled structure of DL H-BSi3 (Figure 2a). For example, two kinds of hollow sites are available in SL H-BSi3 (H1 and H2), while there are 12 hollow sites in DL H-BSi3: four on the top external surface, four in the interior of DL H-BSi3, and four on the bottom external surface. We considered all these nonequivalent stable Li adsorption sites in this work. First, we examined the most favorable Li adsorption configurations on DL H-BSi3 and their energetics at different Li contents. When a unit cell contains only a single Li atom (DL-H-Li0.06B0.24Si0.71, x = 0.06) Li prefers to adsorb at the hollow site H2 of the bottom external surface with Eads = 2.84 eV and OCV = 1.13 V (Figure 5a). With increasing Li content (x = 0.11−0.33), Li atoms gradually cover the two external surfaces followed by the hollow sites in the interior (Figure 5b− d). At the Li content of x = 0.43 (DL-H-Li0.43B0.14Si0.43) (Figure 5e), 12 Li atoms occupy all the exterior and interior hollow sites of DL H-BSi3, with Eads = 3.07 eV and OCV = 1.38 V. In this case, the average interlayer distance is 3.92 Å, indicating that the two atomic layers are still bound. Further increasing the Li content leads to a very large volume change of the DL hBSi3; for example, in DL-H-Li0.50B0.13Si0.38, the average distance between two atomic layers reaches 4.81 Å. Therefore, we used DL-H-Li0.43B0.14Si0.43 to represent the fully lithiated structure of DL H-BSi3. In the fully lithiated DL H-BSi3 (DL-H-Li0.43B0.14Si0.43), the charge capacity is 846 mA·h/g (see Figure S5 in the Supporting Information), which is higher than that of DL silicene (715 mA·

We next explored the Li diffusion on the SL H-BSi3. The lithium contents of 0.11 and 0.38 were chosen as representatives for low and high lithiation. Both in-plane diffusion (Li diffuses on the same side) and out-of-plane diffusion (Li diffuses from one side to another side through the SL H-BSi3) were considered. Figure 4 summarizes the optimized structures and the energy profile when Li is adsorbed at different sites together with the corresponding diffusion pathways and barriers for the in-plane diffusion process. At low Li content (SL-H-Li0.11B0.22Si0.67) (Figure 4a−d), the Li can easily diffuse from H2 to another H2 through the TB site (energy barrier for H2 → TB is 0.38 eV); however, the diffusion between H2 and H1 is relatively hard (energy barriers for H1 → H2 and H2 → H1 are 0.47 and 0.63 eV, respectively). At high Li content (SL-H-Li0.38B0.15Si0.46) (Figure 4e−h), the energy barriers for H1 → H2 and H2 → H1, which are high in the low Li content case, significantly decrease to 0.27 and 0.25 eV, respectively. Note that the distance (>4.2 Å) between two neighboring centers of the hexagonal rings of H-BSi3 is much larger than that of the Li−Li bond, and the interaction between Li atoms on the surface of H-BSi3 is rather small. Therefore, the effect of lithium to block the movement of other lithium ions of the H-BSi3 surface is not significant, similar to the case on silicene.22 In general, Li can diffuse inplane with relatively low energy barriers (typically less than 0.4 eV), which are smaller than those of bulk crystalline silicon (∼0.6 eV)15 and silicene (