Lithiation of Two Dimensional Silicon Carbide-Graphene van der

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C: Energy Conversion and Storage; Energy and Charge Transport

Lithiation of Two Dimensional Silicon Carbide-Graphene van der Waals heterostructure: A First Principles Study Bijoy Thoyikkottu Kuttiyattu, and Murugan Palanichamy J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b12492 • Publication Date (Web): 01 Apr 2019 Downloaded from http://pubs.acs.org on April 1, 2019

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The Journal of Physical Chemistry

Lithiation of Two Dimensional Silicon Carbide-Graphene van der Waals heterostructure: A First Principles Study T.K Bijoyab, P. Muruganab*

a

Academy of Scientific and Innovative Research (AcSIR), CSIR-Central

Electrochemical Research Institute, Karaikudi, Tamil Nadu, India, 630003

b

Functional Materials Division, CSIR- Central Electrochemical Research Institute,

Karaikudi, Tamil Nadu, India, 630003

ABSTRACT: In this work, we employed first principles density functional theory (DFT) calculations to understand the structural, electronic, and Li intercalation properties of 2D van der Waals (vdW) heterostructure consisting of single layers of graphene and silicon carbide (SiC). Our calculations show that both layers are interacting through weak vdW force, with the interlayer separation of 3.40 Å. It also reveals that Li atoms intercalate preferentially in between the SiC/graphene layer rather than adsorbing onto anyone of layers. As lithiation proceeds, the intercalated Li atoms interact each other and forming planar lithium clusters in between layers, rather than stabilizing in its stable 3D isomeric form, which indicates the suppression of lithium dendrite growth in the heterostructure. The insertion of this cluster does not cause any significant structural changes in the graphene, whereas the SiC layer is slightly distorted owing to the regaining of sp3 hybridized like Si. The Bader charge analysis and electronic structure calculations infer that most of the lithium’s charge is transferred to graphene until occupying its Dirac cone, beyond this limit, both layers are equally gain the charges. The average intercalation voltage of this heterostructure is deduced to be ~0.54 V, demonstrating that it can be used as an anode of Lithium Ion Batteries.

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INTRODUCTION

In present-days, the scientific research on various energy storage materials gets much attention as it provides fruitful solution to limiting usages of fossil fuels, which enable us to control the global climatic changes.1-3 Among various energy storages, the rechargeable battery is considered as a potential candidate for durable applications. In this category, Lithium Ion Battery (LIB) outperforms the conventional energy storage devices in terms of operational voltage, specific capacity, energy density, cycle life, and compact size.4 The major components of LIB are anode, cathode, and electrolyte; in which the performance of the battery mainly depends upon the nature of anode and cathode materials.5 Various transition metal oxide based materials are the well established for the cathode, while carbonaceous materials are commonly used in anode of LIB.6-15 However, for employing LIB as the high power electric devices, it is essential to enhance its overall specific capacity.16 In this regard, the considerable attention has been focused on various anode materials, for which the capacity can be improved far better than that of the cathode.17 After the rise of the well celebrated two dimensional (2D) graphene, which is having large surface-to-volume ratio and unique electronic properties, it is widely employed for LIB applications. The capacity of these graphene based anode is found to be much higher than that of graphite owing to its high Li uptake capability.18-21 However, one of the main drawbacks associated with graphene is faulty restacking owing to having weak π-π interaction between the layers.22-23 This resulted to decrease in lithium uptake capability, thereby capacity fading has occurred in LIB. Succeeding to the successful isolation of graphene from graphite, several other vdW 2D materials, such as silicene, germanene, boron nitride (BN) have been predicted theoretically and some of them are realized experimentally in laboratory.24-28 Hitherto, as the


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anode of LIB, the electrochemical performance of many of these materials are not improved significantly due to having their weak binding with Li ions.29-31 Recently, the vdW heterostructure consisting of stacked two or more individual 2D materials has shown to have improvement in the electrochemical performance. For instance, hexagonal-boron nitride (hBN)/black phosphorene heterolayers was predicted to have excellent anodic properties.31 Our previous work on SiC (111) surface demonstrate that 2D layers of SiC are exfoliated by lithiation.32 In addition, in recent times the 2D SiC, which is also known as silagraphene attracted many researchers due to the growing interest in diverse fields.33-35 Similar to graphitic SiC, various other tetrahedrally coordinated wurtzitic compounds such as ZnO, ZnS, GaN and AlN also reported being existed in graphitic phase.

36-38

Recently, the graphitic SiC

based nanostructures such as nanotube, nanosheets were realized in laboratory.39-40 Hence with these intuitions, we anticipated that this layered material could be a probable candidate for making heterostructure with graphene. By means of this inspiration, here we considered the heterostructure consisting of graphene and SiC layers. Although, Li intercalation in graphene on thick (0001) SiC surface was reported by few groups.41-43 To the best of our knowledge, there is no study to explain the structural stability and lithiation mechanism in SiC/graphene heterostructure. Although SiC is a direct band gap semiconductor; the superior electronic conductivity of the graphene may enhance the electrochemical performance of the heterostructure. Our results on this system demonstrate that the Li atoms preferentially occupy in between these two layers and this lithiation does not cause any significant structural change to the graphene, while the Si atom of the SiC is slightly buckled to regaining sp3 hybrid. Our calculations also show that the planar shaped lithium clusters are formed during the lithiation and they found to be more stable as compared to respective lithium 3D isomers. The deduced electronic band structures suggest that the system has excellent electronic conductivity and computed the average intercalation voltage (AIV) is found to be ~0.54 V, indicating anodic behavior of the heterostructure. COMPUTATIONAL METHODOLOGY:

In this work, first principles density functional theory (DFT) calculations are performed within the framework of plane wave based approach as implemented in the Vienna Ab-initio Simulation


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Package (VASP) code.44 Here all the atoms are described by projector-augmented wave (PAW) pseudopotential45 and generalized gradient approximations (GGA) are employed for correlating the electron-electron interactions.46 All the ions are relaxed self-consistently without considering any symmetry, and the iterative relaxation processes are repeated out until absolute forces on each ion are converged to less than 0.01 eV/Å. For the optimization of SiC/graphene heterostructure, we have chosen Monkhorst-Pack 3 × 3 × 1 k-meshes. To understand the effect of vdW interaction we have used nonlocal DFT-D2 functional throughout calculations.47 The convergence of energy is set to be 10−5 eV in all the calculations. In order to avoid the interaction of this heterostructure with its periodic image along z-axis we kept sufficiently large vacuum (≈15 Å) along this direction. The spin-polarized calculations are performed for all the systems which prefer to be stabilized in magnetic solution. Denser k-mesh (25× 25 × 1) is used for deducing the electronic density of states (DOS) and band structures. Further we calculated Bader charges for all the system by generating all electron charge density.48-49 Moreover, to understand the bond orders of the heterostructure we performed DDEC6 bond order analysis using Chargemol program.50 RESULTS AND DISCUSSION At the outset, we have optimized single layer model of graphene and graphitic SiC independently using first principles calculations. The obtained results are well matched with the previous experimental and theoretical reports.51-54 By comparing their lattice parameters (a = b = 2.46 Å for graphene and a = b = 3.09 Å for SiC), we understood that the unit cell model of these layers has ~20.1% lattice mismatch. To minimize this, we constructed the supercell structure of both graphene and SiC. In this regard for graphene sheet we have taken (5×5) and for SiC (4×4) supercells were constructed. These layers are stacked together to form SiC/graphene heterostructure. The optimized lattice parameter of this heterostructure is found to be 12.35 Å (shown in Figure S1, Supporting Information). Here, the optimum Si-C and C-C bond distances are found to be 1.78 and 1.42 Å, respectively. Along z-direction the interlayer distance between of SiC/graphene heterostructure is 3.82 Å (without vdW), and the same obtained by including vdW correction is reduced to 3.40 Å. Moreover, both layers remain perfectly planar without any buckling of constituent atoms. Further to understand the strength of binding between these two


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layers in the heterostructure, we calculated the binding energy/atom (Eb) of the heterostructure as follows, 𝐸𝑏 =

[𝐸(𝑆𝑖𝐶/𝑔𝑟𝑎𝑝ℎ𝑒𝑛𝑒)] ― [𝐸(𝑆𝑖𝐶) + 𝐸(𝑔𝑟𝑎𝑝ℎ𝑒𝑛𝑒)] 𝑁

(1)

Here, E(SiC), E(graphene), E(SiC/graphene) and N are the dispersion corrected total energies of single layer SiC, graphene, their combined layers, and total number of atoms present in the heterostructure, respectively. Our calculated Eb is found to be -28 meV/atom, indicating that the SiC/graphene heterostructure is energetically stable. This optimized heterostructure is chosen for studying the interaction with Li atoms. For this, we considered three possible sites, such as, (1) above the graphene layer, (2) in between the SiC/graphene layers, and (3) above SiC layer. For understanding the most feasible site for Li adsorption, we calculated Li adsorption energy (Ead) as follows. 𝐸𝑎𝑑 =

[𝐸(𝑆𝑖𝐶/𝑔𝑟𝑎𝑝ℎ𝑒𝑛𝑒) + 𝐸(𝐿𝑖)] ― 𝐸(𝑛𝐿𝑖_𝑆𝑖𝐶/𝑔𝑟𝑎𝑝ℎ𝑒𝑛𝑒 ) 𝑛

(2)

Here E(SiC/graphene), E(nLi_SiC/graphene), E(Li) and n are the total energy of SiC/graphene heterostructure, lithiated SiC/graphene heterostructure, atomic energy of Li atom and number of added Li atoms, respectively. The calculated Ead for aforesaid three cases are 1.72, 2.68, and 2.07 eV, respectively. It infers that the first Li atom preferentially occupies at the interlayer of the heterostructure and the optimized structure is shown in Figure 1(a). To compare the binding strength of Li atom with this heterostructure, we also carried out the calculations for Li atom inserted in between the bilayered SiC and graphene (homostack), independently and the optimized structures are presented in Figure S2 (refer Supporting Information). The corresponding Ead values are estimated to be 1.73 and 2.07 eV, respectively. It clearly proves that the heterostructure consisting of graphene and SiC has a stronger binding affinity towards Li atoms rather than their individual bilayers.


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Figure 1. Optimized structures of (a) non-lithiated, (b) one, (c) two, (d) three, (e) four, (f) five, (g) six, (h) seven lithium atoms intercalated SiC/graphene heterostructures are shown. Grey and green colored balls represent C and Li atoms, respectively. Graphitic SiC layer is shown as stick model.

This enhancement of Ead while making the heterostructure may be due to the synergistic effect of its constituent layers; the similar behavior was also observed in the case of various other vdW heterostructure investigated earlier.55-56 From the optimized structure of lithiated SiC/graphene heterostructure, it is also evident that the Li atom establishes the bond with six carbon atoms from the graphene layer and one C atom from the SiC. Here the mean values of measured C-Li bond distances for graphene and SiC are 2.30 and 2.41Å, respectively, indicating that the Li atom strongly interacts with the graphene layer. At the same time observed Si-Li bond distance is estimated to be 2.51 Å.


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Figure 2. Ead values for lithium adsorption are given. Black line shows all lithium atoms occupied at interface of SiC/graphene heterostructure. Red (blue) line indicates one lithium adsorbed on graphene (on SiC) and remaining lithium atoms are intercalated at interfaces.

Following to the intercalation of first Li atom, we introduced more Li atoms, one by one, to occupying the various possible sites of the heterostructure. The optimized structures of the stable configurations are presented in Figure 1. The Ead values for various lithiated SiC/graphene heterostructure is also reported in Table 1 (also shown in Figure 2). The added second Li atom is also preferred to occupy at the interface and it is ~4.68 Å apart from the first Li atom. However, for three Li added case, the most stable configuration is one in which two Li atoms form the lithium dimer and another Li atom remains non-bonded with those two atoms. It is interestingly observed that an introduction of third Li atom causes slight upward buckling of Si atom of the SiC layer as shown in Figure S3 (refer Supporting Information). As a result the mean value of local Si-C bond distances is increased from ~1.79 Å (graphitic SiC) to ~1.89 Å, which is almost equal to the Si-C bond distance in sp3 hybridized SiC32. We also noted that in this case, the system shows magnetism with spin moment of ~0.71 µB and it is mainly located on the buckled Si atom as shown in Figure S4 (refer Supporting Information). In a similar way the magnetic solution is energetically favored for Lin clusters with n = 5, 6, and 7 and magnetic moments are reported in Table 1. As lithiation proceeds, we could observe that the clustering of Li atoms is also increased. For instance, the Li atoms form a rhombus shaped Li4 cluster at the interface of SiC and graphene layers. Similar observation was already reported on the surface of graphene in earlier study.57 Nevertheless, for ensuring the stability of this particular Li4 rhombus cluster at SiC/graphene interface, we studied several other configurations four Li atoms in the heterostructure and their relative energies are compared as demonstrated in Figure S5 (Supporting Information). From this study we confirmed that the intercalation of four Li atoms into the heterostructure lead to the formation of Li4 rhombus at the interfacial site of heterostructure. From Figure 1, it is very clear that the beyond n=4, the Li atoms prefer to occupy in the vicinity of this Li4 rhombus cluster as we introduce more Li into this heterostructure. Therefore, for n=7 case, they stabilizes in a planar hexagon shaped geometry in between the SiC and graphene layer as bestowed in Figure 1(h); here the mean Li - Li bond distance is calculated to be 2.79 Å. As previously stated, the interlayer region is more preferred for lithiation, we have also carried out the calculations on two configurations, such as, 1) one Li


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adsorbed on graphene and remaining Li atoms are intercalated at the interlayer and 2) one Li adsorbed on SiC layer and remaining Li atoms occupied at interface. The obtained Ead values for all three configurations are compared in Figure 2, which infers that as lithiation continues, the difference in Ead values of former and latter cases is gradually reduced, however those values are lesser when compared to all Li atoms occupied at interface. Table 1. Adsorption energy per Li atom (Ead), C-C, Si-C, C-Li, Si-Li, Li-Li lithiated SiC/graphene heterostructure are provided. Here * denote the case enlisted. Ead C-Li C-C Si-C C-Li (eV/Li (Å) System (Å) (Å) (Å) atoms)

bond distances and spin moment of where the local bond distances are Si-Li (Å)

Li-Li (Å)

Spin moment(μB)

SiC/graphene

-

1.43

1.78

-

-

-

-

0.00

SiC/graphene-1Li

2.68

1.43*

1.79*

2.30

2.41

2.51

-

0.00

SiC/graphene-2Li

2.33

1.44*

1.79*

2.33

2.46

2.60

4.68

0.00

SiC/graphene-3Li

2.34

1.44*

1.85*

2.36

2.37

2.64

2.54

0.71

SiC/graphene-4L

2.28

1.44*

1.90*

2.40

2.38

2.61

2.65

0.00

SiC/graphene-5Li

2.31

1.44*

1.89*

2.40

2.37

2.64

2.75

1.44

SiC/graphene-6Li

2.27

1.44*

1.92*

2.41

2.39

2.63

2.80

0.79

SiC/graphene-7Li

2.26

1.44*

1.91*

2.40

2.45

2.65

2.79

2.65

As discussed earlier, even though the Li intercalation leads to slight buckling on the local structure of SiC layer, it does not rupture of any of the Si-C bonds as observed in lithiation of bulk or surface of SiC32. In overall, we have noticed that the Ead values vary from 2.68 to 2.26 eV for n = 1 – 7, suggesting that the first Li atom binds strongly with the heterostructure. Howbeit, as more Li atoms are intercalated we could observe accountable reduction in Ead on comparing to that of first Li atom, owing to interaction between the Li atoms. Obviously, this clustering is unavoidable during the lithiation of SiC/graphene heterostructure. It is worthy to note that the planar shaped cluster is formed in the heterostructure is having the mean Li-Li bond distance of ~2.80 Å (refer Table.1), which is quite similar to those Li-Li atomic separation in well established cathode material like LiCoO2.58 Further to compare the relative stability of planar (2D) and 3D shaped lithium clusters, we also optimized 3D shaped Li5, Li6 and Li7 cluster inserted SiC/graphene heterostructures. The optimized structures are shown in Figure 3. The relative stability is quantified by calculating the relative energy (∆𝐸) as, ∆𝐸 = 𝐸(𝐿𝑖𝑛𝑆𝑖𝐶/𝑔𝑟𝑎𝑝ℎ𝑒𝑛𝑒 ) ―𝐸(𝑛𝐿𝑖_𝑆𝑖𝐶/𝑔𝑟𝑎𝑝ℎ𝑒𝑛𝑒 )

(3)


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Here, 𝐸(𝐿𝑖𝑛𝑆𝑖𝐶/𝑔𝑟𝑎𝑝ℎ𝑒𝑛𝑒 ) and 𝐸(𝑛𝐿𝑖_𝑆𝑖𝐶/𝑔𝑟𝑎𝑝ℎ𝑒𝑛𝑒 ) denote the total energies of 3Dshapeed Lin cluster inserted SiC/graphene heterostructure and corresponding successively lithiated SiC/graphene heterostructure, respectively.

Figure 3. Optimized structures of pristine Li5 (a), Li6 (d) and Li7 (g) clusters. (b), (e) and (h) correspond to aforesaid cluster inserted SiC/graphene heterostructure; (c), (f) and (i) is the energetically more favored structure obtained by successively lithiating SiC/graphene heterostructure for four, five and five Li atoms.

The calculated ∆E (provided along with Figure 3) indicates that in all the three cases, the 3D shaped Lin inserted heterostructure is energetically less stable than that of corresponding 2D cluster formed cases even though in the pristine form all three aforesaid clusters prefer 3D geometry. Albeit, for the pentagonal bipyramidal shaped Li7 cluster inserted case, we found that the initial geometry of the cluster is almost converted into planar shaped one even though it is relatively higher in energy (∆E = 1.02 eV) than that of sequentially lithiated case. Thus, our calculations clearly demonstrate that during lithiation, forming 3D lithium clusters in the heterostructure is less probable; instead of the Li atoms prefer to stabilize as a planar sheet form. This result reveals the controlling lithium dendrite growth in this heterostructure, which is the important criterion for usage of materials as electrode of LIBs. Further, to study the charge transfer in this lithiated heterostructure we calculated Bader charge analysis and the obtained results are shown in Figure 4 (c). Note that, the charge transfer from Li to SiC is computed by considering the Bader charge of both Si and C after lithiation. According to our calculations, each Li atom added to the heterostructure donates ~ 0.84 e-; which is almost the complete charge transfer from the 2s electron of Li. In the commencement Li to


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graphene charge transfer is predominant since the added Li atom is strongly interacting with the delocalized π electron cloud of the graphene sheet via cation-π interaction.59-60 From Figure 4(c), it is clear that for the intercalation of first two Li atoms, the SiC layer gains marginally fewer electrons from the Li atoms, when compared to graphene. However, by the addition of the third Li atom onwards, SiC layer gains the charge from the Li. It is also noted that when n = 4 onward, the charge transfer from Li to graphene as well as SiC layer is almost equal. The schematic of charge transfer from Li to SiC/graphene heterostructure is shown in Figure 4(a) and (b), which also depicts that both layers receive charges from Li atoms. Therefore the intercalated Li atoms stro ngly adso rbed with in the hete rostr uctu re.


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Figure 4. The top (a) and lateral (b) view of charge transfer from the intercalated Li7 to SiC/graphene heterostructure. The yellow and cyan isosurface denote excess and depletion charge densities, respectively. (c) the Bader charge of the SiC and graphene layers of the lithiated heterostructure, the blue(red) lines in the graph correspond to charge transfer from Li to SiC(graphene) layers, δQB = QB(LinSiC/graphene) - QB(SiC/graphene) (d) partial charge density calculated in the energy range –0.40 to –0.30 eV of an SiC/Gr-Li4 heterostructure.

Table 2. The average bond orders various bonds in pristine and lithiated SiC/graphene heterostructure. System

BO of Si-C

BO of C-C

BO of Si-Li

BO of C-Li

BO of Li-Li

Diamond (sp3- C)

-

0.77

-

-

-

Graphene (sp2- C)

-

1.19

-

-

-

Acetylene (sp- C)

-

2.87

-

-

-

Bulk SiC (sp3- C & Si)

0.82

-

-

-

-

1.19

-

-

-

-

Pristine Li4 cluster

-

-

-

-

0.36

SiC/graphene SiC/graphene-1Li SiC/graphene-2Li SiC/graphene-3Li SiC/graphene-4Li SiC/graphene-5Li SiC/graphene-6Li SiC/graphene-7Li

1.19 1.18* 1.17* 1.16* 1.16* 1.15* 1.14* 1.14*

1.19 1.19 1.19 1.19 1.20 1.20 1.20 1.20

0.04 0.03 0.11 0.12 0.11 0.12 0.12

0.05 0.05 0.07 0.07 0.07 0.07 0.07

0.01 0.01 0.01 0.01 0.01

Graphitic SiC (sp2- C & Si)

Furthermore, to explore the nature bonding of this lithiated heterostructure we performed DDEC6 bond order (BO) analysis and the same is enlisted in Table 2. In addition to that, we also performed BO analysis for diamond (sp3 hybridized C), graphene (sp2 hybridized C), acetylene (sp hybridized C), bulk SiC (sp3 hybridized C and Si), graphitic SiC (sp2 hybridized C and Si), and pristine Li4 cluster for reference. Our calculations reveal that the BOs of C-C and Si-C in pristine SiC/graphene is 1.19 and 1.20, respectively. These values are same as those obtained for isolated graphene and graphitic SiC, indicating that they are sp2 hybridized. Table 2 shows that the BO of both C-Li and Si-Li bonds are gradually increased as the results of formation of both C-Li and Si-Li bond. For C-Li bonds, the BO is in the range of 0.05 to 0.07 while for Si-Li the value is in the range of 0.03 to 0.12. The sum of bond order (SBO) of C and Si atoms in the pristine SiC/graphene heterostructure is found to be ≈4 (not provided in Table). It is interesting to note that when more Li atoms intercalated into the


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heterostructure, the SBO of the buckled Si atoms are marginally increased from 4 to 4.7. In such cases, the BO of the particular Si-C bonds in the Li interacting region is gradually reduced due to the increase of Si-C bond distances. The BO of the formed Li-Li bonds in this heterostructure is found to be relatively less (≈0.01), which is far lesser than that of pristine Li4 cluster, this is due to the almost complete charge transfer from the Li to heterostructure, so that only fraction of electrons are involved in forming Li-Li bonds at the interface of heterostructure. Next, the electronic band structures (BS) of pristine and lithiated SiC/graphene heterostructure are deduced to unveil their electronic properties and the same is shown in Figure 5. Both DOS (shown in Figure S6, Supporting Information) and BS of pristine SiC/graphene indicate that the system is metallic in nature. Since the layer to layer interaction is weak in this heterostructure, the Dirac cone of the graphene layer is not affected and it is exactly located at the -point of the Brillouin zone. By comparing the band structure pristine SiC/graphene heterostructure with that of single layer graphitic SiC (shown in Figure S7, Supporting Information), we confirmed that the valence band maximum (VBM) and conduction band minimum (CBM) of the SiC is located at Ef - 0.26 eV (just below the Dirac point) and Ef +1.57 eV respectively, and these bands show excellent dispersion. However, it can be seen from Figure 5 (b) that the Dirac point of the graphene is shifted to the valance band region upon Li intercalation of this heterostructure. This is attributed to the charge transfer from Li to graphene. Albeit, we have observed significant changes in the conduction band of the SiC as lithiation continues. We have plotted the spin polarized band structures for all cases, which favor magnetic solutions (n = 3, 5, 6 and 7). Notably, from n = 3 onward a discrete band is appeared in the BS and more such discrete bands are increased for further introduction of Li atoms into this heterostructure. To identify the origin of such discrete bands, we plotted partial charge densities in the energy interval of –0.40 to –0.30 eV for four Li intercalated SiC/graphene and the same is shown in Figure 4(d). This depicts that the band is mainly coming from the buckled Si atom. To some extent, the charges are also distributed on surrounding C atoms. Overall, the BS analysis shows that the SiC bands are significantly affected as more Li atoms are intercalated to this heterostructure due to the Li to SiC charge transfer. This is well consistent with the Bader charge as well as structural analysis discussed earlier.


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Figure 5. Electronic band structures of (a) non-lithiated, (b) one, (c) two, (d) three, (e) four, (f) five, (g) six, (h) seven lithium intercalated SiC/graphene heterostructure. The spin polarized band structures are plotted for cases where magnetic solutions are energetically more favored. In the case of spin polarized calculations, the upspin(downspin) bands are shown by blue(red) lines.


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Figure 6. Average intercalation voltage plot for SiC/graphene heterostructure.

Next, to explore the electrochemical properties of this heterostructure we calculated Average Intercalation Voltage (AIV)61 as AIV = ―

[𝐸𝐿𝑖𝑢@(𝑆𝑖𝐶/𝐺𝑟𝑎𝑝ℎ𝑒𝑛𝑒 ) – 𝐸𝐿𝑖𝑣@(𝑆𝑖𝐶/𝐺𝑟𝑎𝑝ℎ𝑒𝑛𝑒 )] –(𝑢 ― 𝑣)µ𝐿𝑖 (𝑢 ― 𝑣)𝐹

Here 𝐸𝐿𝑖𝑚@(𝑆𝑖𝐶/𝐺𝑟𝑎𝑝ℎ𝑒𝑛𝑒 ), 𝐸𝐿𝑖𝑛@(𝑆𝑖𝐶/𝐺𝑟𝑎𝑝ℎ𝑒𝑛𝑒 ), µ𝐿𝑖and F represent total energies of u and v Li intercalated SiC/graphene heterostructure, chemical potential of bulk Li and the Faraday constant respectively. The obtained AIV profile is presented in Figure 6; which infers that for the first Li intercalation the corresponding AIV is comparatively larger (≈0.96 V) and this value is reduced to 0.61 V for the second Li added case. This higher value for the first two Li atom intercalation is attributed to the strong Li adsorption within the heterostructure. However, the AIV is gradually reduced for further lithiation and the value is almost saturated to ≈0.54 V beyond n = 3. This is possibly due to the formation of Si-Li bonds, for which lithiationdelithiation voltage is lower.62 Since the obtained AIV is below 1.0 V, this material could be coupled with high voltage cathode material for designing LIB with higher operational voltage. CONCLUSIONS In summary, we performed dispersion corrected DFT calculations to explore the atomic structure, electronic and electrochemical properties of SiC/graphene vdW heterostructure for LIB anode applications. Our systematic investigations demonstrate that SiC and graphene layers form stable vdW heterostructure and both layers are separated by ~3.40 Å. The non-bonded interface between the graphene and SiC layers is found to be the most favorable site for deposition of lithium atoms. As insertion of more lithium atoms proceed, the buckling in SiC layer was observed, while such distortion is not observed in the graphene layer. Importantly, we have observed the formation of 2D-shaped Li clusters during Li intercalation and they were found to be relatively more stable as compared to their 3D isomers, indicating that the suppression of growth of Li dendrites. The Bader charge analysis shows that each Li atom transfers ~0.84 e-, which is shared by both graphene and SiC layers. The deduced electronic structure evidences this charge transfer by shifting of Dirac cone of the graphene and occupying of Si-3p-band from the conduction band region. In addition by performing DDEC6 BO analysis, we ensured the


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formation of C-Li and Si-Li bonds during the lithiation of this heterostructure. It is also found from this analysis that the calculated BO of Li-Li bond is relatively low, confirming that Li atoms are weakly interacted each other in these clusters. To understand the electrochemical behavior, AIV of heterostructure is calculated to be ~0.54 V, indicating its anodic behavior. ASSOCIATED CONTENT

Supporting Information. The following files are available free

of charge.

(1) Lattice constant vs energy plot for the SiC/graphene heterostructure; (2) the optimized structures of Li intercalated bilayers of graphene and SiC; (3) optimized structure of three Li atoms intercalated SiC/graphene heterostructure, various bond distances are labeled in respective sites; (4) spin density plot of various lithiated SiC/graphene heterostructure; (5) Atomic structures of various configurations of four Li intercalated SiC/graphene heterostructure and corresponding relative energy plot (6) Electronic density of states of pristine and lithiated SiC/graphene heterostructure (7) Electronic band structure of single layer SiC. AUTHOR INFORMATION

Corresponding Author

*Dr. P. Murugan. Email: [email protected] Phone: +91-4565-241443. Fax: +91-4565-227779. Author Contributions The manuscript was written through equal contributions of both authors. All authors have given approval to the final version of the manuscript. Notes


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The authors declare no competing financial interest. ACKNOWLEDGMENT Authors acknowledge CSIR-CECRI for providing High Performance Computing facility. TKB is grateful to Dr. Vijay Kumar Foundation, Gurgaon, India for the excellent hospitality where a part of this manuscript was written. REFERENCES (1) Wang, H.; Hu, Y. H. Graphene as a counter electrode material for dye-sensitized solar cells. Energy & Environmental Science 2012, 5 (8), 8182-8188, DOI: 10.1039/C2EE21905K. (2) Suh, S. Are Services Better for Climate Change? Environmental Science &

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TOC: Charge transfer from Li to SiC/graphene heterostructure.


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Charge transfer from Li to SiC/graphene heterostructure. 24x19mm (300 x 300 DPI)


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Lithiation of Two Dimensional Silicon Carbide-Graphene van der

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