Article pubs.acs.org/JPCC
Exploring the Mechanism of Spontaneous and Lithium-Assisted Graphitic Phase Formation in SiC Nanocrystallites of a High Capacity Li-Ion Battery Anode T. K. Bijoy,†,‡ J. Karthikeyan,†,‡ and P. Murugan*,†,‡ †
Functional Materials Division, CSIR-Central Electrochemical Research Institute, Karaikudi 630003, India Academy of Scientific and Innovative Research (AcSIR), CSIR-Central Electrochemical Research Institute, Karaikudi 630003, India
‡
S Supporting Information *
ABSTRACT: Herein, we employed first-principles density functional theory calculations to understand the structural, electronic, and magnetic properties of pristine and lithiated zinc blende (ZB) SiC(111) surface slabs. Our calculations on below four layer thick slabs reveal the spontaneous formation of a graphitic SiC layer which mimics the two-dimensional boron nitride structure. Though this monolayer shows a direct band gap, the energy bands in bi- and trilayer slabs are nondegenerated owing to weak van der Waal’s interaction between the layers, and they show indirect band gap for those cases. In a pristine slab, the surface states presented in both sides originate magnetism, and they are coupled antiferromagnetically. Its strength decreases with increasing layer thickness. This magnetism is quenched during lithiation and exfoliation of layers. The latter is observed, even for thicker ZB slabs during lithiation. The average lithium intercalation potential is calculated to be 0.20 V, which is quite comparable with the anodic potential of high capacity SiC nanoparticles as reported in experiment. Thus, the mechanism of lithiation in SiC nanoparticles is proposed to be intercalation, rather than alloying.
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INTRODUCTION In the current scenario, the serious concern about energy and environment provided a huge momentum for researchers to explore new materials with tunable properties to meet the demands of energy conversion and storage devices.1 Especially, lithium-ion battery (LIB) is the most preferred option for electrical energy storage in portable electronics and plug-in hybrid electric vehicles, due to its higher specific capacity, better operational voltage, and compact size.2,3 Though LIBs have been widely used, the improvement in the performance of its components, particularly that of anode, is still essential because the theoretical capacity of silicon having the highest value of 4200 mAh/g has not been achieved so far.4 Even so, its poor interparticle conductivity and colossal volume change during the lithiation process limit the application in practical devices due to the capacity fading within few redox cycles. Thus, graphite is well commercialized as it has good intercalation chemistry and long cycling life.5 Regardless, its lower specific capacity (372 mAh/g) impedes the application in high energy devices.6 Hence, there were many attempts using other carbon nanostructures having porous nature and large surface area, such as carbon nanotube and graphene, respectively. Even though these nanostructures have significantly improved the capacity up to 1000 mAh/g,7,8 its commercialization is not viable. An alternative to these, the nanostructures of silicon are considered to enhance the capacity toward the maximum value.9,10 In this direction, theoretical studies demonstrated that © 2017 American Chemical Society
silicene, a cousin of graphene with similar honeycomb layered structure,11,12 has better Li-ion adsorption ability and volumetric efficiency,13−15 though freestanding silicene is not so far achieved in experiment owing to its weak π bonding character in the sp2-hybridized state. However, its formation was theoretically evidenced as an intermediate structure during lithiation of Si nanowires that shows the highest capacity among various morphologies of silicon nanoparticles.16 As discussed earlier, the poor interparticle conductivity in such nanostructures is overcome by synthesizing the carbon−silicon nanocomposites or carbon coating over them.17−22 In such attempts, Kumar and his group22 unexpectedly noticed the formation of SiC nanoparticles showing a decent capacity of 1200 mAh/g with long cycle life. Being a wide band gap semiconductor and good heat resistor, bulk SiC is commonly used as electrical and thermal insulators.23 However, it becomes the ugly duckling for LIB applications after unveiling its electrochemical activity in the nanoregime.22,24,25 Interestingly, the polytype conversion from cubic to hexagonal was observed in the SiC nanoparticle after the complete cell discharge.22 Hitherto, there is no attempt to understand the mechanism of lithiation in SiC nanoparticles. In this study, first-principles density functional theory (DFT) calculations are employed to understand the structural and Received: May 10, 2017 Revised: May 28, 2017 Published: June 21, 2017 15106
DOI: 10.1021/acs.jpcc.7b04489 J. Phys. Chem. C 2017, 121, 15106−15113
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Figure 1. Ball and stick models of optimized structures of SiC (a) monolayer, (b) trilayer, and (c) four-layer ZB slab with spin density distribution. Gray and purple colored balls represent C and Si atoms, respectively. Cyan and yellow isosurfaces refer to up- and down-spin charge densities. (d) Energy per layer (EL) values are plotted for various thicknesses of ZB and graphitic SiC slabs, along with the exchange energy (J) values of respective ZB slabs. One layer of SiC is also illustrated in (c).
optimization of the surface slabs is performed in such a way that they are allowed to relax in all directions without constraining the lattice parameter. The spin-polarized calculations are performed for all the systems which prefer to be stabilized in magnetic solution. Further, denser k-mesh (150 × 150 × 1) is employed for deducing the electronic density of states (DOS) and band structures.
electrochemical properties of pristine and lithiated zinc blende (ZB) SiC(111) surface slabs that can mimic the nanocrystallite. Our calculations on pristine slabs having distinct thicknesses show that the ZB phase is spontaneously transformed to the graphitic phase in ultrathin slabs as reported earlier.26 Similar to this, exfoliation of graphitic SiC layers is observed during lithiation in thicker ZB surface slabs. The average intercalation potential (AIV) is calculated to be 0.20 V, which is consistent with that of the experimental observation on SiC nanoparticles.22 This work reveals that the Li atom is preferably intercalated in between the SiC layers, and the graphitic SiC layers are evidenced as an intermediate structure between two polytypes.
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RESULTS AND DISCUSSION Initially, we deduced the structural and electronic properties of bulk ZB SiC using the aforesaid methodology to verify the consistency of our calculations. The obtained cell parameters are a = b = c = 4.37 Å and α = β = γ = 90°, and the energy band gap is found to be 2.40 eV, which is in good agreement with previous experimental and theoretical works.31−33 In bulk SiC, C(Si) atoms are tetrahedrally coordinated with four sp3hybridized Si(C) atoms. The measured Si−C bond length (1.91 Å) and Si−C−Si or C−Si−C bond angle (109.4°) are uniform throughout the structure. Though it is electrochemically inactive, we inserted Li atoms to understand the structural stability during lithiation. The lithiation of 12.5% per SiC unit collapses the structural network by forming Si clusters, which resulted in large volume change, as shown in Figure S1 (refer to Supporting Information). However, wurtzitic 6H-SiC is found to be fairly stable toward lithiation. In this case, the Li atom occupies the tetrahedral void presented between the SiC layers, without affecting the Si−C bonds along the x and y directions. However, the cell parameter along the z-axis is increased by 1.95 Å (refer to Supporting Information Table S1). This persuaded us to study the interaction of Li atoms with (111) ZB SiC surface slabs that can mimic the crystallite of SiC nanoparticles. Here, (111) facets are only considered as they are structurally similar to that of the (0001) surface of 6H-SiC.
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COMPUTATIONAL METHODS All the computations are performed within the plane wave based DFT approach as implemented in the Vienna Ab-initio Simulation Package (VASP).27 All the atoms are described by projector-augmented wave pseudopotentials, and electron− electron interactions are correlated by generalized gradient approximations.28,29 For sampling the Brillouin zone, Monkhorst−Pack 9 × 9 × 9 and 15 × 15 × 1 k-meshes are chosen for optimizing the bulk ZB SiC and (111) surface slabs (both pristine and lithiated cases), respectively. All the ions are relaxed self-consistently without considering any symmetry, and the iterative relaxation processes are carried out until absolute forces on each ion are converged to less than 0.01 eV/Å. We have also included the nonlocal DFT-D2 functional30 for incorporating van der Waal’s correction wherever it is required. The convergence of energy is set to be 10−5 eV throughout our calculations. Sufficiently large vacuum (≈15 Å) is kept along the z axis in the case of surface slabs to avoid the interaction between the slab and its periodical images. The geometry 15107
DOI: 10.1021/acs.jpcc.7b04489 J. Phys. Chem. C 2017, 121, 15106−15113
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The Journal of Physical Chemistry C In fact, it is the most stable due to the close packing of constituent atoms. In this regard, we constructed ZB SiC(111) surface slabs with various thicknesses, n = 1−10, where n represents the number of Si−C layers as depicted in Figure 1. The optimized structures of ultrathin slabs (n < 4) clearly show the spontaneous structural transition from ZB to graphitic layers as observed in earlier work.26 The structure of the latter is perfectly planar, and the Si−C bond distance is shorter (1.79 Å) than that of bulk (= 1.91 Å). Note that here the C (Si) atoms bind with three Si (C) atoms through sp2 hybridization, evidenced from the Si−C−Si (or C−Si−C) bond angle of 120°. In addition, we have considered the van der Waal’s (vdW) correction for understanding the layer to layer interaction. For bi- and trilayer cases, the dispersion-corrected interlayer distance is found to be ≈3.30 Å, indicating the weak vdW interaction between them. The calculated layer to layer interaction energy for graphitic SiC layers is very small which also supports the vdW interaction (refer to Supporting Information ST 2). Though such a structural transition is not observed for n ≥ 4 cases, we compared the energy per layer (EL) of stacked graphitic SiC layers with that of the ZB slab, having an equal number of atoms. The calculated EL of the four-layer graphitic case is 0.1 eV/f.u. (formula unit) higher than that of the corresponding ZB slab. To compare further, the EL values of graphitic and ZB phases are together plotted against n in Figure 1. The EL of the graphitic phase is not significantly changed with n, because of the weak interaction between the layers, whereas for the ZB slab, it is decreased exponentially as they are firmly bonded in the bulk region. In the stoichiometric (111) slabs (n ≥ 4), one side of the surface is terminated with three coordinated C(3c) atoms, and the opposite side is with Si(3c) atoms. Since the electronegativity of C is higher than that of Si, the charge distribution and atomic relaxations are expected to be different for these two surface terminations. For the detailed study, a six layer thick ZB SiC(111) surface slab is considered as a representative case. The interlayer Si−C distances in this slab are calculated for the optimized structure and reported in Table 1. It shows that this distance on the C(3c) side (= 1.97 Å) is longer as compared to that on the Si(3c) side (= 1.92 Å). In fact, the latter is almost equal to the bulk value. Further, the Si−C bond along the surface (x and y directions) on the C(3c) side (= 1.84 Å) is quite a bit shorter than on the Si(3c) side (= 1.92 Å) and bulk SiC (= 1.91 Å). Due to this variation in Si−C bond distances of surface atoms, the Si−C(3c)−Si and C−Si(3c)−C bond angles are observed to be 115.10° and 110.10°, respectively. Note that the latter is not much deviated from the bulk value (109.38°), while the former is close to that of sp2. Thus, both the Si−C bond distance and bond angle on the C(3c) side confirm the greater tendency for graphitic phase formation on this side. To understand the role of charge transfer in these uneven relaxations, the Bader charge34 is calculated for the bulk and surface slabs, and the values are reported in Tables 1 and S3 (Supporting Information). In bulk SiC, the charges of C and Si atoms are 6.53 and 1.47 e−, respectively. This infers a sum of 2.53 e− charge transfer (δq) from Si to C. On comparing the Bader charge values of bulk, the charge on the C(3c) atom is deficient of 0.46 e−, while the Si(3c) atom has an excess of 0.56 e− in ZB surface slabs. It is noted that on the C(3c) side the charge perturbation is extended up to the next layer (refer Figure 1), which causes an elongation (1.97 Å) of the Si−C
Table 1. Si−C Interlayer Distance (Id) and Bader Charges on C (qC) and Si (qSi) Atoms Shown for Bulk and Surface Slabs system SiC-cubic bulk SiC-bilayer
Id (Å)
qC (e−)
qSi (e−)
1.91 3.57
6.53 6.48 6.48 6.07 6.54 6.52 6.52 6.54 6.50 1.62 6.59 6.54 6.52 6.53 6.50 6.08 6.48 6.54 6.54 6.53 6.49
1.47 1.52 1.52 1.45 1.46 1.48 1.46 1.48 2.08
6L-SiC-(111)
6L-SiC-(111)-Li@V1
1.97 1.91 1.91 1.91 1.92 6.51 3.45 1.91 1.90 1.91 1.91
6L-SiC-(111)-Li@V5 1.97 1.91 1.90 1.89 4.12
1.49 1.48 1.46 1.48 2.11 1.39 1.45 1.45 1.45 2.71 1.69
bond normal to the surface and contraction (1.84 Å) of the same along the surface, as discussed earlier. On the other hand, there is no significant charge perturbation observed on the Si(3c) side of surface slabs, thus this side has less tendency to form the graphitic SiC layers. Interestingly, we observed that the spin moment of ≈±1 μB is originated from undercoordinated C(3c) and Si(3c) atoms of the surface slab. On each side, the surface atoms are ferromagnetically coupled, which is confirmed by constructing a 2 × 1 × 1 supercell of the six-layer ZB SiC(111) slab and passivating one side of the slab by H as shown in Figure S2 (refer to Supporting Information). Albeit, the unpaired electrons in C(3c) and Si(3c) atoms are coupled to each other antiferromagnetically as shown in Figure 1. For these slabs of various thicknesses (n = 3−10), the exchange energy (J) is calculated from J = EFM − EAFM, where EFM and EAFM represent the total energies of surface slabs with ferromagnetic and antiferromagnetic configurations, respectively. As expected, the J value reduces with n, due to the increase of separation between the spin-polarized atoms. To explore the electronic properties of pristine SiC(111) surface slabs, we deduced the density of states (DOS) and band structure (BS), which are shown in Figure 2. It infers that all graphitic SiC layers are semiconducting; the dispersion correction included band gap is found to be 2.40, 1.95, and 1.40 eV for mono-, bi-, and trilayer cases, respectively. It is observed from DOS that the sp2-hybridized states of Si and C atoms are located between −6.0 and 3.0 eV. Further, the top of the valence band (VB) is mainly originated from C-2pz orbitals, while the bottom of the conduction band (CB) is from Si-3pz, which confirms the charge transfer from Si-3pz to C-2pz. The SiC monolayer has a direct band gap; however, an indirect gap is observed for multilayer graphitic SiC. In the case of bilayer (trilayer) SiC, the top of the VB and the bottom of CB states are nondegenerate due to weak van der Waals force between the layers (refer to Figure S3, Supporting Information). Note that in the trilayer case (refer Figure 2) the CB minimum 15108
DOI: 10.1021/acs.jpcc.7b04489 J. Phys. Chem. C 2017, 121, 15106−15113
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Figure 2. DOS and BS diagrams of SiC (a) monolayer, (b) trilayer, and (c) four-layer thick ZB SiC(111) surface slabs. Black, red, and green lines correspond to total DOS, C-2p, and Si-3p states, respectively. Black and red colored bands in (c) differentiate up (↑)- and down (↓)-spins. In (d), partial charge densities (green colored isosurface) are shown for states marked in (c).
total energies of the lithiated SiC slab, pristine slab, and Li atom, respectively. The calculated Ei profile is shown in Figure 3. It infers that the C(3c) atoms strongly interact with Li as compared to Si(3c) atoms since the unoccupied states of the former are lower in energy as compared to that of the latter. The optimized structure of Li@C(3c) shows that the adsorbed
corresponding to one of the layers is lowered at the M point which resulted in direct to indirect band gap transition. Besides, for the other two layers, it is not shifted from the K point; thus, the direct optical transition is still possible. Since these layers have weakly interacted, there is a possibility of tuning the indirect to direct gap as demonstrated in previous reports.35,36 As discussed earlier, the ZB surface slabs are stabilized with antiferromagnetic configuration beyond n = 3. Hence, the spinpolarized DOS and BS are plotted for n = 4 in Figure 2. In contrast to graphitic SiC, a small energy gap is observed due to the presence of localized surface states around the Fermi level (Ef). Among these states, up-spin of C(3c) and down-spin states of Si(3c) are occupied. The occupied and unoccupied states arising from both sides of the slabs are oppositely spin polarized, thus bipolar magnetism can be observed on both sides of the surface slab, which can be used in spintronic devices.37,38 It is worth noting that unoccupied states of C are lower in energy when compared to that of Si by ≈0.5 eV. From BS it is understood that the states near Ef have low dispersion; thus, the heavy Fermions are expected to be presented on the surface of SiC. For studying the interaction of Li atoms with this surface slab, we considered a six-layer ZB slab as the host for lithiation in various surface sites and bulk void sites (Vi). Here i is varied from 1 (below to C(3c) atoms) to 5 (above Si(3c) atoms). The interaction energy (Ei) is deduced from Ei = (E(SiC) + E(Li)) − E(Li@SiC), where E(Li@SiC), E(SiC), and E(Li) are the
Figure 3. Ei values of the Li atom at various sites in the six-layer ZB SiC(111) surface slab. Corresponding ball and stick model along with the spin density distribution is shown as insets. Green, gray, and purple colored balls represent Li, C, and Si atoms, respectively. Cyan and yellow isosurfaces refer to up- and down-spin charge densities. 15109
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Figure 4. BS of the six-layer ZB surface slab: (a) pristine, (b) Li@C(3c), (c) Li@V1, (d) Li@V2, (e) Li@V3, (f) Li@V4, (g) Li@V5, and (h) Li@ Si(3c). The surface states near Ef are shown by thick lines, and violet bands correspond to sp2 bands of graphitic SiC layers.
induced pairing of electrons. The pairing of electrons at the surface results in quenching the magnetic moment.38 Since the above calculations are carried out on the unit cell, a 2 × 2 × 1 supercell is considered to understand the layer formation for different Li coverage, and the relaxed structures are shown in Figure S6 (Supporting Information). For 25% and 50% Li coverages, the graphitic SiC layer is not formed distinctly, as the measured interlayer distances are found to be 1.98 and 2.10 Å, respectively. Afterward, the coverage is increased to 75% that is capable of exfoliating SiC layers. Accordingly, we conclude that at least 75% coverage is required to exfoliate the SiC layer. In addition, here we analyzed the staging of SiC during lithiation, especially in voids by adding Li in various sites one by one as shown Figure S7 (refer to Supporting Information). The energetics show that here Li atoms prefer to occupy at the same layer until the first layer is completely saturated by Li. Apart from a six-layer ZB slab, we considered a nine-layer ZB surface also to verify the possibility of layer exfoliation from thicker ZB during lithiation. It was found that only three graphitic layers are formed from this slab when the Li atom occupies the V3 site, whereas no such layers are formed for the V4 site; instead the nine-layer ZB slab is separated into four and five layer thick ZB slabs which are interfaced by the Li atom (refer to Figure S8, Supporting Information). Thus, our calculations show that the graphitic layers could be exfoliated from the SiC nanocrystallite with ≈75% coverage of Li atoms in voids near the surface. It is worthy to mention that, for other primary surfaces such as (100) and (110), we have verified the graphitic SiC layer formation by Li intercalation. The optimized structures of lithiated SiC(100) and (110) surfaces are presented in Figure S9 (refer to Supporting Information). However, unlike the (111) surface, since the SiC layers in (100) and (110) slabs are stacked in a different direction, here the graphitic SiC layers are
Li ion mainly interacts with three C atoms. In this case, Si−Li and C−Li bond distances are found to be 2.56 and 2.14 Å, respectively. As a result of adsorption of the Li atom on the C(3c) site, the Si−C bond distance normal to the surface is contracted from 1.97 to1.91 Å, which reduces the tendency for graphitic SiC layer formation, whereas on the Li@Si(3c) side, the Li atom prefers to interact strongly with Si but weakly with C in the same layer. The calculated respective bond distances are 2.55 and 2.45 Å (refer to Figure S4, Supporting Information). In addition, the adsorption of Li@C(3c) or the Si(3c) side quenches the magnetic moment (≈±1 μB) of the corresponding site as shown in Figure 3. Further, it is observed that the Ei value decreases with increasing distance of the void from the surface atoms. Interestingly, the lithiation in bulk Vi sites leads to the exfoliation of the graphitic SiC layer, and the number of such exfoliated layers is one for Li@(V1, V5), two for Li@(V2, V4), and three for Li@V3 sites (refer to Figure S5, Supporting Information). Note that during lithiation a maximum of three graphitic layers is formed from the six-layer ZB SiC(111) surface slab, and this can be explained by the higher stability of those layers in pristine form. The interlayer separations in such exfoliated layers are found to be 3.30−3.33 Å. The measured C−Li bond distances at the interface of ZB (= 2.16 Å) and graphitic SiC phases (= 2.33 Å) suggest that the Li atom strongly binds with C in the former phase as compared to latter, owing to the presence of its partially unoccupied state. This structural feature is quite applicable for Li@V1, V2, and V3 cases, whereas, for Li occupying V4 and V5 sites, the C−Li and Si−Li bond distances are found to be 2.45 and 2.60 Å, respectively. Similar to adsorption of the Li atom in the surface, magnetism is quenched for lithiated or exfoliated surface slabs due to the charge transfer from Li to the surface slab which 15110
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The Journal of Physical Chemistry C formed accordingly with changes in lattice parameter a and b values. From this, we concluded that the graphitic SiC layers can be formed from other surfaces of SiC also in a similar fashion by Li intercalation. These results increased our curiosity to understand the effect of sodiation on SiC surface slabs. Hence we calculated the Ei for sodiation of a six-layer surface slab, and it is plotted in Figure S10 (Supporting Information). It shows that the Ei value for the Na case is smaller than that of Li, due to larger radius Na. In fact, this value becomes negative for the Na atom occupying V3 and V4 sites, which indicates unfavorable diffusion of Na to interior voids. We also studied the exfoliation of graphitic layers from the ZB surface slab by sodiation in the 2 × 2 surface slab. This reveals the possibility of graphitic SiC layer exfoliation even by 25% coverage of the sodium atom (refer to Figure S11 in the Supporting Information). Our calculations infer that by lithiation or sodiation the graphitic SiC layer could be exfoliated from ZB surface slabs. Toward this direction, we also studied the effect of surface passivation by H and F atoms on a twolayer thick slab which originally formed graphitic SiC layers in the pristine form (results are given in Table S3, Supporting Information). It is obvious that passivation by these elements quenches surface magnetism as well as suppresses graphitic layer formation due to the pairing of electrons on the surface. To understand the charge distribution and charge transfer in lithiated ZB surface slabs, the Bader charge values are deduced and reported in Tables 1 and S3 (Supporting Information). The adsorption of the Li@C(3c) side leads to the charge transfer from Li to C atoms, and it results in an increase in the qC value from 6.04 e− to 6.71 e−. Similarly, due to charge transfer from Li to Si(3c) atoms, qSi becomes 2.86 e−, from its original value of 2.1 e−. In the case of Li @Vi sites (for i = 1−3), it transfers charge predominantly to the C atoms and a small amount of charge to Si atoms. On the other hand, for Li at V4 and V5 cases, there is charge repopulation that occurrs at Si atoms that are interacted with the Li atom. As an example, the Si atom presented in the ZB side has gained the charge of 1.2 e−, while that in graphitic SiC loses 0.4 e− (refer to Tables 1 and S3, Supporting Information). By deducing the DOS of the lithiated six-layer (111) SiC slab (shown in Figure S12, Supporting Information), it is understood that the electronic conductivity varies with respect to occupancy of the Li atom in different sites. In the Li@C(3c) or Si(3c) case, the less dispersed surface states from the respective C-2p or Si-3p orbitals lying near Ef are occupied to open a significant energy gap of 0.30 eV. In addition, BS (shown in Figure 4) demonstrates the occupancy of surface states during lithiation, and it is well consistent with the Bader charge analysis that confirmed the charge transfer from Li to SiC surface slab. However, when Li occupies at Vi sites, the sp2 bands of graphitic SiC layers start appearing along with the ZB bands, and the former is highlighted in Figure 4. The number of these bands are in agreement with graphitic SiC layers formed in those cases. Moreover, these bands are nondegenerated as similar to the pristine case. In order to reveal the suitability of SiC nanocrystallites as an anode of LIBs, the average intercalation voltage (AIV) of the (111) surface slab is calculated by introducing the Li successively on the surface and voids. The AIV is calculated from39 AIV = −
where E(Lix1@SiC), E(Lix2@SiC), ELi, and F denote the total energies of x1 and x2 Li intercalated in the surface slab, chemical potential of lithium metal, and the Faraday constant, respectively. The order of lithiation is adopted from Ei values of various sites, as shown in Figure 3. The optimized structure of Li added at various sites in a six-layer ZB SiC(111) slab is shown in Figure S14 (refer to Supporting Information). From these structures, it is very clear that surface lithiation does not result in graphitic SiC layer formation, whereas lithiation in bulk voids leads to the formation of the graphitic phase with lattice expansion only along the z-direction. However, during delithiation, there is a possibility of fault restacking which will result in a change of its polytype from cubic to hexagonal as observed in experiment.22 Thus, the formation of graphitic SiC layers is proposed to be an intermediate structure. In this regard, the experimental validation of this theoretical observation requires an in situ structural analysis during the charge−discharge process. The total number of Li intercalated in this surface slab is 12, so that it can form the Li2SiC composition for which the specific capacity is calculated to be 1336 mAh/g which is close to the experimentally observed value of 1200 mAh/g.22 This difference is possibly due to the presence of irreversibly settled Li on SiC. This lithiation in SiC is classified mainly into two stages, namely, surface and bulk lithiation. On the surface the Li adsorption energy is high with huge AIV of 1.70 V as observed in the experimental study. Due to strong adsorption these Li cannot be easily reverted back. For the bulk lithiation stage, the AIV is gradually reduced, and beyond X = 5 the voltage profile shows steady voltage of 0.20 V as shown in Figure 5. Overall, the calculated AIV and specific capacity match with the experimental observation, and this illustrates the lithiation mechanism and anodic behavior of SiC.
Figure 5. Optimized structure of Li12(SiC)6 slabs with the AIV profile.
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CONCLUSION In summary, we extensively carried out first-principles calculations to study the structural, electronic, magnetic, and electrochemical properties of pristine and lithiated ZB SiC(111) surface slabs with various thickness. The structural transformation in ultrathin SiC(111) surface slabs to the graphitic phase is understood based on the Bader charge analysis. The SiC monolayer has a direct band gap, while the biand trilayers have indirect band gap due to symmetry breaking and interlayer interaction. In contrast to antiferromagnetically coupled surface unpaired electrons in the ZB slab, the graphitic SiC layer is nonmagnetic due to charge transfer from C-2pz to Si-3pz orbitals. These ZB slabs exhibiting the bipolar magnetism
[E(Li x2@SiC) − E(Li x1@SiC) − (x 2 − x1)E(Li)] (x 2 − x1)F 15111
DOI: 10.1021/acs.jpcc.7b04489 J. Phys. Chem. C 2017, 121, 15106−15113
Article
The Journal of Physical Chemistry C
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are attractive for spintronic devices. To understand the intriguing electrochemical activity of SiC nanoparticles, the Li interaction energy is calculated for different sites of ZB slabs, which shows strong adsorption over the surface without significant structural changes. It is a discernible observation that the graphitic layers are exfoliated during lithiation or sodiation in voids. The AIV of the SiC surface slab is calculated to be ≈0.20 V, and maximum possible Li uptake by SiC per formula unit is found to be 2, which are in agreement with the experimentally observed voltage for SiC nanoparticles. Thus, the mechanism of lithiation in nanocrystallites is proposed as intercalation, rather than alloying. We believe that this work will open up a new avenue for the experimentalist to design superior anode materials having a similar structure of SiC for LIB applications.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b04489. (1) Lithiation of 3C-SiC and 6H-SiC; (2) layer−layer interaction between graphitic SiC layers; (3) spin density distribution of a hydrogenated six-layer ZB SiC(111) surface slab; (4) band decomposed charge density plot; (5) structural analysis of lithiated SiC; (6) lithiation at various sites of the six-layer ZB-SiC(111); (7) lithiation on the nine-layer ZB-SiC(111) surface slab; (8) lithiation on the 2 × 2 × 1 supercell of the six-layer ZB-SiC(111) surface slab; (9) staging of Li in the six-layer ZBSiC(111) surface slab; (10) (10) Lithiation of (100) and (110) surface slabs of ZB-SiC surface slab; (11) Interaction energy plot for Na with the ZB-SiC(111); (12) sodiation on the 2 × 2 × 1 supercell of the six-layer ZB-SiC(111) surface slab; (13) DOS for Li added on various sites of the ZB-SiC(111); (14) DOS for completely lithiated SiC; and (15) Li intercalation in the ZB-SiC(111) surface slab; (16) Bader charge analysis (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +91-4565-241443. Fax: +91-4565-227779. ORCID
P. Murugan: 0000-0003-0062-4828 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by CSIR, India, through the TAPSUN NWP-56 project. The authors acknowledge the use of the CSIR-CECRI, CSIR-NCL, and CSIR-CMMACS high performance computing facilities. We acknowledge T. Premkumar for initiating this work.
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DOI: 10.1021/acs.jpcc.7b04489 J. Phys. Chem. C 2017, 121, 15106−15113
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