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. 2021 Oct 29;6(44):29577–29587. doi: 10.1021/acsomega.1c03771

Density Functional Theory Study of Ethylene Carbonate Adsorption on the (0001) Surface of Aluminum Oxide α-Al2O3

Brian Ramogayana †, David Santos-Carballal ‡,§,*, Khomotso P Maenetja †, Nora H de Leeuw ‡,§,∥, Phuti E Ngoepe †,*
PMCID: PMC8582038  PMID: 34778629

Abstract

graphic file with name ao1c03771_0010.jpg

Surface coating is one of the techniques used to improve the electrochemical performance and enhance the resistance against decomposition of cathode materials in lithium-ion batteries. Despite several experimental studies addressing the surface coating of secondary Li-ion batteries using α-Al2O3, the reactivity of the material toward the electrolyte components is not yet fully understood. Here, we have employed calculations based on the density functional theory to investigate the adsorption of the organic solvent ethylene carbonate (EC) on the major α-Al2O3(0001) surface. During adsorption of a single EC molecule, it was found that it prefers to bind parallel to the surface through its carboxyl oxygen. As the surface coverage (θ) was increased up to a monolayer, we observed larger adsorption energies per EC molecule (Eads/NEC) for parallel interactions and a reduction for perpendicular interactions. We also noted that increasing the surface coverage with both parallel and perpendicularly interacting EC molecules led to a decrease of the surface free energies and hence increased stability of the α-Al2O3(0001) surface. Despite the larger Eads/NEC observed when the molecule was placed parallel to the surface, minimal charge transfer was calculated for single EC interactions and at higher surface coverages. The simulated scanning tunneling microscopy images are also presented for a clean corundum α-Al2O3 surface and after adsorption with different coverages of parallel and perpendicularly placed EC molecules.

1. Introduction

Global interest is growing in the development of cheap, renewable, and clean energy storage systems which will facilitate moving away from the reliance on fossil fuels. Alternative energy sources include solar,1−3 wind,4,5 and biofuel energy.6,7 Among the most researched energy storage systems is lithium-ion battery technology, which has application in portable devices, electric vehicles, and stationary smart grids.8 However, current Li-ion batteries use LiMn2O4 as a cathode material, which over time needs replacement as it experiences degradation due to the interactions with the electrolyte, which leads to capacity fading.9−11 Various strategies have been implemented to improve the electrochemical performance of LiMn2O4, including optimization of the electrolyte,12,13 using alternative cathode materials14−17 and electrolyte salts,18−20 doping the cation sites of the cathode,21,22 and coating the surface to create an artificial barrier that limits direct cathode–electrolyte contact.23−27

Various types of coating materials have been explored, for example, fluorides28−30 (AlF3, SrF2, and LaF3), phosphates,31−33 and oxides (ZnO,34−36 ZrO2,37 MgO,38,39 and Al2O328−41), which have shown improved capacity retention and electrochemical performance compared to the uncoated cathode material. Among these, aluminum oxide (α-Al2O3) is considered one of the best candidates for coating materials because of its good electrochemical properties.42 Kannan and Manthiram43 and Eftekhari44 initially reported that α-Al2O3 significantly improves the electrochemical performance of the LiMn2O4 spinel at elevated temperatures. Numerous subsequent studies indicated that α-Al2O3 also enhances the cycling capacity by reducing the dissolution of the Mn transition metal.40,41,45−47

In other technological applications, α-Al2O3 is mainly used for its high thermal conductivity, resistance against extreme temperatures and corrosion, and excellent electrical insulation properties.48−51 Several studies have reported the adsorption of various molecules, including H2O,52−54 HCl,55−57 CO2,58 and methanol,59 onto the α-Al2O3 surfaces as these are of interest for different applications. Quan et al.(60) employed density functional theory (DFT) simulations to study the adsorption of hydrofluoric acid (HF) on the α-Al2O3(0001) surface and found that, upon relaxation, a new Al–F bond was formed. This finding indicates that α-Al2O3 creates both a protective barrier on its own between the cathode and the electrolyte components and an additional passivating AlF3 coating layer following the reaction with the HF acidic electrolyte component.61

Ethylene carbonate (EC) is the most stable and effective electrolyte solvent with better electrochemical performance than propylene carbonate, dimethyl carbonate, and diethyl carbonate.62−64 Compared to other organic solvents, EC has boiling and melting points, which are approximately 248 and 36.4 °C, respectively.65 In our previous study,66 we employed DFT calculations with a Hubbard Hamiltonian and long-range dispersion corrections to study the adsorption of EC onto the low-Miller-index surfaces of LiMn2O4, where we found that the EC weakly adsorbed on the Mn/Li sites of the spinel surfaces via the carbonyl or ethereal oxygen, thus allowing the molecule to detach easily to expose the (111) surface. However, the reactivity of EC with the coating material α-Al2O3 is equally important but not yet fully understood.

In this paper, we have employed DFT calculations to investigate the reactivity of EC at the major α-Al2O3(0001) surface. We discuss the adsorption energies and binding configurations from a single EC molecule up to full monolayer coverage on the α-Al2O3(0001) surface. We have also carried out an analysis of the charge transfer and vibrational frequencies and finally have reported calculated scanning tunneling microscopy (STM) images for future comparison with the experiment.

2. Results and Discussion

2.1. Bulk and Surface Models

The crystal structure of α-Al2O3 has hexagonal symmetry with the space group R3̅c (no. 167). The structure consists of a hexagonal close-packed array of oxygen atoms along the [001] direction with the Al atoms occupying two-thirds of the octahedral interstices; see Figure 1a. Following a full bulk optimization, the predicted equilibrium lattice parameters were a = b = 4.75 Å and c = 12.97 Å, which agrees within 0.21% with the values of a = b = 4.760 Å and c = 12.989 Å reported in the literature.76 As shown in Figure 1b, EC is an organic molecule with formula C3H4O3 with a high symmetry and large dielectric constant (ε ≈ 90.5)77,78 compared to other organic solvents.

Figure 1.

Figure 1

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(a) Hexagonal crystal structure of aluminum oxide (α-Al2O3) and (b) representation of the EC molecule, indicating the carbonyl (Oc) and ethereal (Oe) oxygen atoms.

The corundum α-Al2O3(0001) surface terminations studied in this work were created by cleaving a fully optimized (2 × 2) supercell of the bulk unit cell using the method implemented in the METADISE code79 to create nondipolar surface models. We chose to model the α-Al2O3(0001) surface, since it has been reported as the dominant plane in corundum nanoparticles.80−82 The (0001) surface simulation cells contain 120 atoms (48 Al and 72 O) distributed in atomic layers of 6 formula units (f.u.) in both Al- and O-terminated slabs with a surface area of 90.3 Å. A vacuum region of 15 Å was added perpendicularly to the surface termination to avoid interactions between the periodic images. The atoms in the upper layers were allowed to relax (i.e., 3 f.u.), whereas the other atoms in the bottom layers were kept fixed at their relaxed α-Al2O3 bulk positions. The two slabs were stoichiometric, symmetric, and nonpolar along the z-direction. Figure 2 shows the side and top views of the Al- and O-terminated α-Al2O3(0001) surfaces before and after relaxation. Convergence tests were carried out to determine the required total number of layers, relaxed atomic layers, and the vacuum thickness until the energy change dropped below 1 meV. Dipole corrections perpendicular to the surface plane were applied during our calculations to enhance the convergence of the electronic energy. Geometry optimizations were conducted using the conjugate-gradient technique and were considered converged when the Hellmann–Feynman forces were below 0.01 eV/Å. The stacking sequence of the two terminations consists of alternating Al and O planes, which are reconstructed Tasker type III surfaces.83 The stoichiometric slab terminating in the Al layer has the sequence Al2, O6, Al4, O6, ..., O6, Al4, O6, Al2, and the stacking sequence for the slab terminating in the oxygen layer can be represented as O3, Al4, O6, Al4, ..., Al4, O6, Al4, O3.

Figure 2.

Figure 2

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Top and side views of the (a) Al- and (b) O-terminated α-Al2O3(0001) surfaces before and after relaxation. The stacking sequence for both Al- and O-terminated surfaces are shown on the far right of each panel. The crystallographic direction for the top view of (0001) surface terminations is [001] for the abscissae toward the right.

2.2. Surface Energies

The unrelaxed (γu) and relaxed (γr) surface energies were calculated using eqs 1 and 2, respectively

2.2. 1
2.2. 2

where Eu,slab, Er,slab, and Ebulk are the total energy of the unrelaxed slab, half-relaxed slab, and the bulk, respectively, all with the same number of formula units, whereas A is the surface area of the slab. Unrelaxed surface energies were evaluated from static calculations of a slab created from a freshly cut bulk, while the surface energies of the half-relaxed slabs were calculated after geometry optimization, where the 3 f.u. uppermost atomic layers were allowed to relax, while the remaining bottommost layers were kept fixed at their relaxed bulk positions. The degree of relaxation (R) was also calculated for all surface terminations as

2.2. 3

As summarized in Table 1, the lowest unrelaxed and relaxed surface energies were obtained for the Al-terminated slab with γr = 0.11 eV/Å2, which accounts for the most stable termination. The predominant stability of the Al-terminated (0001) surface is consistent with previously reported studies54,58,84−87 and the experimental work by McHale et al.,88 who found that γ = 0.16 eV/Å2. For both Al- and O-terminated surfaces, we observed a large geometry relaxation, with the largest R = 65.8% obtained for the O-terminated slab.

Table 1. Surface Energies for the Unrelaxed (γu) and Relaxed (γr) Al- and O-Terminated α-Al2O3(0001) Facets.

surface terminations γu (eV/Å2) γr (eV/Å2) R (%)
Al 0.25 0.11 57.0
O 0.38 0.13 65.8
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This large difference between the unrelaxed and relaxed surface energies was due to the inward movement of the exposed ions in the Al-terminated facet and an outward ionic movement in the O-terminated slab. During energy minimization, we observed a large relaxation, wherein the Al–O interlayer distance decreased from 1.85 to 1.69 Å and increased from 1.70 to 1.97 Å for the Al-terminated and O-terminated slabs, respectively. This decrease was associated with vertical relaxation of Al–O, wherein the Al-terminated slab experienced an inward relaxation of 0.75 Å along the z-axis, thereby coming closer to the O layer underneath, while the O-terminated slab experienced an outward relaxation of 0.99 Å. Similar findings were previously reported in both experimental and theoretical studies, which also revealed that the Al-terminated surfaces exhibit a large lattice contraction along the z-direction in order to minimize the strong electrostatic attractions.89−91 Al2O3 is a strongly ionic oxide, and the reconstructed surface termination is unstable, since it leaves bare surface Al3+ ions or dangling O-bonds.84,92−95 In this work, the molecular adsorption of EC and the characterization of the physicochemical properties is only performed on the most stable Al-terminated surface, which is the most likely to appear in the corundum crystal morphology in the absence of water.54

2.3. Molecular Adsorption

In this section, we study the adsorption of EC on the major Al-terminated α-Al2O3(0001) surface. Considering the availability of one top-layer Al site on the (1 × 1) Al-terminated surface, the Al surface sites on our modeled (2 × 2) Al-terminated surface are all identical. The initial EC binding configurations on the surface involve molecular coordination via the carbonyl (Oc) and ethereal oxygen (Oe) in parallel, perpendicular, and bridging/bidentate binding orientations. As discussed in our previous work,66 where the structural parameters of our optimized and isolated EC molecule were compared with the available experimental and theoretical data, our computational settings can properly capture the main properties of this molecule. Upon adsorption, the average adsorption energy per EC molecule on the α-Al2O3(0001) surface was calculated as

2.3. 4

where Eslab+EC, and EEC are the total energies of the slab with the adsorbed EC molecules, and the isolated molecule, respectively. NEC corresponds to the number of adsorbed EC molecules.

2.3.1. Single EC Molecule Adsorption

We next studied the adsorption of a single EC molecule placed parallel and perpendicular to the surface, interacting via the carbonyl (Oc), ethereal (Oe) oxygen, and both on the top-layer Al positions of the α-Al2O3(0001) surface. In all different orientations, the EC molecule was initially placed at a distance of 2.0 Å from the surface to favor the attractive forces between the adsorbate and the facet. However, the molecule and surface were allowed to move freely during relaxation. Figure 3 summarizes the adsorption energies calculated for the α-Al2O3(0001) surface with the EC molecule placed parallel, perpendicular, and in a bridging/bidentate binding. The largest adsorption energy was observed when the molecule was placed parallel to the surface with Eads = −1.69 eV. In all adsorptions, the protruding Al atom interacting with the molecule moves upward out of the surface by ∼0.035 Å to form a bond with the EC molecule. Upon relaxation of the bridging/bidentate configuration, the EC molecule preferred to interact with the surface via the carbonyl oxygen.

Figure 3.

Figure 3

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Adsorption energies (Eads) calculated for a single EC molecule placed (a) parallel, (b) perpendicular, and (c) in a bridging/bidentate binding.

2.3.2. Varying Surface Coverages

To consider the effect of different surface coverages, the number of adsorbed EC molecules (NEC) was increased stepwise until a full monolayer was obtained on the Al-terminated α-Al2O3(0001) surface. The surface coverage (θ) was calculated as the ratio between the number of adsorbed EC molecules (NEC) and the total number of available adsorption sites (N) as

2.3.2. 5

where θ = 0 indicates that no adsorption takes place on the surface, while θ = 1 shows that the surface adsorption reached a full monolayer. The effect of surface coverage on the average adsorption energy was explored for parallel and perpendicular surface interactions of the EC molecule, which are summarized in Figure 4. Generally, the value of Eads/NEC becomes progressively smaller with NEC for the parallel binding modes, but the average adsorption energy grows for the perpendicular type of orientations for θEC ≤ 0.75, although it slightly decreases again for θEC = 1.00. The decrease of Eads/NEC observed for the parallel interactions indicates that for θEC > 1, the steric effects due to the lateral interactions between the EC molecules, which occupy a large surface area, destabilize the system. For perpendicular interactions, the value of Eads/NEC increases with respect to the single EC molecule adsorption up to θEC ≤ 0.75 as there are attractive van der Waals forces between the parallelly stacked molecules. However, we observed a decrease of the average adsorption energy (Eads/NEC) when we reached a full monolayer (θ = 1) as the repulsive steric forces between the closely placed EC molecules become larger than the attractive van der Waals forces. Our results indicate that from a thermodynamic point of view, the EC molecule prefers to attach to the surface in a parallel fashion up to θEC ≤ 0.75, whereas a full monolayer is energetically more favorable if all the molecules change to the perpendicular orientation. We rationalize the change of the adsorption configuration based on steric factors as the parallel molecules repel each other, which therefore need to become perpendicular to the surface to allow better accommodation of the full monolayer. Note that adsorption at higher coverages led to the detachment of the fifth molecule from the surface as it experienced strong steric hindrance caused by the EC–EC interactions.

Figure 4.

Figure 4

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Average adsorption energy as a function of the surface coverage for the EC molecules adsorbed parallelly and perpendicularly to the surface.

We also explored the effect of surface coverage (θ) on the work function (ϕ) for the EC molecules placed in parallel and perpendicular configurations onto the surface. The work function (ϕ) measures the energy required to bring a surface electron into the vacuum,93 and it is known to be larger for denser surface crystals than for those with open lattices.96,97 Generally, an increase in the work function was observed as we increase the surface coverage (θEC) for both parallel and perpendicular interactions; see Figure 5. The largest value of the work function (ϕ) was observed for the full monolayer of perpendicularly placed EC molecules with ϕ = 6.37 eV.

Figure 5.

Figure 5

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Work function (ϕ) as a function of the surface coverage for the EC molecules adsorbed parallelly and perpendicularly to the surface.

2.4. Surface Free Energies

We further calculated the surface free energy (σ) for the surfaces after adsorption of different coverages of EC molecules via

2.4. 6

Figure 6 summarizes the effect of surface coverage (θ) on the surface free energies (σ). The calculated surface free energies generally decrease compared to the clean surface (θ = 0), indicating that the system is stabilized upon EC adsorption, with the most stable surface achieved when we reach a full monolayer (θ = 1). For parallel interactions, the surface free energies decrease with respect to the surface energy of the pristine (0001) facet up to θ ≤ 0.75, when σ stabilizes at around 0.06 eV/Å. At θ = 1, repulsive steric forces between the closely placed EC molecules become larger than the attractive van der Waals forces, and as a result, the fourth molecule detached from the surface upon relaxation of the full monolayer-covered surface, hence leading to the same surface energy for θ = 0.75 and θ = 1 for the parallel EC molecules.

Figure 6.

Figure 6

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Surface free energies (σ) for the Al-terminated α-Al2O3(0001) surface as a function of the surface coverage for the EC molecules adsorbed parallelly and perpendicularly to the surface.

2.5. Bader Charge Analysis

To investigate the electronic charge transfers between the EC molecules and the surface, we carried out a Bader charge analysis for the most favorable adsorption configurations found on the α-Al2O3(0001) surface. We used the code developed by Henkelman and collaborators98 to perform the Bader analysis of charges.99 In this method, the atoms are defined as the regions enclosed within surfaces of zero flux of charge density. Table 2 summarizes the charge transfers between the surface and the adsorbed EC molecules. For the adsorption of a single EC molecule, we generally observed minor charge transfers between the surface and the adsorbate. Similar results were found in our previous study, where we investigated the EC adsorption on the major surfaces of the LiMn2O4 spinel.66 The EC molecule gained small charges of −0.094 and −0.071 e– when it was placed in parallel and perpendicular modes onto the surface. As we increased the surface coverage to θ = 0.50, the charge transfer remained constant in both orientations. However, we found a reduction of the charge transfer for θ ≥ 0.75 compared to lower surface coverages of the EC molecule due to the interactions between the closely packed adsorbate molecules. We also calculated the charge accumulation/depletion between the interacting atoms, that is, the carbonyl or ethereal oxygen atoms of the EC molecule and the Al atom of the surface. Our calculations suggest that there is minor charge accumulation for the parallel interactions, whereas there is a minimal depletion of charge for parallel interactions when the coverage is increased up to θ = 0.50. However, we observe a large charge accumulation of 0.761 and 0.836 e– on the interacting oxygen atom for θ = 0.75 and at full monolayer coverage.

Table 2. Charge Transfer between the EC Molecule and the Surface (ΔqEC); Average Value for the Charge Accumulation/Depletion within the Interacting EC Atom (ΔqOc/Oe), i.e., Carbonyl or the Ethereal Oxygen Atom as a Function of the Surface Coverage (θEC)a and Average Stretching Mode for the Carbonyl Group ν(C=O), as Well as the Asymmetric νasy(CH2) and Symmetric Stretching Mode νsys(CH2) and the Bending Mode for the Methylene Group δ(CH2).

          CH2
adsorption orientation θ ΔqEC (e–) ΔqOc/Oe (e–) νC=O (cm–1) νsys (cm–1) νasy (cm–1) δ (cm–1)
isolated EC molecule 1829 2952 2990 1341
liquid film EC66,100 1803 2928 2955 1397
parallel 0.25 –0.094 –0.053 1654 3087 3022 1482
  0.50 –0.094 –0.049 1665 2991 3112 1474
  0.75 –0.021 0.743 1704 3015 3064 1455
  1.00 –0.010 0.766 1753 3054 3053 1468
perpendicular 0.25 –0.071 0.056 1619 3024 3059 1432
  0.50 –0.072 0.054 1851 3026 3086 1456
  0.75 –0.019 0.617 1819 3091 3040 1465
  1.00 –0.019 0.631 1823 3120 3054 1474
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a

Negative charge transfer values indicate a charge accumulation on the adsorbate, while positive values indicate charge transfers to the surface.

To gain more insights into the electronic charge transfers, we analyzed the differential electron density (Δρ). Figure 7 shows the isosurfaces of the electron density difference between the α-Al2O3(0001) surface and the EC molecule at different coverages. The charge density difference was obtained by subtracting the sum of the electron charge densities of the clean surface (ρsurf) and an isolated EC molecule (ρEC), with an identical structure as in the adsorbed form, from the electron density of the total system comprising the surface and the adsorbed molecule (ρsys) according to

2.5. 7

Figure 7.

Figure 7

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Top and side views of the charge density flow (Δρ) for different coverages of the EC molecule adsorbed (a) parallelly and (b) perpendicularly on the α-Al2O3(0001) surface. Density gain and depletion regions are represented in yellow and blue, respectively. Isosurfaces display a value of ±0.099, ±0.190, ±0.018, and ±0.009 e/Å3 for (i) a single EC, (ii) two EC, (iii) three EC, and (iv) four EC molecules.

Figure 7 shows that, although we observe a small charge transfer for the adsorption of a single EC molecule, the electronic charge flow indicates an internal charge rearrangement within the molecule. We observe partial oxidation of the carbon atom and partial reduction of the interacting oxygen due to an intramolecular electronic rearrangement from the C=O π-bond to the newly formed Al–O σ-bond, which is also in agreement with our previous work.66 The isosurfaces of charge density also indicate electronic accumulation on the hydrogen atoms, caused by the interactions between them and the surface oxygen atoms.

2.6. Vibrational Modes

The simulated wavenumbers of the vibrational modes are directly related to the force constant of the intramolecular bonds, calculated as the second derivatives of the total energy with respect to the atomic positions,101,102 which were allowed to move by small displacements to ensure that they fall within the harmonic part of the energy potential well. Table 2 summarizes the vibrational modes for the carbonyl group ν(C=O) and methylene group ν(CH2) {i.e., the asymmetric stretching mode (νasy), symmetric stretching mode (νsys) and the bending mode (δ)}. The stretching modes presented are the modes for the isolated EC molecule and the average vibrational modes at various EC coverages on the Al-terminated α-Al2O3(0001) surface. The wavenumbers of the vibrational modes of an isolated EC molecule are in excellent agreement with the experimental modes for liquid films,100 and thus, we are confident of our predictions for the interacting molecules. The average stretching modes calculated for EC as we increase the surface coverage remain in the same range as for single EC adsorption, indicating that the vibrational modes are not affected by the increase in coverage. For example, our computed vibrational modes for the carbonyl group ν(C–H) for single adsorptions and at higher coverage were within the range 2850–3300 cm–1 reported in the previous studies.103,104 The carbonyl (C=O) stretching modes are red-shifted compared to the isolated adsorbate, suggesting that the carbonyl double bond weakens upon adsorption. The stretching vibrational modes of the methylene (CH2) groups are blue-shifted with respect to the isolated EC molecule as a result of the steric effect and the more restricted mobility of the hydrogen atoms.

2.7. STM Images

We have also derived the topographical STM images from a spatial distribution of the valence band state in the vicinity of the Fermi level of the surfaces containing different coverages of the EC molecule. The STM images were simulated according to the basic formulation of the Tersoff–Hamann approach,105 where the tip of the STM was approximated to an infinitely small point source. We have used the program HIVE106 for the production of our constant-height STM topographic images, where the partial charge density was integrated between the ranges −2.5 to 0 eV.107−109Figure 8 show the STM images for a clean surface and after adsorption with different coverages of parallel and perpendicular EC molecules. The STM for a clean corundum α-Al2O3 surface was obtained at 0.1 Å and a density of ϱ = 0.0017 e/Å3, clearly showing alternating rows of well-defined Al and O atoms. The corundum α-Al2O3 adopts the same hexagonal array as the isostructural hematite α-Fe2O3 mineral, displaying similar STM images.110−112 STM images for the surfaces with adsorbed EC molecules clearly show the array and orientation of the molecules on the surface, allowing us to discriminate the orientation and surface coverage of the EC molecules. However, due to the size of the molecule, the molecule(s) in the adsorbed surfaces appear as bright spots as shown by the insets. At higher coverage, the images become less clear since the molecules become closely packed and closer to each other.

Figure 8.

Figure 8

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Simulated STM images of (a) clean Al-terminated α-Al2O3(0001) surface and after adsorption with different coverages of (b) parallelly and (c) perpendicularly placed EC molecules at a sample bias of −2.5 eV. Density (ϱ) and tip distance (d) are also indicated. Insets show the adsorbed EC molecules and atoms on the top surface layer.

3. Conclusions

We have employed DFT calculations to investigate the adsorption of a range of EC coverages onto the major α-Al2O3(0001) surface. Our surface energy calculations indicate that the Al-terminated (0001) surface is the most stable facet, which compares well with the previous works. A single EC molecule prefers to bind in a parallel mode to the Al-terminated α-Al2O3(0001) surface with only minor charge transfer occurring upon adsorption. When the surface coverage was increased up to the full monolayer, the Eads/NEC for parallel interactions decreased with coverage, whereas the average adsorption energy increased for perpendicular orientations compared to the initial adsorption of an isolated EC molecule. The surface free energies decrease with an increase of the EC coverage, indicating increased surface stability when parallel or perpendicular EC molecules interact with the surface. Minor charge transfers between the surface and EC molecules were generally observed for both orientations even at higher surface coverage. However, we found an increase in charge accumulation on the interacting atom (either the carbonyl or ethereal oxygen). We also observed steric hindrance at higher coverages due to the EC–EC interaction, which led to the detachment of EC molecules at high coverages.

As we increase the surface coverage, our simulations show an increasing red shift for ν(C=O) with respect to an isolated EC molecule, suggesting that the carbonyl bonds weaken upon adsorptions. The blue shift of the symmetric and asymmetric ν(CH2) modes indicate steric hindrance. The simulated STM images are also presented for a clean corundum α-Al2O3(0001) surface and surface with adsorbed molecules at different EC coverages. The STM for the clean corundum α-Al2O3(0001) surface compares well with the image for the same facet of the isostructural hexagonal haematite α-Fe2O3 structure, which can be used for comparison with future experiments.

The results obtained in this work indicate that the EC solvent used as an electrolyte is relatively inert toward Al2O3, which is important in the interpretation of the role of alumina as a coating for the cathode materials in Li-ion batteries. Future work will focus on the interaction of EC with the α-Al2O3(0001)/LiMn2O4(111) heteroepitaxial junction and the effect on the mobility of the Li ions through the interface.

4. Computational Methods

The spin-polarized DFT calculations were carried out using the Vienna ab initio simulation package.67 The generalized gradient approximation was employed using the Perdew, Burke, and Ernzerhof exchange–correlation functional.68 The core electrons and their interaction with the valence electrons were described using the projector augmented-wave method69 in the implementation of Kresse and Joubert.70 The frozen core electrons comprised levels up to 2p for Al and 1s for O and C, while the electron of the H atom was treated as a valence electron.

The semiempirical method of Grimme with the Becke–Johnson damping [D3-(BJ)]71,72 was included to model the long-range dispersion interactions. The energy of a surface–adsorbate system was also calculated using a number of functionals that include van de Waals interactions,73,74 but the variation in the calculated results was less than 1%, most likely because the EC molecule is quite polar and the interactions are dominated by electrostatics. The comparison and agreement between these methods shows that the functional chosen in this work [D3-(BJ)] is suitably accurate to describe the surface–adsorbate interactions.

The kinetic energy cutoff was fixed at 560 eV for the expansion of the Kohn–Sham valence states. Γ-Centered Monkhorst–Pack grids of 9 × 9 × 5 and 5 × 5 × 1 k-points were used for the integration in the reciprocal space of the α-Al2O3 bulk and its two (0001) surface terminations, respectively. The Γ point of the Brillouin zone was sampled during energy minimization of the isolated EC molecule using a cell of dimension 10 Å × 11 Å × 12 Å to avoid spurious interactions. To improve the convergence of the Brillouin zone integrations, Gaussian smearing with a width of 0.05 eV was set for all geometry optimization calculations.72 However, the tetrahedron method with Blöchl corrections was used to obtain accurate total energies and electronic properties.75

Acknowledgments

The authors acknowledge funding from the UK Economic and Social Research Council (ESRC grant no. ES/N013867/1) and the National Research Foundation South Africa for funding of a UK-SA Newton PhD partnership program. P.E.N. acknowledges the financial support of the DSI-NRF South African Research Chair Initiative, and N.H.d.L. acknowledges the UK Engineering and Physical Sciences Research Council (EPSRC grant EP/K009567) for funding. The authors acknowledge the use of the Centre for High-Performance Computing (CHPC) facility of South Africa in the completion of this work. They also appreciate the support received from DSI Energy Storage Research Development and Innovation Initiative, South Africa. This work was performed using the computational facilities of the Material Modelling Centre (MMC), University of Limpopo, Centre for High-Performance Computing, Cape Town, and the Supercomputing Facilities at Cardiff University operated by ARCCA on behalf of the HPC Wales and Supercomputing Wales (SCW) projects. All data are provided in full in the Results and Discussion section of this paper.

The authors declare no competing financial interest.

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