Article Cite This: J. Phys. Chem. C 2017, 121, 28235−28248
pubs.acs.org/JPCC
Decomposition of Ionic Liquids at Lithium Interfaces. 2. Gas Phase Computations Justin B. Haskins,†,‡ Handan Yildirim,†,‡ Charles W. Bauschlicher, Jr.,§ and John W. Lawson*,§ ‡
AMA Inc., Thermal Materials Protection Branch, NASA Ames Research Center, Moffett Field, California 94035, United States; Thermal Materials Protection Branch, NASA Ames Research Center, Moffett Field, California 94035, United States
§
S Supporting Information *
ABSTRACT: This is Part 2 of a two part series of papers on decomposition of two ionic liquids at lithium metal interfaces. In Part 1 of this series, ab initio molecular dynamics (AIMD) simulations were used to examine the stability and decomposition of two ionic liquids (ILs), [pyr14][TFSI] and [EMIM][BF4], on Li metal anodes. Here in Part 2, density functional calculations of ions and ion pairs in the gas phase are coupled with model electrode surface effects to provide an in-depth analysis of the results obtained from more computationally expensive AIMD simulations of electrolytes on the Li surface in Part 1. The gas phase approach is used to examine the cathodic and anodic stability, the electrochemical decomposition thermodynamics, and the kinetic barriers to the electrochemical decomposition of the ions on a Li surface. The states of the ILs are shown to mix with those of the Li surface, which leads to the reduction of the cations by one electron and a partial reduction of the anions. Upon reduction, many ion decomposition reactions are found to be thermodynamically favorable and to have small or moderate kinetic barriers. An examination of reaction transition states for reduced ions and ions in the presence of Li atoms suggests that the reductive decomposition of anions is mediated by chemical association with Li surface atoms, while reductive decomposition of the cations need not involve such chemical interactions. Overall, the gas phase results obtained here corroborate and extend understanding of the stability and decomposition behavior of ILs on Li metal anodes noted from the AIMD simulations in Part 1.
I. INTRODUCTION Understanding the interaction of electrolytes with lithium metal anodes is crucial to enabling advanced battery chemistries, such as Li−air and Li-sulfur. Experimental studies have shown that certain IL pairs, namely the N-methyl-N-butyl-pyrrolidinium (pyr14) cation with the bis(trifluoromethanesulfonyl)imide (TFSI) anion and the 1-ethly-3-methyl-imidazolium (EMIM) cation with the boron tetrafluoride (BF4) anion, are stable against an array of negative electrode materials used in conventional lithium ion batteries.1−4 Beyond this, recent investigations have revealed that these ILs may prevent the formation of dendrites on Li metal electrodes.5−8 In particular, the [pyr14][TFSI] liquid with Li[TFSI] was shown to provide a stable SEI layer that led to roughly 1000 cycles in a lithium metal coin cell. The [EMIM][BF4] liquid with the Li[BF4] salt and a vinylene carbonate additive led to stable cycling behavior for up to (∼150 cycles).7 Driven in part by the experimental promise for electrochemical applications, computational investigations of the [pyr14][TFSI] and [EMIM][BF4] ILs have examined bulk properties as a function of Li-salt doping,8−31 the formation and capacitance of the electric double layer,32 and interactions with Li metal27−31 and other33,34 anodes. Interest in understanding the SEI formation on Li metal led us to perform an extensive ab © 2017 American Chemical Society
initio molecular dynamics (AIMD) examination of the decomposition of the [pyr14][TFSI] and [EMIM][BF4] ILs on lithium metal surfaces, as given in Part 1 of this series.35 In Part 1, ion pairs and bulk liquids were examined in contact with a Li metal surface. Prior to decomposition, the unoccupied electronic states of all ions were found to mix with those of the Li surface, which was accompanied by the reduction of the cations by roughly one electron and the anions by up to half an electron. Thus, all ions in these two ILs were found to be cathodically unstable on the Li surface. Additionally, the cations were shown to be physisorbed to the surface, while the anions were chemisorbed. Through AIMD simulations, a variety of decomposition reactions were probed for all ions. The cations remained reduced by roughly one electron and decoupled from the Li surface up to the point of decomposition. The anions, on the other hand, remained fractionally reduced and chemisorbed to the surface up to the point of decomposition, which suggests the reductive decomposition of the anions is mediated by a chemical association with the Li surface. For most ions, high temperatures were required to induce decomposition, while Received: September 28, 2017 Revised: November 17, 2017 Published: November 27, 2017 28235
DOI: 10.1021/acs.jpcc.7b09658 J. Phys. Chem. C 2017, 121, 28235−28248
Article
The Journal of Physical Chemistry C TFSI was shown to decompose at room temperature. The decomposition products observed during the course of the AIMD simulations were verified experimentally through a comparison to available XPS data. The present work uses density functional theory computations of gas phase ions and ion pairs to examine the stability and decomposition reactions of bulk ionic liquids on a Li surface. The gas phase computations are applied to three aspects of the behavior of ILs on Li surfaces. First, the question of cathodic and anodic stability is evaluated on the basis of an analysis of ion pair-Li surface electronic level alignment and state mixing. Standard electronic level alignment procedures are extended to incorporate electrostatic surface effects into gas phase ion pair levels, which is compared to the alignment of ion pairs with an explicit Li surface. Second, the thermodynamics of decomposition are examined on the basis of the thermal and reductive bond dissociation free energetics of individual ions. The relative magnitude of the ion bond dissociation free energies is employed as a qualitative index of the favorability of a given decomposition mode. Third, the relative rates of the reactions are evaluated from decomposition barriers for ions reduced by one electron and for ions in the presence of Li atoms. The barriers from reduced ions are akin to those where reductive decomposition proceeds without chemical interactions with the Li surface, as noted for cations in Part 1. Alternatively, the barriers computed in the presence of Li atoms correspond to those where reductive decomposition is mediated by chemical association with the Li surface, as noted for anions in Part 1. The Li atoms act as an electron source that, through complexation, can enable the fractionally reduced states noted for anions in Part 1. The predictions from the gas phase calculations are shown to be fully in accord with the results of the AIMD simulations in Part 1. To outline this work briefly, details related to the computational treatment of gas phase ions and the Li surface are provided in Section II. We present our results in Section III, with relevant AIMD results being described in each section as a precursor to the gas phase analyses. The oxidative and reductive stability of ionic liquids on a Li surface as determined from the alignment of the electronic levels of gas phase ions and ion pairs with the Li Fermi level is discussed in Section III.1. Evaluation of the thermal and reductive decomposition reactions on the basis of bond dissociation energies is described in Section III.2. The rates of the most promising reactions are evaluated for reduced ions and ions in the presence of Li atoms in Section III.3.
Figure 1. Gas phase optimized structures of the (a) pyr14 cation, (b) TFSI anion, (c) EMIM cation, and d) BF4 anion. The color-coding used for the atoms in the figures is C (gray), S (yellow), F (purple), O (red), N (blue), H (white), and B (pink).
Properties of gas phase ions and ion pairs also were obtained from plane wave DFT calculations as performed using the Vienna Ab Initio Simulation Package (VASP).40 The ions and ion pairs were placed in a periodic, cubic box with an edge length of 30 Å. The PBE, HSE06, and B3LYP functionals were used. For these computations, the k-space consisted of only the Γ-point, smearing was performed with a Gaussian scheme and a width of 0.2 eV, and the energy cutoff for the plane-wave (PW) basis set was 500 eV. In addition, the energy, potential, and forces were corrected for periodic charge and dipole interactions. The primary quantities computed were the valence band maximum (referred to here for simplicity as the HOMO) energy and the conduction band minimum (referred to here for simplicity as the LUMO) energy. An ensemble of the bulk ionic liquid configurations obtained from the simulations performed in Part 135 was analyzed in this work with PW DFT using VASP. Different functionals were used, including PBE, HSE06, and B3LYP. The simulation specifications for the bulk liquids can be found in Part 1.35 The quantities computed in this case included the density of states, the valence band maximum (HOMO), the conduction band minimum (LUMO), and the vacuum potential. For each liquid, all properties were averaged over 10 configurations, as obtained from room-temperature molecular dynamics simulations. In particular, computation of the vacuum potential required the generation of slabs from the bulk liquid systems. This was accomplished by breaking the periodicity of the liquid in one direction and inserting a 20 Å region of vacuum along this direction. The potential was corrected to remove the interaction of periodic images of the slab in the direction normal to the slab surface. The potential plateau at distances far from the slab was taken as the vacuum potential. A lithium slab was studied with PW DFT using VASP. The slab was terminated along the (001) plane and was composed of 9 layers. A vacuum region of 20 Å separated slab images. Computations were performed with the PBE and HSE06 functionals. For these systems, 4 k-points were used in each direction parallel to the slab, while only the Γ-point was considered in the direction normal to the slab. Electron smearing was carrier out using the scheme of Methfessel and Paxton41 with a 0.2 eV width, while the energy cutoff for the PW basis set was 500 eV. Energy, potential, and forces were corrected to remove the periodic interaction of slab images in
II. COMPUTATIONAL DETAILS Properties of gas phase ions (see Figure 1a−d) and ion pairs were obtained from density functional theory (DFT) computations using Gaussian0936 (G09). Several functionals were used, including PBE (denoted PBEPBE in G09),37 HSE06 (denoted HSEH1PBE in G09),38 and B3LYP.39 The use of empirical dispersion was examined and found to provide similar energetics to the PBE and B3LYP functionals, as demonstrated in Tables S1 and S2. These computations were performed with the 6-31+G** basis set, referred to here as simply the Gaussian basis set (G). In terms of properties, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies, orbital maps, thermal and reductive bond dissociation energies, and reaction transition states were computed. 28236
DOI: 10.1021/acs.jpcc.7b09658 J. Phys. Chem. C 2017, 121, 28235−28248
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The Journal of Physical Chemistry C Table 1. Electrochemical Windows of Ions, Ion Pairs, and Bulk Ionic Liquidsa Cation
Anionb
Pair
Bulk
pyr14-TFSI PBE HSE06 B3LYP
PW/G 6.77/6.86 8.22/8.35 8.56/8.65
PW/G 4.15/3.10 5.19/4.16 5.44/4.35
PW/G 4.68/4.71 6.03/5.97 6.30/6.10
PW 4.64 6.2516 6.5316
645,46 645,46 645,46
EMIM-BF4 PBE HSE06 B3LYP
PW/G 5.02/5.03 6.41/6.42 6.68/6.69
PW/G 3.83/2.75 5.51/4.40 5.65/4.52
PW/G 4.50/4.67 6.72/6.56 6.98/6.84
PW 3.94 5.1516 5.4616
4.347,48 4.347,48 4.347,48
Experiment
a
All quantities are given in volts. Windows are computed using different functionals with the PW (plane wave) and G (Gaussian) basis sets. The bulk liquid HSE06 and B3LYP values are taken from a previous work.16 bCathodic limit taken as 0 V.
Various methods for computing ion electrochemical windows are examined. The electrochemical window is computed from gas phase ions, ion pairs, and bulk liquids, which were examined in Part 1.35 This allows for the evaluation of the influence of increasing environmental accuracy on the window, with gas phase ions having no effective environment, gas phase ion pairs having potential from the counterion as the environment, and the bulk liquids having a fully solvated environment. One computational peculiarity arises in the treatment of the electrochemical window of isolated, gas phase anions. Unlike the LUMO energy of the gas phase cations and ion pairs, the LUMO energy of the anions is positive, which indicates that the addition of an electron is not favorable. In turn, this leads to the LUMO energy being dependent on the basis set. We demonstrate this in Supporting Information Figure S1. Upon adding additional diffuse s-functions to the basis set, the LUMO energy decreases toward zero. Correspondingly, the LUMO becomes more expansive and extends away from the anion. In the infinite basis set limit, the wave function would allow the electron to be infinitely separated from the anion to minimize electrostatic interaction. The lack of a well-defined LUMO complicates the assignment of a cathodic limit for the level alignment process. We thereby use a cathodic limit of 0 (infinite separation of electron from ion) for the window calculations on isolated, gas phase anions. In a more realistic environment, such as in a liquid, the anion will be closely associated with a cation. The association of these ions has a stabilizing effect on the anion LUMO, as given in Supporting Information Figure S2, which displays the energy of the anion LUMO as a function of the distance from a point countercharge. When the countercharge is closer than 8 Å from the anion, the LUMO energy becomes negative and well-defined. As a precursor to the discussion of the environmental effects on the electrochemical window, we discuss the impact of the level of theory. In Table 1, we summarize the different measures of the electrochemical window using pure exchangecorrelation functionals (PBE) and hybrid functionals (HSE06 and B3LYP) as well as the PW and G basis sets. For all systems, the hybrid functionals leads to larger estimates of the window than pure functionals, with the difference being on the order of 1−2 V. Within the hybrid functionals, B3LYP provides a 0.1− 0.3 V larger estimate of the window than HSE06. The trends for these systems are consistent with previous works16,43 and the common observation that pure functionals typically underestimate the HOMO−LUMO gap.38 Concerning the use of the G and PW basis sets, for cations and ion pairs both basis sets provide similar windows (within 0.1−0.2 V), while for anions the differences are larger (within 1 V). The PW estimate
the direction normal to the slab surface. The quantities obtained from these calculations were the Fermi level and the vacuum potential.
III. RESULTS AND DISCUSSION III.1. Ion Stability at the Li-Metal Surface. III.1.1. Summary of AIMD Results. In these simulations, ion pairs and bulk liquids were placed in contact with a Li surface terminated along the (001) plane. The electronic structure was computed, from which the degree of charge transfer between the surface and the electrolyte was evaluated. The electronic structure was determined from both energetic minima (T = 0 K) as well as from AIMD simulations (finite temperature). Concerning the ion pairs, unoccupied electronic states on both the cation and anion were found to mix with Li surface states. Charge transfer from the surface to the ions accompanied the state mixing, suggesting all ions were reductively unstable on lithium. The charge transfer was configuration dependent, ranging from 0.3 to 0.5e for pyr14, 0.4−0.6e for EMIM, 0.3e for TFSI, and 0.1− 0.2e for BF4. The cations in this case resided roughly 3−4 Å from the surface, while the anions bound strongly with surface Li atoms, residing 1−2 Å from the surface. In the case of bulk liquids, ion/Li surface state mixing was again noted for all species on the lithium surface. The amount of charge transfer to ions at the interface ranged from 0.7 to 1.1e for pyr14, 1.2−1.4e for EMIM, 0.2−0.5e for TFSI, and 0.0−0.1 for BF4. A positioning of the cations and anions above the Li similar to that of the ion pair case was noted for the bulk liquid. III.1.2. Gas Phase Analysis: Ion Stability. The AIMD simulations showed that all ions accept charge from the Li surface. One route to reductive charge transfer to the ions is the LUMO energies being equal to or lower than the Fermi energy of the Li surface. In principle, this can be assessed from a DFT mediated electronic energy level alignment.42 Energy level alignments are typically carried out by first determining the cathodic energy limit (given by the LUMO energy, EL) and the anodic energy limit (given by the HOMO energy, EH) of an electrolyte, the difference between which yields an estimate of the electrochemical window, EL − EH .16,43 These energies are e then shifted based on expected interactions with the electrode surface. An assessment of cathodic or anodic stability can be made based on the position of the limit energies with respect to the electrode Fermi energy. The electrolyte is suggested to be stable on the electrode if the Fermi energy falls between EL and EH. On the other hand, anodic (oxidative) or cathodic (reductive) instabilities are suggested if the Fermi energy is lower than EH or higher than EL, respectively. 28237
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The Journal of Physical Chemistry C of the anion window is likely in error, as the diffuse anion LUMO can be sensitive to the size of the periodic box. For the remainder of the electrochemical window and level alignment discussion, we focus on the windows obtained from the HSE06 functional and the PW basis set, which provides a full set of results spanning single ions to bulk liquids and is expected to be similar in accuracy to other hybrid functionals. In terms of environment effects, the windows obtained from ions, ion pairs, and bulk liquids can exhibit large differences. The pyr14 cation electrochemical window is ∼3 V larger than that of the TFSI anion. The window of the pyr14TFSI pair (6.03 V) is more similar in value to that of the anion, while the bulk [pyr14][TFSI] window (6.25 V) is only 0.22 V larger than that of the pair. The EMIM cation electrochemical window is ∼1 V larger than that of the BF4 anion. The EMIMBF4 pair window (6.72 V) is 0.31 V larger than that of the cation, while the bulk [EMIM][BF4] window (5.15 V) is 1.57 V smaller than that of the pair. In this regard, the environment can be rationalized as influencing the ion window both electrostatically and structurally. The electrostatic interactions between neighboring ions can shift the relative alignment of the electrochemical window limits. Structural changes the ion undergoes in response to interactions with neighboring ions can change the magnitude of the window, further altering the level alignment. At the bulk level, [pyr14][TFSI] liquid has a 1 V larger window than [EMIM][BF4] liquid. This is in general agreement with trends noted from experiment.44−47 In terms of magnitude, our [pyr14][TFSI] window is within 0.5 V agreement with the experimental measurements.44,45 The [EMIM][BF4] window is within 1 V agreement with the experimental measurements.46,47 The experimental windows of both systems, however, are bounded by the maximum and minimum computational estimates, corresponding to hybrid and pure functionals, respectively. Using the computed electrochemical windows, we apply and compare two different approaches for energy level alignment with the Li surface Fermi level. The first approach is to subtract the workfunction of the Li surface from the ion cathodic and anodic energy limits, which equates to the ions only interacting with the surface through the vacuum potential (referred to here as the noninteracting approach). This is the most commonly used approximation for level alignments. The second approach subtracts the Li surface workfunction from the ion energy limits and then incorporates an additional energy shift attributed to surface interactions, which includes image charges and dipoles that ions would feel above an ideal metallic surface (referred to as the interacting approach). This provides the alignment of the energy limits of cathodic and anodic energy stability with Fermi energy as a function of distance from the electrode surface. The alignment of the ion window limits with respect to the Fermi energy of the Li surface in the noninteracting regime is shown in Figure 2. To avoid the complications posed by isolated anions, we use EH and EL of the ion pairs and the density of states from bulk liquids for alignment with the Li surface. For the [pyr14][TFSI] liquid, presented in Figure 2a., the ELi,f is seen to be 1.5 eV above the HOMO level of the liquid (valence band maximum) and 4.2 eV below the LUMO energy (conduction band minimum). For the gas phase ion pair, ELi,f is centered in the window 1.9 and 4.4 eV from EL and EH, respectively. For the [EMIM][BF4] liquid, the EH is 0.5 below ELi,f, while EL is 3.5 eV above ELi,f. In the gas phase, ELi,f is
Figure 2. Alignment of the electronic levels of gas phase ion pairs and bulk liquids with the Fermi-energy of the Li surface (ELi,f). Results for the pyr14 cation and TFSI anion are given in (a), while results for the EMIM cation and BF4 anion are given in (b). Solid lines represent the density of states (DOS) of cations and anions in the bulk liquid, while the dashed lines are the HOMO and LUMO energies of the gas phase ion pairs. The occupied and unoccupied regions of the liquid DOS are labeled. Energies and the DOS are determined from the HSE06 functional and the PW basis set.
centered in the window at 1.6 and 5.4 eV from EL and EH, respectively. The noninteracting approach to level alignment thus predicts that bulk liquids and gas phase ion pairs interfaced with the Li surface would be stable. This is in direct conflict with the results of the AIMD simulations performed in Part 1.35 Indeed, we have noted that all ions exposed to the lithium surface exhibit signs of hybridization and reduction, which suggests EL should be similar to ELi,f. Worse still, the liquid densities of state suggest near anodic instability, where charge transfer would occur from the ions to the lithium slab. A potential improvement to the standard procedures employed for level alignment incorporates a measure of surface physics into the gas phase ion pair results - the interacting approach. We propose that the leading influence of the surface on the ion pair level energies is the image dipole present on the metal surface. To model this in a gas phase system, we represent the expected image effect as a point dipole of the same magnitude as that of the gas phase ion pair, placed at twice the distance from the ion to the surface. Classical electrostatic interactions are assumed between the ion pair image and the electronic levels. Specifically, a molecule with a ⎯⇀ ⎯ dipole ( pI ) at a given distance from the surface (rs) would act ⎯⇀ ⎯
|p |
to shift the energy of the electronic levels by ± 16πεI r 2 , where ε0 0 s
is the permittivity of free space and |···| indicates the magnitude of a vector. We assume in this expression that the ion dipole is ⎯⇀ ⎯ normal to the surface, resulting in only the magnitude | pI | appearing in the energy expression. When the dipole points away from the surface (−), the image dipole points to the molecule and maximally decreases the energy of the electronic
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DOI: 10.1021/acs.jpcc.7b09658 J. Phys. Chem. C 2017, 121, 28235−28248
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The Journal of Physical Chemistry C
Figure 3. Comparison of the HOMO and LUMO energies of gas phase ion pairs (thin dashed lines) and the HOMO and LUMO energies of gas phase ions pairs under the influence of image effects from an ideal metal surface as a function of distance from the surface (thick dashed lines) for (a) pyr14-TFSI and (b) EMIM-BF4. The LUMO energy is provided for the case of the ion dipole pointing away from the surface, while the HOMO energy is provided for the case of the ion dipole pointing toward the surface. Energies are obtained from the HSE06 functional and the PW basis set.
levels. Oppositely, the energy of the levels is maximally increased when the dipole points away from the surface (+). Though positioning the dipole normal to the model surface may not represent the minimum energy configurations of ions on a true surface, entropic effects in a liquid can put the ions into high energy configurations, as noted from AIMD simulations.35 If these maximally biased configurations lead to the surface Fermi energy falling outside the limits of the electrochemical window, we posit that the ions will hybridize with the surface and donate or accept some degree of charge. The level alignment obtained from the interacting approach is shown in Figure 3 for gas phase ion pairs. We look at the interaction of EH for the case of the dipole pointing toward the surface (maximum increase in level energy) and EL for the case of the dipole pointing away from the surface (maximum decrease in level energy) to assess the likelihood of anodic and cathodic instability with respect to the Li surface. For all ions, the value of EH is far below the Fermi level, and surface effects do not lead to energies where anodic instability becomes realistic. Even at 2 Å from the surface, the EH is more than 1 eV below ELi,f for both pairs. On the other hand, EL of the ion pairs is shown to decrease below ELi,f at 2−3 Å from the surface. This is similar to the distances the ions were shown to reside with respect to the surface in AIMD simulations. The decrease in energy upon approaching the surface indicates that the ion LUMO energy can be brought to a value that allows hybridization with the lithium surface and reductive charge transfer. An examination of the LUMO orbitals for the ion pairs, as given in Figure 4, shows that the LUMO orbital resides on both the cation and anion. This indicates charge reductive charge transfer would involve both ions. An important factor that has not been addressed in this discussion is the nature of the mixed ion/Li surface states arising below the Li surface Fermi energy. A comparison of the level alignment predictions made using the interacting approach with the energetics obtained from explicit computations of ion pairs on Li surfaces is given in Figure 5 and Figure S3. For the case of the ion pair dipole pointing away from the surface, energetic trends obtained from the interacting approach to level alignment and the explicit computations agree well. There is also no hybridization below the Fermi energy until the ion pair is near the surface, roughly 2 Å. For the dipole pointing toward the surface, the energetic trends predicted from our level alignment are followed until 6−7 Å from the surface, where numerous mixed ion-surface states are found to develop below
Figure 4. Lowest unoccupied molecular orbitals (LUMOs) of the (a) pyr14-TFSI and (b) EMIM-BF4 ion pairs as computed using the B3LYP functional and the G basis set. The green and orange regions of the LUMOs represent positive and negative contributions, respectively. Cation and anion orbitals are determined at different isovalues to highlight their distributions.
the Fermi energy. The primary difference between these cases is that the anion is closer to the surface in the former case, and the cation is closer to the surface in the latter case (see Figures 5 and Figure S3 for a visualization of the atomic configuration). The ion that is closer to the surface appears to dictate how the ion pair states mix with those of the surface. When the anion is closer to the surface, the ion pair does not readily form occupied, mixed states with the surface. The ion pair in this case must therefore be electrostatically poised for reduction by the surface image effects described in Figure 3. On the other hand, when the cation is closer to the surface, the ion pair readily forms mixed states regardless of the electrostatic bias from surface images. For the question of ion stability, we have shown the interacting approach to level alignment predicts cathodic instability of the ion pairs. An analysis of the LUMO orbital indicates that both ions would be involved in any corresponding reductive charge transfer. These results agree well with our previous findings from the AIMD simulation and comparisons to computations of ion pairs above explicit Li surfaces. Alternatively, level alignment using the noninteracting approach incorrectly predicts the ion pairs and bulk liquids to be stable on the Li surface. Therefore, our analysis demonstrates how the incorporation of classical surface electrostatics into gas phase results can be used to provide a reasonable indication of ion electrochemical stability on the electrode surface. 28239
DOI: 10.1021/acs.jpcc.7b09658 J. Phys. Chem. C 2017, 121, 28235−28248
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The Journal of Physical Chemistry C
Figure 5. Comparison of the HOMO and LUMO energies of gas phase ion pairs (thin dashed lines), the HOMO and LUMO energies of gas phase ions pairs under the influence of image effects from an ideal metal surface (thick dashed lines), and electronic energy levels of ion pairs at various distances from an explicit Li surface (shaded regions). Energies are shown for the ion pair dipole pointing away from the surface (anion down) for (a) pyr14-TFSI and (b) EMIM-BF4. Energies are obtained from the HSE06 functional and the PW basis set.
III.2. Thermal and Reductive Decomposition Reactions. III.2.1. Summary of AIMD Results. Several decomposition reactions were observed in both room temperature and high temperature AIMD simulations of ions on a Li surface. At room temperature, the TFSI anion readily decomposed. The TFSI decomposition was initiated by the near simultaneous scission of the S−N and S−C bonds. This led to the formation of metastable SO2 and CF3 species and an NSO2CF3−Li complex. The SO2 and CF3 species were rapidly atomized (within 1 ps) and incorporated into the lithium slab. The remaining NSO2CF3−Li complex eventually decomposed (after several ps) into N, SO2, and CF3 species. All decompositions processes were found to be highly exothermic. Room temperature decomposition events for EMIM, pyr14, and BF4 were not observed. High temperature (T = 1000− 2500 K) simulations of these species on the Li surface were used to accelerate decomposition. For the pyr14 cation, the butyl chain was the initial species to be removed from the pyrrolidinium ring. For the EMIM cation, the ethyl chain was the first species to be severed from the imidazolium ring. The BF4 anion, naturally, decomposed through the removal of fluorine species. III.2.2. Gas Phase Analysis: Bond Dissociation. The decomposition reactions observed during the course of the AIMD simulations are not exhaustive. Additional decomposition processes could be favorable but restricted by kinetic barriers such that they would not be observed on the picosecond time scale of these molecular simulations. Therefore, possible reactions are probed on the basis of bond dissociation energies (BDEs) and free energies (BDFEs). The BDE is defined as ΔE = EA + EB − EAB, where EA, EB, and EAB are the total energies, without zero-point energies, of A, B, and AB, respectively, which gives the energy change when a bond is cleaved in molecule AB to form separate species A and B. The definition for the BDFE follows by replacing energy with Gibbs free energy (G), which is computed at standard conditions here. Using these quantities, the relative favorability of specific decomposition reactions may be ranked, with lower values of BDE and BDFE being more favorable. For all ions, these quantities were computed for a variety of thermal and reductive decomposition reactions. In addition, a variety of radical reactions involving the EMIM cation are studied for comparison to decomposition reactions due to their proposed importance upon reduction, as determined in a previous work.48
We briefly discuss the influence of the computational method on our results as well as how the values of BDEs compare to BDFEs. The bond dissociation quantities for cations and anions are provided in Tables 2−4 using both the PBE and B3LYP Table 2. Thermal Bond Dissociation Energies (ΔE) and Free Energies (ΔG) for Cation Decomposition Reactionsa Reactions pyr14 (C9H20N+) C9H20N+ → C4H9+ + C5H11N C9H20N+ → C4H9 + C5H11N+ C9H20N+ → CH3 + C8H17N+ C9H20N+ → CH3+ + C8H17N C9H20N+ → CH3 + C8H17N+ C9H20N+ → CH3+ + C8H17N EMIM (C6H11N2+) C6H11N2+ → C2H5 + C4H6N2+ C6H11N2+ → C2H5+ + C4H6N2 C6H11N2+ → CH3 + C5H8N2+ C6H11N2+ → CH3+ + C5H8N2 C6H11N2+ → CH3 + C5H8N2+ C6H11N2+ → CH3+ + C5H8N2
B3LYP
PBE
Bond
ΔΕ
ΔG
ΔΕ
ΔG
C−N
79.4
57.3
70.8
48.4
C−N
72.5
49.4
76.0
53.2
C−N C−N C−C C−C
70.0 133.5 91.4 160.3
49.5 115.3 72.0 134.9
73.3 141.7 96.3 168.2
53.0 123.8 77.2 144.3
C−N
101.8
81.5
106.8
86.7
C−N
96.1
78.9
98.3
81.5
C−N
101.2
82.9
106.4
88.3
C−N
136.5
120.9
144.0
128.6
C−C
89.5
71.3
93.6
75.8
C−C
194.5
176.6
197.9
180.4
ΔG is computed at standard conditions. All values are given in kcal/ mol and are computed using the G basis set.
a
functionals. In general, the qualitative trends between the energetics provided by the two functionals are the same, with differences being on the order of a few kcal/mol. Similarly, BDEs provide qualitatively similar trends in the favorability of decomposition reactions when compared to BDFEs. Due to these similarities, our analyses and discussion will focus only on the BDFEs evaluated using the B3LYP functional. The BDE and BDFE results for thermally activated decomposition processes in cations are summarized in Table 2. As expected for stable species, none of the proposed thermal bond breaking reactions are favorable. For the pyr14 cation, neutral butyl group (C4H9) removal from the cation by C−N bond cleavage has the lowest BDFE (49.4 kcal/mol). The next 28240
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lowest BDFE corresponds to the neutral methyl (CH3) removal by C−N cleavage (49.5 kcal/mol). Other processes, including the removal of cation methyl and butyl groups, are significantly less favorable. BDFEs for thermally activated bond breaking for the EMIM cation are similar to those of the pyr14 cation. The BDFEs for the EMIM cation are higher overall than those for the pyr14 cation. Among the reactions studied, the neutral methyl group (CH3) removal from the ethyl chain has the lowest BDFE (71.3 kcal/mol), followed by the removal of the entire ethyl group as a cation (C2H5+) via the C−N bond cleavage (78.9 kcal/mol). The removal of neutral ethyl and removal of neutral methyl side chains by C−N cleavage are competitive with the other processes (81.5 and 82.9 kcal/mol, respectively). Reductive decomposition reactions were examined for the pyr14 cation. The chemical formulas for these decompositions are given in Figure 6, with the corresponding BDEs and BDFEs
Table 3. Reductive Bond Dissociation Energies (ΔE) and Free Energies (ΔG) for Cation Decomposition Reactions (see Figure 6 for reaction description)a Reactions
B3LYP
pyr14 (C9H20N+) P1 P2 P3 P4 EMIM (C6H11N2+) E1 E2 E3 E4 E5 E6 E7
PBE
ΔΕ
ΔG
ΔΕ
ΔG
−96.8 −98.3 −94.1 −56.9
−108.2 −98.0 −103.7 −60.8
−90.9 −93.7 −88.0 −59.8
−102.0 −93.2 −97.2 −64.3
−92.7 −91.0 −67.3 −89.0 −46.3 −41.2 10.2
−100.3 −96.6 −61.7 −80.3 −28.9 −38.4 29.6
−87.7 −85.7 −63.4 −88.5 −48.9 −39.0 −7.6
−95.0 −90.9 −57.8 −79.7 −31.9 −36.2 12.0
ΔG is computed at standard conditions. All values are given in kcal/ mol and are computed using the G basis set.
a
Table 4. Thermal and Reductive Bond Dissociation Energies (ΔE) and Free Energies (ΔG) for Anion Decomposition Reactionsa Reactions
B3LYP
PBE
Bond
ΔΕ
ΔG
ΔΕ
ΔG
C−F
118.1
105.8
124.6
112.9
C−Fb
76.7
62.7
82.1
68.9
C−F
112.0
101.8
105.7
95.5
C−S
100.3
83.5
97.4
80.7
C−S
53.9
36.8
55.8
39.3
N−S
71.9
54.7
73.3
56.2
N−S
98.8
81.7
96.8
79.9
90.0
86.0
c
c
C−S
10.3
3.3
14.7
8.1
C−F
37.1
36.9
41.4
41.4
C−Fb
−4.3
−6.2
−1.1
−2.6
N−S
−10.6
−16.4
−2.6
−8.6
B−F B−F
80.1 169.8
70.8 157.5
80.1 170.8
70.9 159.0
B−F
123.6 88.8
120.3 88.5
121.9 87.6
117.9 87.5
−
TFSI (C2F6NO4S2 ) thermal C2F6NO4S2− → F + C2F5NO4S2− C2F6NO4S2− → F + C2F5NO4S2− C2F6NO4S2− → F− + C2F5NO4S2 C2F6NO4S2− → CF3− + CF3NO4S2 C2F6NO4S2− → CF3 + CF3NO4S2− C2F6NO4S2− → NSO2CF3− + SO2CF3 C2F6NO4S2− → NSO2CF3 + SO2CF3− reductive C2F6NO4S2− + e- → C2F6NO4S22− C2F6NO4S2− + e- → CF3− + CF3NO4S2− C2F6NO4S2− + e- → F− + C2F5NO4S2− C2F6NO4S2− + e- → F− + C2F5NO4S2− C2F6NO4S2− + e- → NSO2CF3− + SO2CF3− BF4 thermal BF4− → BF3 + F− BF4− → BF3− + F reductive BF4− + e- → BF42− BF4− + e- → BF3− + F−
Figure 6. Cathodic reductive reactions for the pyr14 cation (P1−P4) and EMIM cation (E1−E4) as well as the radical reactions for EMIM cation (E5−E7).
provided in Table 3. Three possible reductive decomposition reactions were considered, labeled P1, P2, and P3. The first reaction corresponds to pyr14 decomposition, via electron addition, into a methylpyrrolidine and a butyl radical (P1). The second corresponds to the radical decomposing into a dibutylmethlyamine radical via ring opening (P2). The final reaction is a reductive dissociation of the pyr14 cation into a butylpyrrolidine and a methyl radical (P3). The process of reduction without decomposition is labeled P4. The BDFEs summarized in Table 3 for pyr14 cation indicate that all the reduction reactions studied are favorable. This is significantly different than the thermal reactions detailed in Table 2 for this cation. The most favorable reaction is the formation of a butyl radical (P1, −108.2 kcal/mol). This is followed by formation of a methyl radical (P3, −103.7 kcal/ mol) and then ring opening (P2, −98.0 kcal/mol). The BDFEs associated with the reactions are thus in the following order, P1 < P3 < P2. The largest energy difference among these reactions is 10.2 kcal/mol. The high exothermicity and small energetic variance of P1−P3 lead us to expect them to all be competitive,
ΔG is computed at standard conditions. All values are given in kcal/ mol and are computed using the G basis set. bThe bond energy is computed for the product that has rearranged. cNot a local minimum; dissociates into two fragments.
a
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functional leads to the dissociation to two anions. As previously stated, our analyses and discussion will focus on the results of the B3LYP functional. Thermal decomposition reactions of TFSI were examined, as given in Table 4. As expected, the BDEs and BDFEs show that thermal bond breaking is not favorable. For TFSI, the smallest BDFE corresponds to C−S bond breaking (36.8 kcal/mol), which results in the formation of a neutral CF3 (trifluoromethyl) fragment and a CF3NO4S2− ion. The next lowest BDE corresponds to S−N bond breaking (54.7 kcal/mol), which results in SO2CF3 and NSO2CF3− products. We find that C−F bond breaking can be competitive with C−S and S−N bond breaking, although we find two solutions. The larger BDFE corresponds to the solution for which the C2F5NO4S2− radical looks very much like the TFSI parent (105.8 kcal/mol). For the smaller solution, the CF2 group shifts from the S to the N atom, and the SO2 group becomes loosely bound to the rest of the molecule (62.7 kcal/mol). This rearrangement stabilized the molecule by 43.1 kcal/mol, therefore making the loss of F more competitive compared with other bond breaking processes. These results are consistent with a recent DFT study on the decomposition of LiTFSI salt in organic solvents.50 BDEs and BDFEs for the reductive decomposition reactions of TFSI are provided in Table 4. BDFEs associated with cleavage of C−S, S−N, and C−F bonds are dramatically reduced when compared to thermal decomposition processes. The BDFE associated with the cleavage of the S−N bond now becomes the most favorable (−16.4 kcal/mol). The free energies show that the C−S bond cleavage becomes competitive (3.3 kcal/mol). The breaking of the C−F bond also becomes more favorable for the rearranged configuration (−6.2 kcal/mol). Similar results have been reported for the reduction of LiTFSI salt in earlier experimental studies.51,52 A previous computational study has reported that C−S bond cleavage was the most favorable path for LiTFSI salt decomposition.33 The thermal and reductive decomposition of BF4 is also examined in Table 4. The thermal removal of an F atom from the tightly bound anion is not favorable. Note, however, that the thermal removal of an F− ion has a much lower BDFE than that of a neutral F atom. Reductive decomposition by removal of an F anion is associated with a high BDFE, suggesting that this reaction is not favorable as well. Kinetics may ultimately control these reactions;53 thus, the reaction barriers may impact the viability of these particular decomposition processes. We can relate the distribution of the LUMO to the favorability of a given reductive decomposition reaction. The LUMO orbital plots for the ions pairs are shown in Figure 4. For the pyr14 cation (Figure 4a), the LUMO level is localized to the C atoms on the ring as well as to chain carbons close to N. These atoms likely adopt a large amount of charge when the cation is reduced, weakening the C−N bonds. This is in agreement with BDFEs associated with reductive decomposition of the pyr14 cation (see Table 3), which indicates ring opening, butyl group loss, and methyl group loss are favorable. For the EMIM cations (Figure 4b), the LUMO is localized on the C and N atoms in the aromatic ring. Charge transferred to these atoms would invariably weaken the N bonds to the methyl and ethyl chains. BDFEs (see Table 3) associated with reductive decomposition of the EMIM cation indicate that ethyl group as well as methyl group removal are the most favorable reactions. For the TFSI anion (Figure 4a), the LUMO
barring large differences in kinetic barriers. Previously, Kroon et al.,48 using semiempirical calculations (PM3 level),49 reported the most favorable reactions for this cation to be P1, followed by P2, and finally P3, with a difference of about 5 kcal/mol between each reaction. The differences between their results and those presented here can be attributed to the different levels of theory used and our inclusion of vibrational effects. Reductive decomposition reactions were examined for the EMIM cation. The structures for these reactions are shown in Figure 6, with the corresponding BDE and BDFE values given in Table 3. Three purely reductive decomposition reactions were considered for the EMIM cation, labeled E1, E2, and E3. These include the cation decomposition into methylimidazole and an ethyl radical (E1), decomposition into dimethylpyrazole and a methyl radical (E2), and decomposition via ring opening (E3). The reduction of EMIM without decomposition is labeled E4. As with the pyr14 cations, the BDFEs indicate that the reductive decomposition reactions of EMIM (E1 to E3) are highly exothermic. Generally speaking, the chain dissociation reactions (E1 and E2) are more favorable than the ring opening (E3), likely due to the strong bonds in the aromatic ring. The most favorable reaction among the three corresponds to decomposition into a methylimidazole and an ethyl radical (E1, −100.3 kcal/mol). The energy difference between reactions E1 and E2 is 1.7 kcal/mol, while the energy difference between E1 and E3 is 25.4 kcal/mol. Because all three reactions are highly exothermic, we expect them to be competitive provided the barriers to decomposition are comparable. In an earlier quantum chemistry study, reductive radical reactions were explored for the BMIM cation.48 These radical reactions are examined here using the EMIM cation. The structures corresponding to these reactions are provided in Figure 6, with the corresponding BDEs and BDFEs being given in Table 3. Following the previous work, three reactions involving two EMIM radicals were considered, labeled E5, E6, and E7. The first reaction involves the formation of a dimer by the interaction of two radicals (E5). The second reaction is a disproportionation reaction, where an H atom is taken from one of the radicals (E6). The final reaction corresponds to two radicals forming a cage-like structure (E7). The results presented here indicate that the most thermodynamically favorable radical reaction is disproportionation (E6, −38.4 kcal/mol), followed by formation of a dimer (E5, −29.6 kcal/ mol), and finally the formation of the cage-like structure (E7, 29.6 kcal/mol). These observations are similar to those reported for the BMIM cation, where the disproportionation reaction was also shown to be the most favorable.48 A comparison of free energies shows that the reductive decomposition reactions (E1-E3) are significantly more favorable than the radical reactions (E5-E7). Thus, calculations using gas phase ions suggest that decomposition is a more likely outcome of reduction that radical reactions. In Table 4, BDEs and BDFEs for TFSI and BF4 anions are presented. These BDEs are for both thermally activated decomposition processes, which do not involve charge transfer, and to reductive decomposition processes, which result in the formation of two anions. The BDEs and BDFEs are computed using the PBE and B3LYP functionals; energetics from both functionals are in generally good agreement. However, one difference between the functionals is noted when adding an electron to the TFSI anion. Use of the B3LYP functional leads to the formation of a metastable dianion, while use of the PBE 28242
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Table 5. Energy of Reaction (ΔE), Energy of Transition State (ΔE*), Free Energy of Reaction (ΔG), and Free Energy of Transition State (ΔG*) for the Decomposition of Reduced Cations and Cations in the Presence of a Li Atoma Reactions pyr14 reductive C9H20N → C4H9 + C5H11N C9H20N → C9H20N C9H20N → CH3 + C8H17N Li-mediated LiC9H20N+ → C4H9 + LiC5H11N+ LiC9H20N+ → LiC9H20N+ LiC9H20N+ → CH3 + LiC8H17N+ EMIM reductive C6H11N2 → C2H5 + C4H6N2 C6H11N2 → CH3 + C5H8N2 C6H11N2 → C6H11N2 Li-mediated LiC6H11N2+ → C2H5 + LiC4H6N2+ LiC6H11N2+ → CH3 + LiC5H8N2+ LiC6H11N2+ → LiC6H11N2+
Bonds
ΔE
ΔE*
ΔG
ΔG*
Qr
C−N (butyl; P1) C−N (ring; P2) C−N (methyl; P3)
−39.9 −41.4 −37.2
7.2 4.1 4.5
−47.4 −37.2 −42.9
3.5 2.0 2.3
1.0 1.0 1.0
C−N (butyl; P1) C−N (ring; P2) C−N (methyl; P3)
−5.9 −19.7 −9.1
33.7 28.5 34.3
−22.7 −22.5 −23.7
26.6 24.6 30.2
1.0 1.0 1.0
C−N (ethyl; E1) C−N (methyl; E2) C−N (ring; E3)
−3.7 −2.0 21.7
17.9 18.7 37.2
−20.0 −16.3 18.6
14.6 15.3 33.7
1.0 1.0 1.0
C−N (ethyl; E1) C−N (methyl; E2) C−N (ring; E3)
−10.9 −10.1 9.6
14.1 15.6 24.0
−25.6 −24.0 8.1
11.7 13.6 23.3
1.0 1.0 1.0
The degree of reduction of the cation transition states (Qr) is provided in number of electrons. Quantities are given in kcal/mol. ΔG is computed at standard conditions. Values are computed using the B3LYP functional and the G basis set.
a
found to be reduced by one electron and not closely bound to the Li surface. This corresponds to a reductive decomposition that is decoupled from chemical association with the Li surface. On the other hand, the anions are only partially reduced and closely bound to the Li surface. In this case, the reductive decomposition involves a strong chemical coupling to Li atoms on the surface. III.3.2. Gas Phase Analysis: Reaction Barriers. The AIMD simulations show possible decomposition processes that agree well with the thermodynamic analyses of decomposition presented in Tables 2−4. However, these simulations provide only qualitative insight into the relative rates of decomposition based on the use of observed rates at high temperatures. The room temperature decomposition of the TFSI anions, T = 1000 K decomposition of the pyr14 anion, and T = 2500 K decomposition of the EMIMBF4 suggest barrier height likely follows the trend of TFSI < pyr14 < EMIMBF4. We, therefore, compute transition states and associated barriers for many of the reductive decomposition reactions examined in Tables 2−4. We take two approaches to the evaluation of decomposition barriers. The first is simply through the addition of an electron to isolated, gas phase ions and the determination of the transition states associated with the bond dissociations discussed previously, which is referred to as the reductive approach to transition states. This approach is reasonable for cations, which were noted to accept roughly one electron from and not be closely bound to the Li surface in AIMD simulations. For gas phase anions, the ill-defined nature of the LUMO level (as demonstrated in Figure S1) along with the partial reduction by and close coupling to the Li surface (noted in Part I) make this procedure unattractive. Therefore, an additional route is adopted that examines the transition states for bond dissociation in the presence of Li atoms, which is referred to as the Li-mediated approach to transition states. In this way, the Li atoms provide a source of electrons for reduction during bond dissociation in a manner that can be compared to the role of the Li surface in the AIMD simulations performed in Part 1. This approach obviates the questionable
level is localized primarily on the SO2 and N groups. Thus, we expect charge to be distributed to the SO2 and N groups upon reduction. The orbital maps presented here are consistent with the BDFEs in Table 4. The S−N and C−S bond cleavages were found to be the favored reductive decomposition reactions. The LUMO of the BF4 anion (Figure 4b) is localized to the F atoms. Upon reduction, this would assist B−F bond cleavage, which is supported by the BDFE analysis in Table 4. The evaluation of decomposition reactions on the basis of bond dissociation energetics corroborates the results obtained from and extends our understanding of the AIMD results reported in Part 1. The BDE and BDFE analyses show that reductive decomposition is highly favored over thermal decomposition. The BDFE analysis indicates the decomposition reactions observed from AIMD simulations are among the most thermodynamically favorable. There are, however, a number of favorable decomposition reactions that were not observed in AIMD simulations. This is due to time scales accessible to quantum simulations being inadequate to overcome kinetic reaction barriers, a well-known limitation of molecular dynamics approaches to kinetically limited processes. In reality, the unobserved reactions in fact will occur, however, perhaps at a lower rate that depends on the size of the energetic barrier, which will be assessed in the following section. III.3. Barriers to Reductive Decomposition. III.3.1. Summary of AIMD Results. Differences in the decomposition rates of the different ion species were observed from the AIMD simulations. The TFSI anion undergoes rapid decomposition at room temperature within the first picosecond of being exposed to a Li surface. The decomposition occurred by the near simultaneous scission of the S−C and N−C bonds. The other species required higher temperatures to initiate decomposition on the picosecond time scale of AIMD simulations. For pyr14, the butyl side chain removal required simulation at T = 1500 K. For EMIM and BF4, the removal of the ethyl side chain and F atom, respectively, required simulation at T = 2500 K. Furthermore, the process of reductive decomposition was found to differ between cations and anions. The cations were 28243
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The Journal of Physical Chemistry C process of adding a full electron to a gas phase anion, while allowing for partially reduced ions at the decomposition transition state that may follow from a chemical involvement of the Li surface. We treat all ions with both approaches for comparative purposes. The reaction energy (given as energy of the products minus energy of the reactants) and free energy (ΔE and ΔG), barriers associated with these reactions (ΔE* and ΔG*), and the degree of reduction of the ions at the reaction transition states (Qr) in number of electrons are shown in Tables 5 and 6 for both the Table 6. Energy of Reaction (ΔE), Energy of Transition State (ΔE*), Free Energy of Reaction (ΔG), and Free Energy of Transition State (ΔG*) for the Decomposition of Reduced Anions and Anions in the Presence of Li Atomsa Reactions TFSI reductive C2F6NO4S22− → CF3− + CF3NO4S2− Li-mediated LiC2F6NO4S2− → LiCF3NO4S2− + CF3 Li2C2F6NO4S2− → Li2CF3NO4S2− + CF3 BF4 reductive BF42− → BF3− + F− Li-mediated Li2BF4− → BF3 + Li2F− Li3BF4− → BF3− + Li3F
Bonds
ΔE
ΔE*
ΔG
ΔG*
Qr
S−C
−79.7
0.1
−82.7
−0.1
1.0
S−C
−73.4
11.4
−87.4
10.1
0.2
S−C
−93.6
2.6
−97.2
0.2
0.5
B−F
−35.4
34.2
−31.8
33.2
1.0
B−F
10.9
21.9
−5.3
18.0
0.2
B−F
−1.2
16.7
−14.0
16.7
0.5
Figure 8. Transition states of the anion decomposition reactions in the presence of Li atoms. The CF3 removal from TFSI in the presence of (a) one and (b) two Li atoms is shown. The F− abstraction from BF4 in the presence of (c) two and (d) three Li atoms is shown as well. Bonds made between Li atoms and the anion as well as bonds broken within the anions are fully colored red.
The relative rates of the reactions are assessed qualitatively on the basis of the magnitude of their barriers, with larger values occurring less frequently. The charge transferred to the ion transition states is given as 1 au for the reductive approach, while a Bader analysis is performed to assess the charge transferred from Li atoms to the ion transition states the Li-mediated approach. Multiple geometries of the ions in the presence of Li atoms were examined, the lowest energy structure of which was used as a starting point for the Li-mediated transition state calculation. Each Li atom has a spin of 1/2. The following discussion focuses on the value of ΔG and ΔG*. Barriers resulting from the reductive approach to transition states are shown in Table 5 for the previously examined decomposition processes of pyr14 and EMIM. Transition states for the removal of the butyl chain, removal of the methyl chain, and by ring opening were computed for the pyr14 cation. These processes are all thermodynamically favorable. All of these processes also have small, similar barriers. The lowest
a The degree of reduction of the anion transition states (Qr) is provided in number of electrons. Quantities are given in kcal/mol. ΔG is computed at standard conditions. Values are computed using the B3LYP functional and the G basis set.
reductive and Li-mediated transition states. The values of ΔE* and ΔG* are obtained from verified transition states (one imaginary frequency). The transition state structures associated with the Li-mediated approach are provided in Figures 7 and 8.
Figure 7. Transition states of the cation decomposition reactions in the presence of a Li atom. Processes shown for the pyr14 cation include (a) butyl chain removal, (b) ring opening, and (c) methyl group removal. Processes shown for the EMIM cation include (d) ethyl chain removal, (e) methyl group removal, and (f) ring opening. Bonds made between Li atoms and the cation as well as bonds broken within the cations are fully colored red. 28244
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states. The larger distance between the Li atom and the pyr14 cation presents an obstacle to charge transfer from the Li atom. Though a unit charge is eventually transferred from the Li atom to pyr14, the more difficult process of charge transfer leading to a transition state is less stable than that computed through the reductive approach. Barriers computed through the reductive approach to transition states are provided in Table 6 for the anions. The transition state for the removal of the CF3 group, which is thermodynamically favorable, was considered for TFSI. Decomposition in this way through breaking the S−C bond has a transition state associated with a small energetic barrier (∼0.1 kcal/mol). The inclusion of thermal effects leads to the barrier taking a negative value (−0.1 kcal/mol). Other decomposition processes of the reduced TFSI anion were found to be viable, including C−F or N−S bond breaking. Exploration of the processes led to rapid decomposition of the anion via breaking of the S−C bond. As such the small barrier to S−C bond breaking effectively prevented us from isolating transition states associated with other decomposition processes. The reduced BF4 can only decompose through the abstraction of an F anion. This process, which is not thermodynamically favorable, is associated with a high barrier (33.7 kcal/mol). In terms of rates, the S−C scission in TFSI is effectively barrierless, and decomposition is expected to occur rapidly even at room temperature. On the other hand, the large BF4 barrier will lead to slower decomposition. The Li-mediated transition states and barriers to the anion decomposition reactions were evaluated, as provided in Table 6. In this case, the anions strongly associate with Li, which allowed the examination of the influence of up to three Li atoms on barrier height. For the TFSI anion, reaction energetics show the removal of the CF3 group in the presence of Li atoms is thermodynamically favorable, with coordination of TFSI with more Li atoms increasing the favorability. The decomposition barrier in the presence of two Li atoms (0.2 kcal/mol) is negligible and significantly smaller than that in the presence of one Li atom (10.1 kcal/mol). This corresponds to more charge being transferred to the anion at the transition state in the presence of more Li atoms; one Li atom transfers 0.2 au, while two Li atoms transfer 0.5 au For the decomposition of the BF4 anion, decomposition is only favorable in the presence of 3 Li atoms. As with TFSI, the barrier to BF4 decomposition decreases upon adding more Li atoms. For a single Li atom, the decomposition is unfavorable and uphill. The barriers in the presence of two (18.0 kcal/mol) or three (16.7 kcal/mol) Li atoms are significantly smaller than that found from the reductive approach to transition states. The reduction in barrier height again corresponds to a larger degree of charge transfer from more Li atoms; two Li atoms transfer 0.2 au, while three Li atoms transfer 0.5 au. The charge transfer results indicate that anions do not accept a full electron and can favorably decompose after being partially reduced, which is akin to the cooperative reduction−decomposition effect seen in AIMD. The transition states associated with the anion reactions in the presence of Li atoms are shown in Figure 8. The TFSI anion transition states, given in Figure 8a and b, show that a single Li atom binds closely to a single O atom in the SO2 group participating in S−C bond breaking. For two Li atoms, one Li atom binds as noted before, with the second Li atom forming a bidentate bond that bridges the two SO2 groups. The transition state stretches the S−C bond by 0.2 Å, roughly 10%
barrier process is associated with ring opening (2.0 kcal/mol), followed by methyl group abstraction (2.3 kcal/mol) and, finally, butyl group abstraction (3.5 kcal/mol). Transition states for the removal of the ethyl side chain, removal of the methyl side chain, and by ring opening were computed for the EMIM cation. All processes except ring opening are favorable (this is different from the results in Table 3 because Table 3 includes the energy of electron attachment, while Table 5 neglects this and assumes the electron is already attached). The barriers associated with these processes are higher than those of pyr14, which is likely due to stabilization of the reduced species via the aromatic ring. The lowest barrier is associated with ethyl removal (14.6 kcal/mol), which is closely followed by methyl removal (15.3 kcal/mol). Ring opening is a significantly higher energy process (33.7 kcal/mol), which can again be attributed to stronger ring bonds resulting from aromaticity. The decomposition barriers for the EMIM cation are three times larger than those for the pyr14 cation. This suggests pyr14 decomposition occurs more rapidly. Barriers associated with the Li-mediated approach to cation transition states also are provided in Table 5. For these processes, a single Li atom was placed in proximity to the cations and barriers to the aforementioned decomposition processes were determined. For the pyr14 cation, the decomposition energetics in the presence of Li are all favorable. However, the barriers are up to an order of magnitude larger than those found through the reductive approach. The relative order of the barriers now changes to favor butyl abstraction (26.6 kcal/mol) over ring opening (24.6 kcal/mol) and methyl abstraction (30.2 kcal/mol). At the transition states corresponding to these decomposition processes, all pyr14 cations have accepted 1 au of charge from the Li atom. For the EMIM cation, only the ring opening decomposition process is not favorable in the presence of Li atoms. In this case the barriers are found to be somewhat smaller than those obtained from the reductive transition states. The relative order of the barriers remains unchanged, with ethyl removal (11.7 kcal/mol) and methyl removal (13.6 kcal/mol) being highly preferred over ring opening (23.3 kcal/mol). At the transition states corresponding to these decomposition processes, all EMIM cations have accepted 1 au of charge from the Li atom. The different response of the pyr14 and EMIM decomposition barriers to the introduction of Li atoms can be understood based on an examination of the transition states, as provided in Figure 7. The pyr14 transition states, given in Figure 7a−c, show the Li atom does not strongly associate with cation during decomposition. The Li atom is sterically hindered from coordinating with the N or C atoms on the cation by the H atoms. For methyl removal, the Li atom manages to associate with the C atom on the methyl group, while at the same time introducing a significant deformation of the methyl C−H bonds. For the EMIM transition states, given in Figure 7d−f, the Li atom is seen to closely coordinate to the N atoms on the imidazolium ring. In this case, the C atoms of the ring have a single H atom, which resides in the plane of the ring. Thus, the Li atoms can approach and bind with N from above or below the plane of the ring. To summarize, the large number of H atoms on the pyrrolidinium ring of the pyr14 cation restrict Li atom access to potential C and N coordination sites, while such H-mediated obstructions are not present on the ring of the EMIM cation. The complexation of the Li atom with EMIM allows for effective charge transfer from the Li atom to the cation and a small stabilization of the decomposition transition 28245
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Article
The Journal of Physical Chemistry C
reductive decomposition processes are ranked on the basis of the free energetic favorability of bond dissociation in gas phase ions. The reactions noted from ab initio molecular dynamics simulations are found to be favorable and exothermic. Third, a number of decomposition reactions are thermodynamically favorable but not observed with ab initio molecular dynamics. To assess the possible kinetic limitations to the unobserved reactions, decomposition barriers are computed for ions that have been reduced by one electron and ions that are complexed with Li atoms. Barriers computed from reduced ions conform to the case of a reductive decomposition process chemically decoupled from the Li surface, as exhibited by cations in Part 1. Barriers from ions complexed with Li atoms conform to the case of a reductive decomposition that involves a chemical coupling to the Li surface, as exhibited by anions in Part 1. In both cases, the reactions observed in Part 1 are found to have among the smallest barriers computed. Overall, this work agrees with the findings of Part 1 and provides a clear understanding of the stability and decomposition of ions on Li metal anodes. The approach taken here can be adapted to nonionic electrolytes and a range of electrode materials.
of the bond length. For the transition states to the BF4 decomposition, given in Figure 8c and d, the Li atoms are shown to aggregate around the removed F atom. In the presence of two Li atoms, the two Li−F bonds are arranged to form a large angle of 167°. For three Li atoms, the Li−F bonds are arranged to form a trigonal planar structure, akin to BF3. Of interest, the transition state for F removal takes the F atom roughly 3.2−3.7 Å away from the B atom, roughly three times larger than the B−F bonds in BF4. The close association of Li atoms to the anions, then, allows for effective charge transfer and stabilization of the decomposition transition states. Qualitative trends in decomposition do not change, with TFSI being expected to decompose readily on the Li surface. The quantitative size of the barriers, however, does change. In particular, the barrier to BF4 decomposition becomes significantly smaller than that found using the reductive approach. The barrier computations confirm the observations from AIMD simulations. The pyr14 cation has lower barriers than the EMIM cation, which corresponds to the observed decomposition of pyr14 at lower temperatures (T = 1500 K) when compared to EMIM (T = 2500 K). The practically nonexistent barrier to the TFSI decomposition through the S− C bond agrees well with the rapid, room temperature decomposition of TFSI in AIMD simulations. The F anion extraction from the BF4 has a large barrier, which can be reduced in the presence of Li. The barrier qualitatively explains why temperatures of T = 2500 K were needed to initiate any BF4 decomposition in AIMD simulations. Therefore, all the kinetics implied from our room/high temperature AIMD simulations on the Li surface are supported by the thermodynamic and barrier analyses conducted on gas phase systems. In terms of the transition states, the extremes of purely reductive and Li-mediated decomposition were examined. For the pyr14 cation, the Li atom cannot effectively complex due to steric hindrance by H; thus, the barriers from the purely reductive transition states are lower. For the EMIM cation, Li can complex with the ring N atoms, and barriers from the Limediated transition states are slightly lower. For both anions, pure reduction is energetically unfavorable and likely ill-defined in computations due to the positive nature of the LUMO energy (see Figures S1 and S2). In the presence of Li atoms, anion decomposition can be made thermodynamically favorable, with more Li atoms reducing the barrier for decomposition.
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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.7b09658. Details of the influence of dispersion on reaction energetics, electronic structure of reduced ions, and influence of configuration on ion pair alignment with a lithium surface (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Handan Yildirim: 0000-0002-3060-7896 Charles W. Bauschlicher Jr.: 0000-0003-2052-332X Author Contributions †
J.B.H. and H.Y. contributed equally to the manuscript.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported through the NASA Aeronautics Research Mission Directorate’s (ARMD) Convergent Aeronautics Solutions (CAS) project.
IV. CONCLUSIONS Density functional theory calculations of ions and ion pairs in the gas phase are used to evaluate the stability and decomposition of the [pyr14][TFSI] and [EMIM][BF4] ionic liquids on Li metal anodes, complementary to the extensive ab initio molecular dynamics study performed in Part 1 of this series.35 The results of the gas phase calculations are found to corroborate and explain three aspects of ionic liquid stability and decomposition noted from Part 1. First, the electrochemical stability of the ionic liquids on a Li surface is treated by aligning the electronic levels of the gas phase ions with the Fermi level of the Li surface. Standard electronic level alignment procedures are adapted to include the influence of surface polarization on electronic level. This approach successfully describes the anodic instability of the ion pairs on the Li metal surface that was noted from Part 1. Second,
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REFERENCES
(1) Hassoun, J.; Fernicola, A.; Navarra, M. A.; Panero, S.; Scrosati, B. An Advanced Lithium-Ion Battery Based on a Nanostructured Sn−C Anode and an Electrochemically Stable LiTFSi-Py24TFSI Ionic Liquid Electrolyte. J. Power Sources 2010, 195, 574−579. (2) Fernicola, A.; Croce, F.; Scrosati, B.; Watanabe, T.; Ohno, H. LiTFSIBEPyTFSI as an Improved Ionic Liquid Electrolyte for Rechargeable Lithium Batteries. J. Power Sources 2007, 174, 342−348. (3) Markevich, E.; Sharabi, R.; Borgel, V.; Gottlieb, H.; Salitra, G.; Aurbach, D.; Semrau, G.; Schmidt, M. A. In Situ FTIR Study of the Decomposition of N-butyl- N-methylpyrrolidinium Bis(trifluoromethanesulfonyl)Amide Ionic Liquid During Cathodic Polarization of Lithium and Graphite Electrodes. Electrochim. Acta 2010, 55, 2687−2696. 28246
DOI: 10.1021/acs.jpcc.7b09658 J. Phys. Chem. C 2017, 121, 28235−28248
Article
The Journal of Physical Chemistry C (4) Lee, K.-H.; Song, S.-W. One-Step Hydrothermal Synthesis of Mesoporous Anatase TiO2 Microsphere and Interfacial Control for Enhanced Lithium Storage Performance. ACS Appl. Mater. Interfaces 2011, 3, 3697−3703. (5) Schweikert, N.; Hofmann, A.; Schulz, M.; Scheuermann, M.; Boles, S. T.; Hanemann, T.; Hahn, H.; Indris, S. Suppressed Lithium Dendrite Growth in Lithium Batteries Using Ionic Liquid Electrolytes: Investigation by Electrochemical Impedance Spectroscopy, Scanning Electron Microscopy, and In Situ 7Li Nuclear Magnetic Resonance Spectroscopy. J. Power Sources 2013, 228, 237−243. (6) Basile, A.; Hollenkamp, A. F.; Bhatt, A. I.; O’Mullane, A. P. Extensive Charge−Discharge Cycling of Lithium Metal Electrodes Achieved Using Ionic Liquid Electrolytes. Electrochem. Commun. 2013, 27, 69−72. (7) Bhattacharyya, R.; Key, B.; Chen, H.; Best, A. S.; Hollenkamp, A. F.; Grey, C. P. In Situ NMR Observation of the Formation of Metallic Lithium Microstructures in Lithium Batteries. Nat. Mater. 2010, 9, 504−510. (8) Lvovich, V.; Wu, J.; Bennett, W.; Demattia, B.; Miller, T. Applications of AC Impedance Spectroscopy as Characterization and Diagnostic Tool in Li-Metal Battery Cells. ECS Trans. 2014, 58, 1−14. (9) Borodin, O.; Smith, G. D. Development of Many−Body Polarizable Force Fields for Li-Battery Applications: 2. LiTFSIDoped Oligoether, Polyether, and Carbonate-Based Electrolytes. J. Phys. Chem. B 2006, 110, 6293−6299. (10) Liu, H.; Maginn, E.; Visser, A. E.; Bridges, N. J.; Fox, E. B. Thermal and Transport Properties of Six Ionic Liquids: An Experimental and Molecular Dynamics Study. Ind. Eng. Chem. Res. 2012, 51, 7242−7254. (11) Canongia Lopes, C. J. N. A.; Padua, A. A. H. Using Spectroscopic Data on Imidazolium Cation Conformations To Test a Molecular Force Field for Ionic Liquids. J. Phys. Chem. B 2006, 110, 7485−7489. (12) Hunt, P. A. The Simulation of Imidazolium-Based Ionic Liquids. Mol. Simul. 2006, 32, 1−10. (13) Borodin, O. Polarizable Force Field Development and Molecular Dynamics Simulations of Ionic Liquids. J. Phys. Chem. B 2009, 113, 11463−11478. (14) Borodin, O.; Smith, G. D.; Kim, H. Viscosity of a Room Temperature Ionic Liquid: Predictions from Nonequilibrium and Equilibrium Molecular Dynamics Simulations. J. Phys. Chem. B 2009, 113, 4771−4774. (15) Borodin, O.; Smith, G. D. Structure and Dynamics of N-MethylN-propylpyrrolidinium Bis(trifluoromethanesulfonyl)imide Ionic Liquid from Molecular Dynamics Simulations. J. Phys. Chem. B 2006, 110, 11481−11490. (16) Haskins, J. B.; Bauschlicher, C. W., Jr.; Lawson, J. W. Ab Initio Simulations and Electronic Structure of Lithium-Doped Ionic Liquids: Structure, Transport, and Electrochemical Stability. J. Phys. Chem. B 2015, 119, 14705−14719. (17) Haskins, J. B.; Bennett, W. R.; Wu, J. J.; Hernández, D. M.; Borodin, O.; Monk, J. D.; Bauschlicher, C. W., Jr.; Lawson, J. W. Computational and Experimental Investigation of Li-Doped Ionic Liquid Electrolytes: [pyr14][TFSI], [pyr13][FSI], and [EMIM][BF4]. J. Phys. Chem. B 2014, 118, 11295−11309. (18) Bauschlicher, C. W., Jr.; Haskins, J. B.; Bucholz, E. W.; Lawson, J. W.; Borodin, O. Structure and Energetics of Li+−(BF4−)n, Li+− (FSI)n, and Li+−(TFSI)n: Ab-Initio and Polarizable Force Field Approaches. J. Phys. Chem. B 2014, 118, 10785−10794. (19) Borodin, O.; Smith, G. D. Development of Many−Body Polarizable Force Fields for Li-Battery Components: 1. Ether, Alkane, and Carbonate-Based Solvents. J. Phys. Chem. B 2006, 110, 6279− 6292. (20) Borodin, O.; Smith, G. D.; Henderson, W. Li+ Cation Environment, Transport, and Mechanical Properties of the LiTFSI Doped N-Methyl-N-alkylpyrrolidinium TFSI Ionic Liquids. J. Phys. Chem. B 2006, 110, 16879−16886.
(21) Liu, H.; Maginn, E. Effect of Ion Structure on Conductivity in Lithium-Doped Ionic Liquid Electrolytes: A Molecular Dynamics Study. J. Chem. Phys. 2013, 139, 114508. (22) Li, Z.; Smith, G. D.; Bedrov, D. Li+ Solvation and Transport Properties in Ionic Liquid/Lithium Salt Mixtures: A Molecular Dynamics Simulation Study. J. Phys. Chem. B 2012, 116, 12801− 12809. (23) Méndez-Morales, T.; Carrete, J.; Bouzón-Capelo, S.; PérezRodriguez, M.; Cabeza, O.; Gallego, L. J.; Varela, L. M. MD Simulations of the Formation of Stable Clusters in Mixtures of Alkaline Salts and Imidazolium-Based Ionic Liquids. J. Phys. Chem. B 2013, 117, 3207−3220. (24) Valencia, H.; Kohyama, M.; Tanaka, S.; Matsumoto, H. Ab initio Study of EMIM-BF4 Crystal Interaction with a Li(100) Surface as a Model for Ionic Liquid/Li Interfaces in Li-Ion Batteries. J. Chem. Phys. 2009, 131, 244705. (25) Buchner, F.; Forster-Tonigold, K.; Bozorgchenani, M.; Gross, A.; Behm, R. J. Interaction of a Self-Assembled Ionic Liquid Layer with Graphite(0001): A Combined Experimental and Theoretical Study. J. Phys. Chem. Lett. 2016, 7, 226−233. (26) Buchner, F.; Forster-Tonigold, K.; Uhl, B.; Alwast, D.; Wagner, N.; Farkhondeh, H.; Groß, A.; Behm, R. J. Toward the Microscopic Identification of Anions and Cations at the Ionic Liquid/Ag(111) Interface: A Combined Experimental and Theoretical Investigation. ACS Nano 2013, 7, 7773−7784. (27) Koch, V.; Dominey, L.; Nanjundiah, C.; Ondrechen, M. J. The Intrinsic Anodic Stability of Several Anions Comprising Solvent-Free Ionic Liquids. J. Electrochem. Soc. 1996, 143, 798−803. (28) Ue, M.; Murakami, A.; Nakamura, S. A Convenient Method to Estimate Ion Size for Electrolyte Materials Design. J. Electrochem. Soc. 2002, 149, A1385. (29) Vollmer, J. M.; Curtiss, L. A.; Vissers, D. R.; Amine, K. Reduction Mechanisms of Ethylene, Propylene, and Vinylethylene Carbonates: A Quantum Chemical Study. J. Electrochem. Soc. 2004, 151, A178−A183. (30) Budi, A.; Basile, A.; Opletal, G.; Hollenkamp, A. F.; Best, A. S.; Rees, R. J.; Bhatt, A. I.; O’Mullane, A. P.; Russo, S. P. Study of the Initial Stage of Solid Electrolyte Interphase Formation upon Chemical Reaction of Lithium Metal and N-Methyl-N-Propyl-PyrrolidiniumBis(Fluorosulfonyl)Imide. J. Phys. Chem. C 2012, 116, 19789−19797. (31) Camacho-Forero, L. E.; Smith, T. W.; Bertolini, S.; Balbuena, P. B. Reactivity at the Lithium−Metal Anode Surface of Lithium−Sulfur Batteries. J. Phys. Chem. C 2015, 119, 26828−26839. (32) Haskins, J. B.; Wu, J. J.; Lawson, J. W. Computational and Experimental Study of Li-Doped Ionic Liquids at Electrified Interfaces. J. Phys. Chem. C 2016, 120, 11993−12011. (33) Sodeyama, K.; Yamada, Y.; Aikawa, K.; Yamada, A.; Tateyama, Y. Sacrificial Anion Reduction Mechanism for Electrochemical Stability Improvement in Highly Concentrated Li-Salt Electrolyte. J. Phys. Chem. C 2014, 118, 14091−14097. (34) Piper, D. M.; Evans, T.; Leung, K.; Watkins, T.; Olson, J.; Kim, S. C.; Han, S. S.; Bhat, V.; Oh, K. W.; Buttry, D. A.; Lee, Se-Hee Stable Silicon-Ionic Liquid Interface for Next-Generation Lithium-Ion Batteries. Nat. Commun. 2015, 6, 6230. (35) Yildirim, H.; Haskins, J. B.; Bauschlicher, C. W.; Lawson, J. W. Decomposition of Ionic Liquids at Lithium Interfaces. 1. Ab Initio Molecular Dynamics Simulations. J. Phys. Chem. C 2017, DOI: 10.1021/acs.jpcc.7b09657. (36) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al.. Gaussian 09; Gaussian, Inc.: Wallingford, CT, 2009. (37) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Atoms, Molecules, Solids, and Surfaces: Applications of the Generalized Gradient Approximation for Exchange and Correlation. Phys. Rev. B: Condens. Matter Mater. Phys. 1992, 46, 6671−6687. 28247
DOI: 10.1021/acs.jpcc.7b09658 J. Phys. Chem. C 2017, 121, 28235−28248
Article
The Journal of Physical Chemistry C (38) Heyd, J.; Scuseria, G. E.; Ernzerhof, M. Hybrid Functionals Based on a Screened Coulomb Potenital. J. Chem. Phys. 2003, 118, 8207. (39) Becke, A. D. Density-Functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648−5652. (40) Kresse, G.; Furthmuller, J. Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15−50. (41) Methfessel, M.; Paxton, A. T. High-Precision Sampling for Brillouing-Zone Integration in Metals. Phys. Rev. B: Condens. Matter Mater. Phys. 1989, 40, 3616. (42) Cheng, J.; Sprik, M. Alignment of Electronic Energy Levels at Electrochemical Interfaces. Phys. Chem. Chem. Phys. 2012, 14, 11245. (43) Ong, S. P.; Ceder, G. Investigation of the Effect of Functional Group Substitutions on the Gas-Phase Electron Affinities and Ionization Energies of Room-Temperature Ionic Liquid Ions using Density Functional Theory. Electrochim. Acta 2010, 55, 3804−3811. (44) MacFarlane, D. R.; Meakin, P.; Sun, J.; Amini, N.; Forsyth, M. Pyrrolidinium Imides: A New Family of Molten Salts and Conductive Plastic Crystal Phases. J. Phys. Chem. B 1999, 103, 4164−4170. (45) McFarlane, D. R.; Sun, J.; Golding, J.; Meakin, P.; Forsyth, M. High Conductivity Molten Salts Based on the Imide Ion. Electrochim. Acta 2000, 45, 1271−1278. (46) Noda, A.; Watanabe, M. Highly Conductive Polymer Electrolytes Prepared by In Situ Polymerization of Vinyl Monomers in Room Temperature Molten Salts. Electrochim. Acta 2000, 45, 1265−1270. (47) Fuller, J.; Carlin, R. T.; Osteryoung, R. A. The Room Temperature Ionic Liquid 1-Ethyl-3-Methylimidazolium Tetrafluoroborate: Electrochemical Couples and Physical Properties. J. Electrochem. Soc. 1997, 144, 3881−3886. (48) Kroon, M. C.; Buijs, W.; Peters, C. J. Witkamp, Geert-Jan. Decomposition of Ionic Liquids in Electrochemical Processing. Green Chem. 2006, 8, 241−245. (49) Hehre, W. J. A. Guide to Molecular Mechanics and Quantum Chemical Calculations; Wavefuntion, Inc: Irvine, 2003. (50) Camacho-Forero, L. E.; Smith, T. W.; Bertolini, S.; Balbuena, P. B. Reactivity at the Lithium−Metal Anode Surface of Lithium−Sulfur Batteries. J. Phys. Chem. C 2015, 119, 26828−26839. (51) Markevich, E.; Sharabi, R.; Borgel, V.; Gottlieb, H.; Salitra, G.; Aurbach, D.; Semrau, G.; Schmidt, M. A. In Situ FTIR Study of the Decomposition of N-butyl- N-methylpyrrolidinium Bis(trifluoromethanesulfonyl)Amide Ionic Liquid During Cathodic Polarization of Lithium and Graphite Electrodes. Electrochim. Acta 2010, 55, 2687−2696. (52) Howlett, P. C.; Izgorodina, E. I.; Forsyth, M.; MacFarlane, D. R. Electrochemistry at Negative Potentials in Bis(trifluoromethanesulfonyl)Amide Ionic Liquids. Z. Phys. Chem. 2006, 220, 1483. (53) Budi, A.; Basile, A.; Opletal, G.; Hollenkamp, A. F.; Best, A. S.; Rees, R. J.; Bhatt, A. I.; O’Mullane, A. P.; Russo, S. P. Study of the Initial Stage of Solid Electrolyte Interphase Formation upon Chemical Reaction of Lithium Metal and N-Methyl-N-Propyl-PyrrolidiniumBis(Fluorosulfonyl)Imide. J. Phys. Chem. C 2012, 116, 19789−19797.
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DOI: 10.1021/acs.jpcc.7b09658 J. Phys. Chem. C 2017, 121, 28235−28248
