Article Cite This: J. Phys. Chem. C 2017, 121, 28214−28234
pubs.acs.org/JPCC
Decomposition of Ionic Liquids at Lithium Interfaces. 1. Ab Initio Molecular Dynamics Simulations Handan Yildirim,† Justin B. Haskins,† 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 work is Part 1 in a two part series that investigates the interfacial decomposition chemistry of [pyr14][TFSI] and [EMIM][BF4] ionic liquids (IL) at Li metal interfaces. Here, the decomposition is probed primarily through ab initio molecular dynamics (AIMD) simulations. For single ion pairs adsorbed on a Li(100) surface, hybrid ion states are found to emerge about the Fermi level. Interestingly, these states have a significant contribution from both ions, which suggests that the cathodic (reductive) stability could in part be governed by the anions. Room temperature AIMD simulations reveal rapid decomposition of the TFSI anion initiated by C−S and/or S−N bond cleavage due to charge transfer from Li to the anion. The unusual phenomenon of reductive decomposition of the anion is supported by recent experimental reports. The reaction products observed included LiF, LiO, Li2F, Li2O, SO2, NSO2, NSO2CF3, etc., which are all in excellent agreement with the XPS results. Initial decomposition reactions for both cations and the BF4 anion were only observed in high temperature AIMD simulations. For bulk ILs interfaces with a Li(100) surface, interfacial decomposition reactions again result from charge transfer to the IL from the Li surface, in particular, to anions at the interfaces. The initial decomposition event at bulk interfaces is found to vary depending on the interface structure. The extensive computational analyses presented in this work provide valuable insights into the fundamental interfacial chemistry of ILs in contact with Li metal. In Part 21 of this series, we consider these results further by systematically examining ion reductive stability, the thermodynamics of decomposition, and kinetic limitations to decomposition using gas phase density functional theory (DFT) computations. Results from these studies can be used for further design of these, or perhaps other, ILs to obtain more stable solid electrolyte interface (SEI) layers to improve cycling in advanced battery chemistries.
I. INTRODUCTION For nearly a decade, rechargeable lithium-ion batteries (LIBs) have dominated the portable electronics market as highly efficient energy storage devices.2−5 Their incorporation into applications requiring high capacity, high power, and high cycle life, such as various electric vehicles (EV), has not been as rapid, however. While electric cars have become more commonplace, they have so far failed to gain widespread commercial success due in part to cost and range issues associated with LIBs. More advanced vehicles, such as electric aircraft, are still in the very early stages of development. Due to power and energy limitations,6,7 the current LIB chemistries are unlikely to satisfy the stringent requirements for electric aircraft and other high performance applications. Therefore, new advanced chemistries are being considered.8−10 Lithium metal has been proposed as a replacement for intercalation anodes, e.g. titanium oxides or carbonaceous materials,11,12 which are standard in current LIBs. Batteries based on Li metal as the anode have the potential to provide the highest energy densities among secondary batteries.13 Li metal has a high volumetric capacity (2046 mAh/cm3) and the highest gravimetric capacity (3862 mAh/g)10× larger than that of graphite (372 mAh/ © 2017 American Chemical Society
g). Despite these advantages, such technology has not yet been realized.14 Among the possible systems involving Li metal, lithium− sulfur (Li−S) and lithium−oxygen (Li−O2) batteries have been the most attractive for next generation high-energy density batteries.15−19 They have the potential to improve considerably the range of EVs. Significant effort has been devoted to developing Li metal anodes for rechargeable batteries.20 However, major challenges have prevented their large-scale application.21,22 In particular, metallic Li electrodes can form dendrites, leading to battery failure. In addition, Li electrodes continuously form SEI layers with cycling, causing loss of Columbic efficiency (CE), significant Li consumption, and increased cell resistance.23 The electrolyte is a critical component that can be tailored to improve battery performance. In particular for Li anodes, studies have been conducted to explore how the electrolyte affects the cycling of Li electrodes.24,25 Electrolytes can have a Received: September 28, 2017 Revised: November 20, 2017 Published: November 27, 2017 28214
DOI: 10.1021/acs.jpcc.7b09657 J. Phys. Chem. C 2017, 121, 28214−28234
Article
The Journal of Physical Chemistry C
indicates a liquid consisting of the indicated molecular cation and anion. A multilayered film composed mainly of Li salts and TFSI fragments was observed.44 Similarly, an earlier study combining X-ray photoemission spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS) showed that anion decomposition products formed the SEI with an electrolyte consisting of [pyr14][TFSI] and LiTFSI salt in an Li−S battery also led to good cycling.65,66 Electron paramagnetic resonance (EPR) spectroscopy experiments examined the differences in the SEI layers and dendrite formation in FSI based IL electrolytes. These studies revealed the ease of mineralization of the reduced FSI without generating organic radicals and/or gas products, which are common for organic electrolytes.67 Boron tetrafluoride (BF4) anion57,68 has been discussed for its high rate and reasonable stability, especially when used in conjunction with organic additives such as vinylene carbonate (VC)although with limited cycling.68 In an earlier experimental study, the cycling stability of an electrolyte consisting of [EMIM][BF4], LiBF4 salt, and a VC additive in a Li battery showed that VC addition improved cycling; however, the morphology of the deposited Li was dendritic. It was reported that VC might be needed in significant quantity to achieve prolonged cycling.68 These examples revealed that anion selection is critical for achieving stable cycling.65 Superior performance was most often reported for the electrolytes consisting of TFSI, whereas extended cycle life with BF4 anions still remains challenging. More recent studies57,69 reported significant differences in cycling stability for two different ILs with Li metal. This behavior was correlated directly with the properties of the SEI layer formed. Electrolytes consisting of [pyr14][TFSI] with LiTFSI salt showed significantly superior cycling stability (∼1000 cycles) compared to electrolytes consisting of [EMIM][BF4] with LiBF4 salt and VC additive (∼150 cycles). Scanning electron microscopy (SEM) images revealed significant differences in the morphology of the Li anode. A mossy conducting SEI layer was formed for the [pyr14][TFSI], whereas a dense insulating layer was observed for the [EMIM][BF4]. EIS results showed much larger interfacial resistance for the latter, indicating that the difference in cycling behavior was related to the characteristics of the SEI layer.69 However, why these ILs result in such a dramatically different SEI layer is not well understood. This is also a motivating question for this study. The choice of Li salt can also have a major impact on cycle life. For Li−O2 batteries, cycling stability with several Li salts was evaluated, including LiTFSI and LiBF4. The best cycling performance was obtained for LiFSI and LiTFSI salts, while the worst performances were observed for LiBr and LiBF4. LiBF4 decomposed into LiF and other products. 70 Using a combination of XPS, Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), and nuclear magnetic resonance (NMR) techniques, SEI formation in binder-free graphite was examined using electrolytes with varying Li salts (1 M LiPF6, LiBF4, LiTFSI, LiFSI, LiDFOB, and LiBOB) dissolved in ethylene carbonate (EC). The Li ion coordination sphere was suggested to play an important role in the formation of the SEI layer. Surface morphology analysis revealed that the films were smooth and uniform for electrodes cycled with LiPF6, LiBOB, LiTFSI, and LiFSI, while for LiBF4 and LiDFOB salts, the films were grainy. The concentration of LiF formed during the reduction process was the highest for
significant impact on the cycling stability of Li metal, and they have become an increasingly attractive field of research in recent years due to the revived interest in Li metal. Previously, Aurbach et al.26 reported that Li is thermodynamically unstable against organic solvent-based electrolytes. Side reactions between these electrolytes and Li metal result in the formation of dendrites during charging.27,28 For carbonate solvents such as propylene carbonate (PC), the plating/stripping efficiency of Li metal is poor and often resulting in dendritic Li deposits and hence low CE.29,30 Furthermore, the SEI formed by electrolyte reduction products cannot accommodate morphological changes of the plated Li, thus leading to penetration of dendrites into the SEI layer and resulting in short-circuits.31 Continuous electrolyte decomposition and dendrite formation can significantly affect the performance, cycle life, and safety of Li batteries. Several strategies have been suggested to overcome these problems, e.g. controlling the morphology of the Li surface using various electrolytes such as carbonates, esters, ethers, ionic liquids, or mixtures.32−37 Li thin films38 and block copolymer electrolytes39 have also been proposed to improve cycling stability. Recently, Qian et al.40 reported that highly concentrated electrolytes consisting of ether solvents and Li salts41 can lead to dendrite-free plating of Li at high rates and with high CE.40 Further details regarding similar approaches can be found in several review articles.42−44 Among the proposed solutions for dendrite problem, of particular interest to the current study, are ionic liquids (IL). ILs are organic salts, which are molten below 100 °C and have unique properties such as negligible vapor pressure, nonflammability, low melting point, and high thermal stability. For energy storage and electrodeposition, electrochemical stability is one of the most important factors affecting performance. This property can potentially be tuned by the appropriate choice of anion/cation.45,46 Many ILs have much wider electrochemical windows (>5 V) than aqueous or organic electrolytes.47 Therefore, they can be considered as potential candidates to replace commonly used organic electrolytes in several electrochemical systems.48−54 Importantly, these systems include Li batteries, where some ILs have been found to form a stable SEI layer with Li metal and to lead to longer cycle life and the suppression of dendrites.50,51,55−57 For stable cycling, the structure and components of the SEI layer are key elements. Therefore, understanding electrolyte reduction, products, and their chemical interaction with a Li anode is critical. This is one of the main objectives of the current work. Previous experimental studies showed that ILs composed of pyrrolidinium (pyr 1,n ) or 1-ethly-3-methyl-imidazolium (EMIM) cations and bis(trifluoromethanesulfon)imide (TFSI) or the bis(fluorosulfonyl)imide (FSI) anions were stable against the most popular negative electrodes such as TiO2 and those based on carbon.58−61 They lead to stable cycling with Li metal.55−57 The TFSI anion was reported to be more stable than the FSI anion51,62 whereas batteries utilizing the FSI anion showed higher discharge rates than those with the TFSI anion.57,63 The pyrrolidinium and piperidinium cations have received much attention due to their intrinsic stability against Li metal.54,64 For Li−S batteries, LiTFSI based electrolytes have been reported to be more stable than LiFSI. The difference in anion bond strength in the presence of polysulfides was correlated with the stability differences for the anode−electrolyte interface. In another study, the composition and structure of the SEI formed on Li with a [pyr13][TFSI] electrolyte were examined. Note the bracket notation [ ] 28215
DOI: 10.1021/acs.jpcc.7b09657 J. Phys. Chem. C 2017, 121, 28214−28234
Article
The Journal of Physical Chemistry C
interfacial reactions observed during the AIMD simulations. Note that the reactions occurring under electrochemical conditions in general will be voltage dependent.103 However, these effects are not considered in this study. In addition, only the initial stages of the decomposition reactions are revealed due to the relatively short time scales of the AIMD simulations (40 ps). In Part 2,1 our findings on decomposition reactions from AIMD simulations are further examined using gas phase quantum chemistry calculations. These calculations provide explanations of the AIMD results and predict potential decomposition pathways for BF4 anion and cations through an analysis of decomposition thermodynamics and barriers. The quantum chemical decomposition results are discussed in light of the observations made in the current AIMD study (Part 1).
LiBF4 and LiDFOB. The high LiF concentration was tied to the strong association of BF4 anion with Li+ and to the reductive decomposition of BF4.71 Several studies also reported that Li salt has an important role in the structure of SEI for different anode electrodes.72−74 Computational investigations of IL electrolytes have focused primarily on structural, thermodynamic, and transport properties. These studies were mostly performed with classical MD simulations using various force fields.75−78 In particular, polarizable force fields79−84 and ionic liquid force fields85−88 were applied to a range of systems including LiTFSI in solvents with pyrrolidinium and imidazolium cations and TFSI anion,82−84,86−88 LiFSI in [pyr14][TFSI],89 LiBF4 with imidazolium cations and BF4 anions,82−84,90 and LiPF6 in [EMIM][PF6].91 In addition to the studies of bulk electrolyte properties, MD simulations of IL electrochemical systems, e.g. supercapacitors, have given detailed information on interfacial structure and properties, especially the formation and capacitance of the electric double layer.92 While these simulations do not include decomposition explicitly, they provide important information regarding interface structures that can likely influence decomposition reactions. To date, only a handful of DFT studies concerning the interaction of ILs with Li metal have been reported. Some of these considered the adsorption of ion pairs on Li and other surfaces,93−95 whereas others compared the electrochemical stabilities of different ion pairs (gas phase computations).96−98 Recently, AIMD studies explored decomposition pathways and products for ILs at Li metal surfaces99 as well as salt decomposition on Li100 and other surfaces.101,102 In the following two part series, we combine gas phase calculations (Part 2) with surface DFT computations and AIMD simulations (Part 1) to investigate electronic structure, interfacial decomposition reactions, and electrochemical stabilities of [pyr14][TFSI] and [EMIM][BF4] ILs at Li surfaces. These ILs were selected due to their very different cycling behavior against Li metal.56,57 As mentioned above, fundamentally different SEI layers were suggested as the reason for this behavior. However, the formation mechanism of the SEI layer, and the ultimate connection to the chemical structure of the original ILs is poorly understood. Such an understanding is important to elucidate the electrolyte design rules that will enable batteries with stable, extended cycling. Our approach is motivated in part by the desire to obtain the most information from the simplest levels of computation, given that interfacial chemistry in highly reactive electrochemical systems is extraordinarily complex. A complete description of these processes is very challenging, requiring the inclusion of additional effects such as salts, additives, and bias, to name a few, which are likely to influence the morphological evolution of the SEI layers at experimentally relevant long time scales. Our results, however, describe only the initial decomposition reactions, reaction orders, and the reaction products for these two specific ILs. Longer time scales, on the other hand, can be accessed using coarse-grained methods, e.g. kinetic Monte Carlo (kMC). The approach developed here can be extended more broadly to other ILs and to other electrolytes to improve performance for these advanced systems. To this end, we both consider gas phase analysis of individual ion pairs and systematically add complexity by including various environment effectsi.e. the effect of Liand finally AIMD simulations of the full solid−liquid interface. In Part 1 of this series, electronic structure computations are used to analyze the
II. COMPUTATIONAL DETAILS II.1. Gas Phase and Surface Adsorbed Calculations of Ion Pairs. The structures of the individual ion pairs were optimized using the Vienna Ab Initio Simulation Package (VASP)104 and Gaussian09.105−107 As discussed below, three adsorption configurations of the ion pairs were considered on the Li(100) surface, namely two on top (OT1 and OT2) and a side-by-side (SS) configuration. Gas phase ion pairs were first optimized in these configurations for comparison with those adsorbed on a Li(100) described in the next section. For the plane wave DFT gas phase calculations, the ion pairs were placed in a cubic box of length 30 Å. A Γ-centered k-point mesh was used for k-space integration. For probing the interaction between the ILs and the Li surface, we performed a combination of T = 0 K DFT and AIMD (T = 298 K) calculations using VASP. As mentioned in the Introduction, we start with the properties of the ion pairs and then explore their chemical interaction with Li metal. For this, and the following steps including bulk IL decomposition at the Li surface, we choose the (100) crystallographic plane. This surface has the lowest surface energy as compared to (110) and (111) surfaces.108,109 We also performed simulations to assess the effect of the surface plane on decomposition reactions using Li(110), which is the next most stable surface.100 Unless otherwise stated, the results reported here were obtained for Li(100). The surface is modeled using nine atomic layers. For the AIMD simulations of single ion pairs on Li(100), the middle three layers are fixed at the bulk termination, while all other degrees of freedom were allowed to relax. Surface adsorbed ion pair simulations were performed for three configurations of the pairs by changing the anion (Figure 1a−b) and cation (Figure 1c−d) position with respect to each other and to the Li surface. The most stable configuration for each pair was determined using T = 0 K simulations by exploring different adsorption sites on Li(100). AIMD simulations of decomposition were performed for a maximum of 40 ps. II.2. Bulk ILs and Interface Calculations. AIMD simulations were performed for bulk ILs interfaced with Li using VASP. The structures were generated with classical MD using the atomic polarizable potential for liquids, electrolytes, and polymers (APPLE&P).79 Bulk ILs were initially constructed from 12 ns APPLE&P simulations on 12 pairs of [pyr14][TFSI] and 22 pairs of [EMIM][BF4]. The simulated bulk ILs have densities of 1.392 and 1.273 g/cm3 (T = 298 K), which are comparable with the experimental densities110,111 of 1.399 and 1.280 g/cm3 for [pyr14][TFSI] and [EMIM][BF4], respectively. An assessment of the electrochemical window for 28216
DOI: 10.1021/acs.jpcc.7b09657 J. Phys. Chem. C 2017, 121, 28214−28234
Article
The Journal of Physical Chemistry C
III. RESULTS AND DISCUSSIONS III.1. Electronic Structure of Ion Pairs in the Gas Phase. We start by discussing gas phase properties of single ion pairs to serve as a reference for discussions in later sections. Decomposition reactions of these pairs, in real systems, depend on such environmental factors as the temperature, ionic environment, liquid state properties, as well as the presence of both reactive and nonreactive surfaces. Therefore, in this section, we discuss the simplest environmental effect by exploring the properties of individual ion pairs. These computations provide the first indications of how environment affects the individual ions; as such, each ion (anion or cation) is effectively the environment of its partner. Then, in the following sections, these pairs are put in contact with a Li surface to study how the surface affects their electronic structure. These computations will lay the foundation for AIMD simulations, which will be presented in later sections. We start by constructing a few configurations of single ion pairs, as we are interested in their adsorption and decomposition on Li(100). Three configurations were created through changing the anion position with respect to the cation and to Li. The main goal behind this effort was to study the properties of a few ion pair configurations that could exist when they are in contact with Li. These configurations are denoted as OT1, OT2, and SS. In the first two configurations (OT1 and OT2), the cation is positioned on top of the anion in two different ways. For OT1, the anion is connected to the surface via N of the TFSI anion, while for OT2, N is not directly on Li but directed toward the cation. For the EMIM-BF4 pair, the OT1 and OT2 configurations correspond to the cases where one or three F atoms of the anion point toward the cation that is on top of the anion, respectively. The SS configuration, on the other hand, has the anion and the cation residing side-byside. This adsorption configuration maximizes the interaction with Li. Before studying the electronic structure properties of these adsorbed ion pairs on Li, gas phase computations for ion pairs using the VASP package were performed to establish points of reference for surface computations. Note that the gas phase configurations examined here may not be exactly the same as those of the surface-adsorbed due to the interaction with Li, which may stabilize/change a given configuration. These configurations were optimized in the gas phase using the PBE and HSE06 functionals. In Table 1, we summarize the HOMO−LUMO gaps of these ion pair configurations (OT1 and SS) in the gas phase, as well as those corresponding to the bulk ILs to serve as reference for later sections and to assess the electronic structure modifications when the liquid environment is introduced. Total and partial electronic densities of states (DOSs) were also computed. The DOSs together with HOMO and LUMO
Figure 1. Gas phase optimized structures of (a) TFSI anion, (b) BF4 anion, (c) pyr14 cation, and (d) EMIM cation. The color-coding used for the atoms in the figures is C (brown), S (yellow), F (gray), O (red), N (gray, small ball), H (light pink), and B (green).
the bulk ILs was obtained from single-point, DFT-PBE calculations on ten APPLE&P equilibrated configurations, without further structural optimization. These structures were then interfaced with Li surfaces, which were separated by a 3 nm region. Li electrode surfaces were modeled using five atomic layers such that the (100) surface of Li was exposed to IL, forming two interfaces. The interfaces were equilibrated for 12 ns at T = 298 K using APPLE&P and a Langevin thermostat. The long (12 ns) simulation time provided statistically independent configurations, which were used as inputs for AIMD simulations. Multiple configurations were then thermalized using AIMD simulations at T = 298 K for 100 fs using velocity scaling. Using these thermalized IL structures, AIMD simulations were performed at T = 298 K using a Langevin thermostat with a time step of 2 fs for 6.5 to 11.2 ps. Deuterium masses were employed for hydrogen atoms to enable a larger time step for the simulations. High temperature AIMD simulations were also performed using VASP to explore possible thermal decomposition reactions for the two cations and the BF4 anion. Note that for these species no decomposition reactions were encountered during the 40 ps AIMD simulations (for single ion pairs) performed at T = 298 K. This suggests that decomposition reactions associated with the two cations and BF4 anion are quite endothermic or kinetically limited in these time scales. Thus, high temperature (T = 700−2500 K) simulations were performed to accelerate the kinetics associated with decomposition reactions for the ion pairs adsorbed on a fixed Li(100). Fixing the positions of Li was required to prevent the melting of Li at high temperatures. These simulations were performed for 1 ps for each temperature. For all AIMD simulations, the projector augmented wave (PAW) method112,113 as implemented in VASP was utilized. The Perdew−Burke−Ernzerhof (PBE) generalized gradient approximation (GGA-PBE)114 was used as the exchangecorrelation functional. Some of the gas phase calculations were also performed using the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional,115 which is well-known to provide more accurate band gaps.116 The energy cutoff for the plane-wave basis expansion was chosen as 400 eV for AIMD simulations and 500 eV for 0 K optimizations. For the surface Brillouin zone integration, a 4 × 4 × 1 Monkhorst−Pack k-point mesh117 was used. For the AIMD simulations, a Γ-centered k-point mesh was used. A conjugate gradient algorithm was employed to relax the ion positions.
Table 1. HOMO−LUMO Gaps (eV) for Ion Pairs in Gas Phase and Bulk Ionic Liquids Using Different Levels of Theory Ion Pair/Method pyr14-TFSI PBE HSE06 EMIM-BF4 PBE HSE06 28217
OT1
SS
Bulk IL
4.63 6.67
4.68 6.69
4.64
4.52 7.11
4.49 7.48
3.94
DOI: 10.1021/acs.jpcc.7b09657 J. Phys. Chem. C 2017, 121, 28214−28234
Article
The Journal of Physical Chemistry C orbital map analyses can provide information on reductive/ oxidative stability as well as on the location of molecular initiation sites for potential decomposition reactions of these pairs. The details and discussions on these will also be reported in Part 2.1 The HOMO−LUMO gaps presented in Table 1 for both ion pairs show small differences between the OT1 and SS configurations. For both ion pairs, the hybrid functional leads to a ∼2 eV increase in the gap when compared to PBE, which is expected due to the well-known shortcomings in commonly used DFT functionals such as PBE when considering the HOMO− LUMO gap and its solid-state counterpart, band gap.118−120 In general, hybrid functionals, e.g. HSE06, are expected to be more accurate; however, they are computationally more intensive and, therefore, are generally only used for small systems, such as ion pairs in this study. These calculations are not possible for larger system size, e.g. surfaces, interfaces, etc., and therefore, we only report results using the PBE functional in those cases. However, the gas phase results reported in Table 1 illustrate the range of values for the HOMO−LUMO gaps that can be obtained when using the HSE06 versus PBE functional. For the pyr14-TFSI pair, the PBE functional gives similar gaps for both configurations, while the hybrid functional leads to the gap of SS being 0.37 eV higher than the OT1. For the EMIM-BF4 pair, HSE06 predicts some effect of geometry on the band gaps, while for pyr14-TFSI such effect does not exist. A comparison between pyr14-TFSI and EMIM-BF4 shows that their gaps are similar for a given functional. The PBE shows a slightly higher gap (0.1−0.2 eV) for pyr14-TFSI, while the hybrid functional shows a higher gap (0.3−0.4 eV) for EMIMBF4. As a further point of comparison, the average gaps for bulk ILs were computed using PBE. The bulk computations yield values of 4.64 and 3.94 eV for pyr14-TFSI and EMIM-BF4, respectively. The pyr14-TFSI gaps for single ion pair and bulk are similar, while the gap for bulk EMIM-BF4 is 0.5 eV smaller than that of the single ion pair. In Figure 2a−f, we present the total and partial DOS for the SS configuration for the pyr14-TFSI and EMIM-BF4 pairs in gas phase decomposed by anion and cation contributions obtained using the PBE functional. Analogous computations of the DOS using the HSE06 functional are reported in the Supporting Information (see Figure S1). For the pyr14-TFSI pair, Figure 2a shows that the HOMO is dominated by the TFSI anion, while the LUMO is formed by the pyr14 cation. The atom projected PDOS shown in Figure 2b−c details which atoms from the anion (Figure 2b) and the cation (Figure 2c) contribute to the HOMO and LUMO levels. According to the PDOS shown in Figure 2b, the largest contribution to the HOMO level comes from the N and O atoms in the TFSI anion, whereas the PDOS in Figure 2c shows that the LUMO originates mainly from C atoms in the pyr14 cation. In Figure 2d, the DOS for the EMIM-BF4 pair is shown. The HOMO level is dominated by the BF4 anion, while the LUMO is formed from the EMIM cation, which is similar to the pyr14TFSI pair. The PDOS analyses (see Figure 2e and Figure 2f) show that the largest contribution to the HOMO comes from F in the BF4 anion, while N and C atoms in the EMIM cation dominate the LUMO level. The DOSs for the OT1 shows the same features as the SS configuration discussed above (see Figure S2). The DOSs was also computed using the HSE06 functional. The DOSs for pyr14-TFSI also predicts the same features for
Figure 2. Electronic DOS for gas phase ion pairs decomposed by ion and atom contributions calculated using the PBE functional for the SS configuration of (a) pyr14-TFSI total DOS, (b) TFSI (anion) partial DOS, (c) pyr14 (cation) partial DOS, (d) EMIM-BF4 total DOS, (e) BF4 (anion) partial DOS, and (f) EMIM (cation) partial DOS. Parts (a) and (b) show the contributions from the anions and cations to the DOS, while parts (b), (c), (e), and (f) show the contributions from each atom in the anions and/or cations to the DOS presented using colored lines, and the DOS of the anion and/or the cation is plotted in the solid (f illed) background.
the HOMO and LUMO levels as those obtained using PBE; see Figure S1.a. On the other hand, the DOSs calculated using the HSE06 functional (from a single point calculation) for the EMIM-BF4 pair shows a clear difference. The DOS (see Figure S1.b) shows that both HOMO and LUMO levels are dominated by the EMIM cation. The HSE06 result was also confirmed by performing orbital map calculations using B3LYP/6-31+G**, which showed the HOMO level to be dominated by the cation states. Furthermore, the orbital maps calculated using HSE06/6-31+G** were also similar to those of the B3LYP/6-31+G**. Therefore, we conclude that the hybrid functionals likely predict a more accurate picture for the EMIM-BF4 pair and that, in addition to the LUMO level, the HOMO level should also be dominated by cation states. Note that this is an interesting observation as it suggests that oxidative stability is not necessarily limited by the anion but also by the cation, therefore indicating that for EMIM-BF4 the anion does not always determine oxidative stability. Our observation supports earlier work of Ong et al.,121 which showed for BMIM-BF4 that both HOMO and LUMO levels were dominated by cation states. The effects of anion and cation states dominating either HOMO or LUMO levels on oxidative stability were explored using orbital map analysis via quantum chemistry computations. As our aim in this manuscript is to present decomposition reactions, further discussions on oxidative stability and gas phase properties including BDEs and reaction barriers 28218
DOI: 10.1021/acs.jpcc.7b09657 J. Phys. Chem. C 2017, 121, 28214−28234
Article
The Journal of Physical Chemistry C
seemingly stable configurations of TFSI. This will be further explored through AIMD simulations in the next section. Three adsorption configurations (OT1, OT2, and SS) are explored for EMIM-BF4 adsorption on Li(100) using PBE, and the results are summarized in Table 2. As with pyr14-TFSI, the SS configuration was found to be the most stable with an adsorption energy of 1.21 eV. Charge transfer from Li to the EMIM-BF4 pair varies with the adsorption configuration and ranges from 0.58 e− to 0.70 e−. For all configurations, charge is transferred to both the anion and the cation. The largest charge transfer (0.7 e−) was found for the SS configuration. For the EMIM-BF4 , most charge is transferred to the cation independent of the configuration, unlike with pyr14-TFSI. The cation receives most charge for the SS configuration, while only small amounts of charge (
