Ab Initio Calculations for Decomposition Mechanism of CH3O

Jul 16, 2013 - We focused our study on N = 4 case where the reaction barrier for the bond ... Ujjal Das , Kah Chun Lau , Paul C. Redfern , and Larry A...
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Ab Initio Calculations for Decomposition Mechanism of CH3O(CH2CH2O)NCH3 (N = 1−4) by the Attack of O2− Anion Yasuharu Okamoto*,† and Yoshimi Kubo‡ †

The Smart Energy Research Laboratories, NEC Corporation, 34 Miyukigaoka, Tsukuba, Ibaraki 305-8501, Japan GREEN, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan

‡

S Supporting Information *

ABSTRACT: Ab initio molecular orbital calculations were done to examine C−O bondbreaking reactions in glyme series CH3O(CH2CH2O)NCH3 (N = 1−4) by the attack of superoxide anion. We focused our study on N = 4 case where the reaction barrier for the bond break becomes the highest among four glymes. Intrinsic reaction coordinate calculations showed that the barrier height measured from the reaction precursor is 1.087 eV. The value is much higher than that of the analogous bond-breaking reaction in propylene carbonate.

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INTRODUCTION Recent technological and industrial trends of expanding the applicability of lithium ion batteries (LIBs) from conventional power supply for mobile phones, portable music players, and notebook computers to new and emerging fields such as electric vehicles and stationary batteries reacknowledge their performance limitations in terms of capacity, safety, cyclability, and above all cost. This situation stimulates the study of various post Li ion secondary batteries.1−5 The lithium−air battery, which was first devised by Abraham and Jiang in 1996,6 is one of the candidates of the post LIBs. The theoretical specific energy of aprotic type lithium−air battery is approximately 6 times larger than that of the battery of LiCoO2 cathode with metallic Li anode.1 The dramatic increase of the specific energy will provide a breakthrough for widening the application of batteries and lead to a better balance between efficiency and stability of energy supply. The basic electrochemical reactions that occur at the anode and cathode of aprotic lithium−air battery are expressed as follows: anode: cathode:

that the basic electrode reactions do indeed occur by changing solvents from PC to ether type such as glyme series CH3O(CH2CH2O)NCH38,11 or to ionic liquids (ILs).12,13 Although ILs have been attracting attention as an electrolyte for Li−air and Li ion batteries, high viscosity of ILs results in relatively low capacity due to a poor impregnation of the electrode by the viscous electrolyte and low ionic conductivity.14,15 Moreover, it is believed that IL-based solutions have a poor cathodic stability limit because imidazolium-based cations tend to be reduced by electrochemical deprotonation around 1.5 V (vs Li/Li+).16 It would be valuable to provide information about the fundamental stability of glyme series because understanding the stability of the solvent molecules is closely linked to achieving appropriate reversible electrochemistry in lithium−air batteries. In particular, ref 7 shows that the decomposition of PC is first triggered by the superoxide anion (O2−) attack to a C−O bond in the [Li−PC]+ complex from the backside of C, which leads to the bond-breaking and ring-opening of PC. This reaction can be regarded as a kind of SN2 reaction as was studied in the case of ethylene carbonate (EC) decomposition by a PF6− anion.17 Examining similar decomposition mechanism with respect to glyme series is thus helpful to estimate to what extent the solvents are stable against O2− attack. Ab inito molecular orbital calculation is a suitable tool for examining such a hypothetical reaction mechanism. Besides several quantum chemical calculations concerning a reductive decomposition or solvation of EC and other solvents for

2Li ⇌ 2Li+solv + 2e− O2gas + 2Li+solv + 2e− ⇌ Li 2O2

Recently, there was a major turning point in the study of lithium−air battery. Several works showed that the above expected electrode reactions do not occur in carbonate ester solvents such as propylene carbonate (PC).7−10 Superficial current observed in charge and discharge processes just corresponds to the side reactions associated with the electrochemical decomposition of PC.7 Then it was found © 2013 American Chemical Society

Received: May 17, 2013 Revised: July 5, 2013 Published: July 16, 2013 15940

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Figure 1. Optimized structures of linear form TEGDME molecule (a), curled form TEGDEM molecule (b), curled form [Li-TEGDME]+ ion (c), and linear form [Li-TEGDME]+ ion (d). Possible bond break positions (bb1−bb5) are also shown in (a) and (c). Sky blue, red, green, and white balls stand for carbon, oxygen, lithium, and hydrogen atoms, respectively.

LIBs,18−23 the decomposition of carbonate ester by the superoxide anion attack was reported in the literature.24,25 The decomposition of glyme series, however, yet remains to be clarified from atomic-level simulations. Although possible reaction mechanisms might be specified by experimental techniques such as isotopic labeling and transition state analogues in some cases, they are often based on an educated guess from the reactant and product of the reaction. In contrast, molecular orbital calculations can directly determine the transition state (TS) of the reaction. The appropriateness of the TS can be judged by the calculation of intrinsic reaction coordinate (IRC)26 which connects the TS with both the reactant and the product. Thus, the examinations of the electrolyte deterioration and decomposition mechanism through ab initio molecular orbital calculations help to understand the essential characteristics of the degradation reactions in the battery and provide complementary information to experimental studies. In this paper, based on ab initio molecular orbital calculations, we have examined the decomposition of glyme series CH3O(CH2CH2O)NCH3 (N = 1−4) by the attack of O2− as inspired by the mechanism proposed for the decomposition of PC in ref 7. In particular, we focused on the N = 4 case, i.e., the decomposition of tetraethylene glycol dimethyl ether (TEGDME). The higher boiling point of TEGDME (275 °C27) in comparison to monoglyme (85 °C28) helps prevent the electrolyte from evaporating in the open environment of Li−air batteries, which seems to be indispensable to improving the cyclability of the battery and practical use.

functional,30,31 unless otherwise stated. The valence double-ζ basis sets augmented with polarization and diffuse functions, 631++G(d,p), were used in this study. It may be noted that Table 1 in ref 24 shows that B3LYP somewhat underestimates the activation energy concerning the superoxide addition to the ethereal carbon atom of ethylene carbonate in the gas phase in comparison to non-DFT-based electron correlation method such as UMP2 and UCCSD(T). It is a well-known behavior that DFT-based calculations underestimate the reaction barriers. A few kcal/mol difference between B3LYP and UCCSD(T) results is a typical and tolerable error in DFT calculations. Solvent effects were treated by using IEF-PCM which performs a reaction field calculation using integral equation formalism model.32 Tetrahydrofuran (THF, ε = 7.425) and dimethyl sulfoxide (DMSO, ε = 46.826) were used as a continuum dielectric material in the IEF-PCM calculations for TEGDME and PC, respectively, because TEGDME and PC are not supported in the solvent library of Gaussian 09. Note that the specific permittivity of THF is very close to that of TEGDME (7.79).33 Stable molecular and ionic geometries of all species in this study were obtained through the geometry optimization, and a transition state was searched for possible bond break positions. It should be emphasized that although a TS is characterized by an extremum of the potential energy hypersurface having one imaginary frequency, the condition does not ensure that the obtained TS corresponds to the reaction in question. This is because there are many extremums that are classified into the TS of some reactions on the potential energy hypersurface of a polyatomic system. Thus, to check the validity of the TS, the intrinsic reaction coordinate (IRC)26 was calculated for important bond-breaking reactions. IRC is a reaction path that connects a reactant to a product through TS. The path can be followed by integrating the reaction coordinate from the TS to a reactant or product direction. If the computed reactant and product are plausible ones for the reaction in question, the reaction path is successfully determined.

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COMPUTATIONAL METHODS All ab initio molecular orbital calculations reported in this paper were carried out with the Gaussian 09 Revision B.01 program.29 The hybrid density-functional theory (DFT) based on Becke’s three-parameter hybrid method was employed with the LYP correlation functional (B3LYP) as an exchange-correlation 15941

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Gibbs energy of solvation of Li+ ion in PC (4.74 eV).37 The difference between the calculated energies and the values in the literature might stem from the entropic term of Li+ ion in gas that is ignored in SCF energy calculations. In addition to the above static ab initio calculations, we then performed ADMP34 calculations with semiempirical PM6 parameters35 to draw insights into the dynamical structure and conformation of TEGDME molecule and [Li-TEGDME]+ ion at room temperature. Starting from the optimized linear structures of TEGDME molecule and [Li-TEGDME]+ ion by using PM6 parameters, the change of the maximum interatomic distance (Rmax) in them with respect to simulation time is plotted in Figure 2. In both cases Rmax rapidly decreases during

To obtain dynamical information that is complementary to the above-described static calculations, molecular dynamics (MD) simulations were done using the atom-centered density matrix propagation (ADMP)34 method. As stated all static calculations in this paper were performed by using ab initio method (B3LYP) with treating solvent effects by IEF-PCM. However, ADMP-MD calculations were done with semiempirical PM635 parameters under vacuum condition to reduce the computational cost. The time step (Δt) of ADMP-MD was set at 0.25 fs, and the temperature was maintained at room temperature (300 K) using the velocity scaling method during the MD simulations.

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RESULTS AND DISCUSSION Stable and Dynamical Structures of TEGDME and [LiTEGDME]+. First, we examined the optimized geometries of linear and curled form of TEGDME molecule and [LiTEGDME]+ ion in a solvent (THF) at the B3LYP/6-31+ +G(d,p) level of theory. Note that considering that the number of coordination bonds in the first solvation shell of Li+ ion is 3− 4 and TEGDME is a polydentate ligand that can form five coordinate bonds with the Li+ ion, the single solvent molecule model ([Li-TEGDME]+) in this study is expected to capture the essential chemistry underlying the solvation of the ion. We made the initial structure of curled [Li-TEGDME]+ ion as being analogous to a chelate complex of 15-crown-5 ether with Li+ ion because the structural formula of TEGDME can be obtained from the ether by breaking one C−C bond of it and attaching H atoms to the dangling bonds. Geometry relaxation of the curled TEGDME molecule was started from the optimized geometry of curled form [Li-TEGDME]+ with removing Li+ from the ion. Calculated optimized geometries are shown in Figure 1. In the case of TEGDME molecule without Li+ ion, we found that the linear form is energetically more favorable than the curled form only by 0.095 eV. On the other hand, in the case of [LiTEGDME]+ ion, the curled form is more stable than the linear one by 0.961 eV. Note that the optimized geometry in Figure 1b (c) is merely an example of countless structures of curled form of TEGDME molecule ([Li-TEGDME]+ ion). Nonetheless, the calculation is useful to estimate the energy difference between linear and curled form. In particular, the relatively large energy difference as much as approximately 1 eV between linear and curled form of [Li-TEGDME]+ ion shows that the former form is obviously unstable. As stated, the structure of curled form [Li-TEGDME]+ ion is analogous to the chelate complex of Li+ with 15-crown-5 ether. The length of five Li−O bonds of the former ion is in the range from 2.053 to 2.200 Å and 2.116 Å in average while that of the latter is in the range from 2.314 to 2.328 Å and 2.321 Å in average. The shorter bond lengths of [Li-TEGDME]+ ion suggest strong interaction between Li+ and O atoms in TEGDME compared to the Li+ and those in 15-crown-5 complex. Indeed, this was confirmed by the solvation energy of Li+ ion defined as follows:

Figure 2. Time evolution of the maximum interatomic distance in TEGDME molecule (pink) and [Li-TEGDME]+ ion (blue) calculated by using the ADMP method with PM6 parameters.

the first 2 ps. After 4 ps, the fluctuation of Rmax of [LiTEGDME]+ ion becomes small, and the averaged Rmax is approximately 9 Å in 4−10 ps. On the other hand, the fluctuation of Rmax in TEGDME molecule is much larger than that in [Li-TEGDME]+ ion due to the lack of coordination bonds between Li+ and O atoms that stabilize the curled structure. This result is consistent with the above ab initio calculation which shows that the linear form is obviously less stable than the curled form in [Li-TEGDME]+ ion. Therefore, it is natural for [Li-TEGDME]+ ion to relax to curled structures as the simulation proceeds. On the other hand, although the linear form is energetically favorable than the curled form by 2.6 × 10−3 eV/atom in TEGDME molecule, the energy difference per atom is much smaller than the thermal noise at room temperature. Thus, the large fluctuation of Rmax in TEGDME seems to be a feasible phenomenon. Reaction Barrier for TEGDME and [Li-TEGDME] + Decompositions by O2−. Next, we examined the bondbreaking position and the reaction barrier for TEGDME molecule and [Li-TEGDME]+ ion by the superoxide anion (O2−) attack. We only have to examine five points in the Figure 1 to count up all the possible C−O bond breaking points because TEGDME is symmetric ether. Here, we defined superficial activation energy as the energy difference between the TS (shown in Figure 3) and the sum of the energies of isolated O2− and TEGDME/[Li-TEGDME]+. Table 1 shows the calculated superficial activation energies with and without zero point vibrational energy (ZPVE) corrections to the SCF energy difference. In the case of TEGDME alone, these reaction barriers are considerably high and are more than 1.411 eV (1.378 eV with ZPVE corrections). In particular, we found that the barrier which corresponds to the bond break at the one

Li+gas + TEGDMEsolv → [Li‐TEGDME]+solv Li+gas + 15‐crown‐5solv → [Li‐15‐crown‐5]+solv

The solvation energy of TEGDME is 5.717 eV, whereas that of 15-crown-5 is 5.489 eV. These energies are comparable with the Gibbs energy of hydration of Li+ ion (4.99 eV)36 and the 15942

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Figure 4. Intrinsic reaction coordinate of C−O bond-breaking reactions by O2− anion attack: (a) [Li-TEGDME]+ + O2− and (b) [Li-PC]+ + O2− (solid line, with Li+) and PC + O2− (dashed line, without Li+). Zero energy is set to the total energy of reaction precursor. Molecular models of reaction precursor and product are also shown in (a) and reaction precursor, TS, and product are also shown in (b) for [Li-PC]+ + O2−. Sky blue, red, green, and white balls stand for carbon, oxygen, lithium, and hydrogen atoms, respectively.

Figure 3. The same as Figure 1, but the optimized TS structures for the reaction of TEGDME molecule with O2− anion at bb5 position (a) and [Li-TEGDME]+ ion with O2− anion at the bb2 position (b).

end (bb1) is the highest whereas the other end (bb5) is the lowest. This suggests the unstable nature of bond breaking product of CH3O−(bb1). On the other hand, it is noteworthy that the superficial activation energies of [Li-TEGDME]+ ion (0.743−0.936 eV in SCF calculations and 0.661−0.851 eV with ZPVE corrections) are approximately half of those of TEGDME molecule. Similar to the TEGDME’s case, the ZPVE corrections lower the height of the reaction barrier. This is because the elongated bonds at the reaction center of TS usually decrease the ZPVE correction, which in turn lowers the barrier. To examine the significant decrease of the barrier heights by chelating the Li+ ion, IRC calculation was performed for the C−O bond break at the bb2 position in [Li-TEGDME]+ ion (Figure 4a). In addition to the relative energy along the reaction coordinate, Figure 4a also shows the structures of [LiTEGDME]+−O2− at the reaction precursor and the product. Note that O2− and [Li-TEGDME]+ form a reaction precursor through electrostatic interaction before the C−O bond breaks. Therefore, the reaction barrier measured from the precursor (1.087 eV) is higher than the superficial activation energy in Table 1 (0.743 eV). To check the influence of dispersion interaction which is ignored in B3LYP functional, the reaction barrier was examined

by calculating the unrestricted second-order Møller−Plesset (UMP2) energy at the geometries of reaction precursor and TS determined by B3LYP functional. Note that the UMP2 can account for the dispersion interaction. We obtained the barrier height of 1.508 eV by UMP2. This means that the barrier by UMP2 is approximately 1.4 times as high as that by B3LYP, which is consistent with the trend observed in the results by Bryantsev et al. (Table 1 in ref 24). As the reaction progress, the net atomic charge of OA (see Figure 4a) calculated by Mulliken population analysis changes from −0.29 e (precursor) to −0.49 e (TS) and to −0.59 e (product) while the net charge of O2 molecule changes from −0.87 e (precursor) to −0.42 e (TS) and to −0.16 e (product). It may be noted that approximately two-thirds of the total charge transfer is observed between the precursor and TS. The fact that the negative charge is transferred from O2 to OA leads to an enhancement of the electrostatic interaction between Li and OA. The result is consistent with the decrease of the Li−OA bond length from 1.997 Å (precursor) to 1.781 Å (product). Thus, a relatively low reaction barrier of [Li-TEGDME]+ ion

Table 1. Calculated Superficial Activation Energies (See Text for the Definition) for C−O Bond Break in TEGDME (in eV) at Five Possible Break Positions in Figure 1 with [ΔEA(+ZPVE)] and without Zero Point Vibrational Energy Correction to the SCF Energy [ΔEA(SCF)] bond break position TEGDME [Li-TEGDME]+

ΔEA(SCF) ΔEA(+ZPVE) ΔEA(SCF) ΔEA(+ZPVE)

bb1

bb2

bb3

bb4

bb5

1.734 1.688 0.857 0.777

1.520 1.483 0.743 0.661

1.558 1.533 0.773 0.697

1.507 1.466 0.772 0.719

1.411 1.378 0.936 0.851

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Figure 5. Energy profile of [Li-TEGDME]+ + O2 + e− system. ULi represents the electrode potential measured from the redox potential of metallic lithium.

of O2− anion than PC, which is suitable character for the use of TEGDME as a solvent of lithium−air batteries. We also examined the C−O bond break in PC by O2− without Li+ ion as was studied in ref 20. The superficial activation energies are 0.540 eV with ZPVE correction and 0.547 eV without ZPVE correction. As shown in Figure 4b, the barrier height from the reaction precursor obtained from the IRC calculation is 0.618 eV, which agrees well with the barrier reported in ref 20 (0.671 eV). Comparing the cases with and without Li+ ion, the presence of the ion decreases the reaction barrier and increases the exothermicity of the C−O bondbreaking reaction. These results show that the possibility of the C−O bond break in PC is enhanced by forming a complex with Li+ ion although the effect of the complex formation may not be so conspicuous as TEGDME’s case. Energy Profile for [Li-TEGDME]+ Decomposition by O2−. Having elucidated the energetics of the elementary reaction of C−O bond break in TEGDME, we are now in a position to examine the energy profile of the whole reaction that starts from O2 in gas phase. In this subsection SCF energy was used unless otherwise stated. We used the eV unit in the argument and dropped elementary charge (e) in the notation for simplicity. The first reaction corresponds to the formation of superoxide anion: O2,gas + e− → O2−solv. The reaction energy is dependent on the electrode potential ULi (that is measured from the redox potential of metallic lithium), and it can be expressed as (ULi − ULi[O2/O2−]). Note that ULi[O2/O2−] (vs Li/Li+) is the redox potential of O2,gas + e− ⇌ O2−solv. Using −0.6339 and −3.0428 V respectively for O2/O2− and Li/Li+ redox potentials measured from the standard hydrogen electrode scale, ULi[O2/O2−] becomes 2.41 V(vs Li/Li+). Although it is possible to estimate ULi[O2/O2−] from ab initio calculations, in that case we have to introduce the absolute potential40 that is found significant ambiguity in the literature.41,42 Thus, it seems to be practical to use the measured value for ULi[O2/O2−]. Then, O2− may form an adduct with [Li-TEGDME]+ ion. It is more stable than the reaction precursor in Figure 4a. However, it is noteworthy that the reaction barrier from the adduct (0.178 + 1.087 = 1.265 eV in Figure 5) is still lower than that of TEGDME alone in Table 1 (1.411 V). It will be valuable to consider the conditions of electrode potential ULi where TEGDME is hard to be decomposed by the attack of O2− anion. We paid attention to the following three conditions: (1) formation of the superoxide anion (O2−) becomes endothermic, (2) TS in Figure 5 has substantial barrier in compassion with the initial state, and (3) the whole

was caused by the increase of the electrostatic interaction between Li and OA as the reaction progresses. Besides the above argument, SN2 nature of the C−O bond break mechanism suggests that negative charge of O2 in the reactant plays an important role in determining the nucleophilicity and reaction activity. To test this hypothesis, we examined the decomposition of [Li-TEGDME]+ ion by the attack of LiO2 and LiO2−. We assume for simplicity that the C− O bond-breaking position is bb2 as the case with O2− attack. The formal oxidation number of O atom in LiO2 is −0.5, which is the same with that in O2−. However, it should be noted that Mulliken charge of O atom is different: The calculated net atomic charge of O is −0.741 e (LiO2−), −0.50 e (O2−), and −0.338 e (LiO2). The barriers for C−O bond-breaking reaction are 0.302, 0.743, and 1.641 eV, respectively for LiO2−, O2−, and LiO2. This suggests that there is an approximately linear relationship between the height of the reaction barrier and the magnitude of the Mulliken charge of O atom. Thus, the hypothesis was confirmed. It may be noted that calculations so far do not consider the influence of counterions in the electrolyte. The concentration of lithium salt such as LiPF6 is usually around 1.0 mol/L in Li− air batteries, where the ion pair Li+···PF6− is entirely dissociated in PC solvents.38 However, in the low-permittivity solvent such as TEGDME, there may be the contact ion pair of Li+···PF6− in the electrolyte although the high donor number of TEGDME (16.6) is favorable for the dissolution of lithium salts in comparison to PC (15.1). The reaction of the [LiPF6− TEGDME] complex with O2− anion was also examined (Figure S1 in the Supporting Information exhibits the optimized structure of the [LiPF6−TEGDME] complex and the TS geometry). The calculated superficial activation energy of the C−O bond-breaking reaction with the presence of LiPF6 is 0.968 eV without ZPVE correction. Although the value is somewhat higher than the case with Li+ ion (0.743 eV) in Table 1, it is still significantly lower than the case TEGDME alone (1.411 eV). A similar IRC calculation was done to a [Li-PC]+−O2− system to evaluate the extent to which TEGDME is stable against O2− attack in comparison with PC. The result is shown in Figure 4b. The reaction barrier measured from the precursor (0.477 eV) is considerably lower than that of [Li-TEGDME]+ in Figure 4a (1.087 eV). Moreover, the superficial activation energies (i.e., the barrier height from the sum of energies of isolated [Li-PC]+ and O2−) are 0.338 and 0.334 eV, respectively, with and without ZPVE corrections. These results clearly indicate that TEGDME is more robust against the attack 15944

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Table 2. Calculated Superficial Activation Energies for C−O Bond Break in [Li−CH3O(CH2CH2O)NCH3]+ (N = 1−3, in eV) at Possible Break Positions with [ΔEA(+ZPVE)] and without Zero Point Vibrational Energy Correction to the SCF Energy [ΔEA(SCF)] bond break position [Li−CH3OCH2CH2OCH3]

+

[Li−CH3O(CH2CH2O)2CH3]+ [Li−CH3O(CH2CH2O)3CH3]+

ΔEA(SCF) ΔEA(+ZPVE) ΔEA(SCF) ΔEA(+ZPVE) ΔEA(SCF) ΔEA(+ZPVE)

bb1

bb2

0.621 0.569 0.633 0.576 0.741 0.686

0.507 0.457 0.628 0.584 0.600 0.566

(1)

ULi − ULi[O2 /O2−] + 0.743 > 0

(2)

ULi − ULi[O2 /O2−] − 0.025 > 0

(3)

0.601 0.570 0.641 0.590

bb4

0.660 0.595

minimum 0.507 0.457 0.601 0.570 0.600 0.566

(bb2) (bb2) (bb3) (bb3) (bb2) (bb2)

(N = 1−4) by the attack of O2− anion. In particular, we focused on the decomposition of TEGDME (N = 4). TEGDME forms a chelate complex with Li+ ion as 15-crown-5 ether. By forming the chelate complex, the reaction barrier for the bond break significantly lowers in comparison with TEGDME alone. However, according to the IRC calculations, the barrier height measured from the reaction precursor is 1.087 eV, which is much higher than that of the analogous reaction in PC (0.477 eV). The reaction barrier of TEGDME is the highest among four glymes within the single solvent model. We examined the conditions of electrode potential ULi where TEGDME is hard to be decomposed by the attack of O2− anion. If the electrode potential is higher than 1.667 V(vs Li/Li+), the bond breaking reaction is expected to be inhibited. Although electrode potential is a principal factor for determining the reactivity in electrochemistry, we did not explicitly include electrode in the computational model. The electrode potential was considered posteriori in drawing the energy profile of Figure 5. Clarifying the effect of the potential in the electronic structure calculation will be one of the leading themes in computational study of Li−air batteries.

reaction is endothermic. From the figure, each condition is expressed by the below inequality: ULi − ULi[O2 /O2−] > 0

bb3

ULi[O2/O2−],

Using 2.41 V for we found that at least condition (2) is satisfied when ULi is higher than 1.667 V(vs Li/Li+) and all three conditions are satisfied when ULi is higher than 2.435 V(vs Li/Li+). Thus, TEGDME is stable to the O2− attack unless for any reason that the cathode potential becomes low during the operation of the battery. It should be noted that the product is less stable than the precursor in TEGDME, which is quite different from PC’s case shown in Figure 4b. We found that a similar analysis as Figure 5 in the case of PC provides a theoretical explanation why PC is decomposed by O2− attack under operating conditions of lithium−air batteries. This will be described in the next study. Dependence of the Reaction Barrier on the Number of (CH2CH2O) Groups in Glyme Series. The discussion so far has focused on TEGDME. However, considering the choice of favorable solvent for lithium−air batteries, it seems to be useful to examine the dependence of the reaction barrier on the number of (CH2CH2O) groups in CH3O(CH2CH2O)NCH3 (N = 1−3). The calculated superficial activation energy for C− O bond break in [Li−CH3O(CH2CH2O)NCH3]+ by the attack of superoxide anion is listed in Table 2. There are N + 1 C−O bond breaking positions in CH3O(CH2CH2O)NCH3. To identify N + 1 C−O bond breaking positions in CH3O(CH2CH2O)NCH3, we used analogous labeling as done in Figure 1c and examined all possible positions. The minimum superficial activation energy of monoglyme (0.457 eV with ZPVE correction) is somewhat lower than other glymes. Nonetheless, the value is still higher than that of PC (0.338 eV), and monoglyme is expected to be stable in comparison with PC. The minimum superficial activation energy of triglyme (0.566 eV with ZPVE correction) is slightly lower than that of diglyme (0.570 eV). Thus, the reaction barrier does not necessarily increase as the number of (CH2CH2O) group increases. However, within the limits of the present single solvent model, the superficial activation energy of TEGDME (Table 1) is the highest among them, which might be advantageous to use TEGDME as solvent for lithium−air batteries in addition to its high boiling point.

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ASSOCIATED CONTENT

S Supporting Information *

Figure S1. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by MEXT Program for Development of Environmental Technology using Nanotechnology.

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REFERENCES

(1) Christensen, J.; Albertus, P.; Sanchez-Carrera, R. S.; Lohmann, T.; Kozinsky, B.; Liedtke, R.; Ahmed, J.; Kojica, A. A Critical Review of Li/Air Batteries. J. Electrochem. Soc. 2012, 159, R1−R30. (2) Girishkumar, G.; McCloskey, B.; Luntz, A. C.; Swanson, S.; Wilcke, W. Lithium−Air Battery: Promise and Challenges. J. Phys. Chem. Lett. 2010, 1, 2193−2203. (3) Oudenhoven, J. F. M.; Baggetto, L.; Notten, P. H. L. All-SolidState Lithium-Ion Microbatteries: A Review of Various ThreeDimensional Concepts. Adv. Energy Mater. 2011, 1, 10−33. (4) Ellis, B. L.; Nazar, L. F. Sodium and Sodium-Ion Energy Storage Batteries. Curr. Opin. Solid State Mater. Sci. 2012, 16, 168−177.

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CONCLUSIONS Ab initio molecular orbital calculations were performed to investigate the C−O bond break in CH3O(CH2CH2O)NCH3 15945

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp404849u | J. Phys. Chem. C 2013, 117, 15940−15946