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Anion-cation, anion-lithium, cation-lithium and ion pair-lithium interactions in alicyclic ammonium based ionic liquids as electrolytes of lithium metal batteries
Journal of Molecular Liquids 242 (2017) 1228–1235
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Anion-cation, anion-lithium, cation-lithium and ion pair-lithium interactions in alicyclic ammonium based ionic liquids as electrolytes of lithium metal batteries L. Maftoon-Azad a,⁎, F. Nazari b a b
Department of Chemistry, Faculty of Sciences, Persian Gulf University, Bushehr 75168, Iran Faculty of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), 5195-1159, Zanjan 45195, Iran
a r t i c l e
i n f o
Article history: Received 28 March 2017 Received in revised form 19 July 2017 Accepted 28 July 2017 Available online 30 July 2017
a b s t r a c t To shed light on the performances of the ionic-liquids (ILs) as electrolyte in lithium metal batteries, we have studied 1-R-1-methyl pyrrolidinium [RmPyrr]+, 1-R-1-methylpiperidinium [RmPiP]+, and 1-R-1-methylazepanium [RmAzp]+ as cations with R = MeOCH2CH2- and [NTF2]- as anion, ion pairs as pure IL's and ion pair plus Li as doped IL's by ab-initio methods. Obtained structural parameters and electronic properties from natural bond orbital, atom in molecule and density of states analyses reveal that in the presence of Li all active electrophile and nucleophile sites of the ion pair vanish. The ion pair binding energy reduces in the doped ion pairs which suggests the elevation of the ILs ionic conductivity in a quantum structure property relationship regime. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Among several anodic redox pairs typically found in commercial batteries, those of Li+/Li and Li+/LixCy Pairs are the most negative. Thus, very high voltages (around twice as high as those of other common batteries) can be produced. In lithium metal batteries, the reaction which occurs at the lithium metal anode is Li(0) ↔ Li+ + e− while the cathode may be composed of a large variety of materials based on diverse chemistries. Li ions are released from the anode, diffuse through the electrolyte and intercalate in the cathode. The electrolyte is most of the time a solution of a lithium salt (such as Li[PF6], Li[ClO4] or Li[TFSI]) in an organic solvent. This solvent might be a cyclic or acyclic hydrocarbon. In spite of the reversibility of Li+/Li, lithium metal batteries are not rechargeable due to high reactivity of pure lithium metal. During charge-discharge cycles, this fact might cause the decomposition of the electrolyte, temperature elevation and fire or explosion hazards. This defect can be removed by utilizing Li ion batteries with intercalated anodes. Here, a protective layer is formed by decomposition of electrolyte in contact with the electrode. This passivating shield, known as solid electrolyte interface (SEI), allows hundreds of charge-discharge cycles to be safely performed [1]. Nevertheless, fire hazards still exist due to over heating or short circuits. Also the charge density will decrease in comparison to Li metal batteries due to heavier anodes. Due to the above facts, electrolytes more resistant to lithium are intensely sought. Among the other alternatives, the attraction to ionic ⁎ Corresponding author. E-mail address: [email protected] (L. Maftoon-Azad).
http://dx.doi.org/10.1016/j.molliq.2017.07.121 0167-7322/© 2017 Elsevier B.V. All rights reserved.
liquids (ILs) is very considerable. ILs are non-flammable and non-volatile. They are at liquid state around or below 100°C (and even for some cases below 0 °C) [2]. Owing to their ionic nature, they have inherent ionic conductivities. Also high electrochemical stability and wide electrochemical windows are of their properties. These characters, make ILs very potential candidates to be used as electrolyte, in many areas such as metal electro-deposition [1,3], fuel cells [1,4], batteries [1,5] or super capacitors [1,6]. Very strong reducing and oxidizing agents can be used as anode and cathode, yielding high cell voltages (if desired) and high energy densities [7]. Given these features, the manufacture of safely rechargeable lithium metal batteries, with higher energy densities than commercial lithium ion cells may still be a demanding goal [1,7]. These materials can be used as standalone electrolytes, additives, plasticizers in gel polymer electrolytes and binders. The most popular ionic liquids used in lithium batteries are those based on fluorinated anions [1] due to their low viscosities and thus, − high conductivities. BF− 4 (bore tetra-fluoride) and PF6 (phosphor hexafluoride) are stable with respect to water but undergo slow hydrolysis in moist air [7]. [NTF2]− (bis(trifluoromethylsulfonyl)amide N(SO2CF3)2]−) is completely hydrolytically stable and a better case [1, 7]. Cation structures also exert influences. Imidazolium based systems tend to be better ionic conductors than pyrrolidinium or piperidinium counterparts but lower resistant to reduction at negative potentials. In fact, due to their aromaticity, their acidic hydrogen atoms and vacant π orbitals, they are prone to be reduced [1,4,8]. Alicyclic ammonium liquid salts are generally better conductors [1,6] and more resistant to
L. Maftoon-Azad, F. Nazari / Journal of Molecular Liquids 242 (2017) 1228–1235
reduction [1,9] than non-cyclic counterparts. The functionalized analogues of these ionic liquids are frequently chosen for battery applications due to their increased conductivities. By this brief view, our focus will be on [RmPyrr]+ (1-R-1-methyl pyrrolidinium), [RmPiP]+ (1-R-1-methylpiperidinium), and [Rmazp]+ (1-R-1-methylazepanium) cations, with R = MeOCH2CH− 2 and [NTF2]− anion. We tend to utilize the ab initio approach to elucidate the ion–ion and ion-metal interactions and ionic structures of the mentioned ionic liquids. These interactions control the transport properties (viscosity, ionic conductivity, and self diffusion coefficients of ions) and stabilities of ionic liquids. The shape and size of ions are important for diffusivity of ions. The magnitude of attraction between cation and anion substantially controls the diffusion of ions in ionic liquids [10]. The hydrogen bonding between counter ions enhances the ionic association of the ILs, the fact that affects the conduction properties in reverse direction. Interactions between the ions of the electrolyte and Li(0) would affect the stability and the electromotive force of the cell. Obviously, this approach will be effective in rational design of new ionic liquids as electrolytes in lithium batteries. 1.1. Computational details The Gaussian09 program [11] is used for first-principle calculations. Geometry optimization for neutral, cationic and anionic species and complexes is carried out in the framework of density functional theory
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[12] by using the hybrid exchange-correlation functionals (B3LYP) [13] and popular 6-311++G (d, p) basis set [14]. The optimized geometries have been characterized by harmonic analysis, and the nature of the stationary points is determined according to the number of negative eigenvalues of the Hessian matrix. Since we were investigating just a trend in the properties of the three ionic liquids and the Li effects, we roughly used one similar geometry for each case on the potential energy surface. The only constraint was to choose the structures to be at local minima. Counterpoise correction [15] has also been applied in determination of ion pair or ion metal binding energies (generally called IPBE here) to avoid basis set superposition error (BSSE) although its contribution to the interaction energy for an ionic salt has been already reported to be insignificant [16–19]. The following expression was used for the IPBE calculation: IPBE ¼ ECP ðC…AÞ−Emin ðC Þ−Emin ðAÞ þ ΔZPVE
ð1Þ
where ECP(C… A) is the counter-poise corrected electronic energy of the ion pair or ion metal complex and Emin(C) and Emin(A) are the electronic energies of the cation, anion or lithium in their minimum energy geometries. To avoid some limitations inherent to other DFT functionals, ion pair binding energies were calculated using M05-2x [20,21]. ΔZPVE is the zeropoint vibrational energy and is calculated here using scaled B3LYP/6-311++G (d, p) vibrational frequencies.
Fig. 1. Optimized structures for [RmPyrr]+, (I); [RmPyrr]Li, (II); [RmPyrr][NTF2]−, (III); [RmPyrr][NTF2]Li, (IV); [NTF2]−, (V); [NTF2]− Li (VI); [RmPip]+, (VII); [RmPip]+ Li, (VIII); [RmPip][NTF2], (IX); [RmPip][NTF2]Li, (X); [RmAzp]+, (XI); [RmAzp]+ Li, (XII); [RmAzp][NTF2], (XIII) and [RmAzp][NTF2] Li, (XIV). Figures are drawn by AVOGADRO [32].
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Fig. 1 (continued).
The bonding features of all the studied species are analyzed by means of the natural bond orbital (NBO) and the natural population analyses (NPA) [22]. We also have analyzed the nature of the bonding by using the atoms in molecules (AIM) [23] approach using AIM 2000 software. Reactivities of different atoms in studied species are investigated by using molecular electrostatic potential (MESP) [24], Fukui functions (FF) [25] and dual descriptors (Δf) [26]. 2. Results and discussion The picture that emerges from what has been reported so far in the literature is that ionic liquids are very complex media, characterized by the simultaneous presence of van der Waals, coulomb, dipole, and in some cases H-bonding interactions. In the following first sections, the geometries and bond characteristics of the ionic liquids are investigated. In the second and third parts we will discuss the dominant contribution of each interaction for the studied cations, anion, ion pairs and their Li doped counterpart. Finally, the most reactive sites will be considered in Sections 2.3 and 2.4. We will report the role of these parameters in structural and electronic properties of candidate ionic liquids and their lithium doped counterparts. 2.1. Structures and bonding features In all optimized cases, Fig. 1, internal coordinates of the isolated anion and cations are prone to change in weak and strong interacting environments. The dihedral angles are the most affected internal coordinates due to the presence of lithium. Lithium is always located close to the etheric oxygen in isolate cations. In the [NTF2]− Li complex lithium is located in the bridge positions above the two sulfur atoms. In [RmPyrr][NTF2]Li and [RmAzp][NTF2]Li complexes Li atom is located very close to the oxygen on [NTF2]− moiety with a close distance of 1:8 9A that can be regarded as a strong interaction. In [RmPip][NTF2]Li, Li
interacts with etheric oxygen in the cation in addition to nitrogen and one of the oxygens of the anion. Compared with [RmPyrr]+ and [RmAzp]+ cations, the presence of [RmPip]+ causes to a kite shaped appearance, see Fig. 2. We should emphasize that the tendency of Li+ ion to be coordinated to the oxygens of [NTF2]− moiety has been approved before by the XPS experiment and calculated DOS spectra [27]. Also Shimizu et al. [28] by means of the Raman spectroscopy and solvation number results have inferred that Li+ ion tends to be coordinated to etheric oxygens of an ether functionalized [RmPip] + based ionic liquid. The changes in two N\\S bonds length in [NTF2]− moiety can be regarded as an indicator of the changes of the interaction in the studied species too. The NBO analysis shows that both N\\S bonds are under strain in all kinds of species but in [NTF2]− Li pair this distortion is maximized for N- S2, Table S1 in Supporting information. This is probably due to the four membered ring formation between Li, N, S2 and O3 atoms which is illustrated in AIM 3D view, Fig. 2. Thus, it is longer than N\\S2 bond in other species. The bond stretches to relief from the tension. This strain is decreased in N\\S1 bond in comparison with N\\S2 since it is in a six member ring between N, S1, O1, F6, C2, S2 atoms. The cation affects this length and shortens the distance by the following trend: [RmPip]+ N [RmAzp]+ N [RmPyrr]+, see Table S1 in Supporting information. Also the S\\N bond which is close to Li is shorter than the further one. In [RmPip][NTF2]Li, again, this bond is larger than the two other Li doped species which decrease the stress of the four member ring which is formed due to the weak interactions. Investigating the role of covalent, van der Waals interactions or hydrogen bonds, the ∠SNS angle in [NTF2]− is also an appropriate index. The ∠ SNSangle in [NTF2]− is pushed inward in the presence of Li atom due to the weak Li- sulfur interactions. Also in [RmPyrr] [NTF2] and [RmPip] [NTF2] ion-pairs this angle is reduced, see Table S2 in Supporting information.
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Fig. 1 (continued).
This reduction is maximized in [RmPyrr][NTF2] where the bonded oxygen to one sulfur (S2) interacts with fluors of bonded CF3 to the other sulfur (S1). This is evident from AIM calculations demonstrated in Fig. S1 in Supporting information. This also can be regarded to the bonded O1\\H8, O3\\H5, O3\\H14, O4\\H7 and O4\\H8 and the strong C6\\O1 interaction. C6 is in the pyrrolidinium cycle and adjacent to the nitrogen atom as illustrated in 3D view of AIM picture of the system in Fig. S1 in Supporting information. The NBO calculations show that in C6-N2 bond 67.63% and 32.37% of the electron density distribution is towards the nitrogen and carbon atoms, respectively. Thus an electron depletion on C6 can cause to C\\O interaction. In the presence of Li, a triangular ring among O3, H28 and Li 29 is seen instead of the C\\O bonding with bent paths which show that the bonds are under stress, see Fig. S2 in Supporting information. Therefore, the angle in doped ion pairs is close to that of [NTF2]−, Table S2 in Supporting information.
In [RmPip][NTF2] again hydrogen bonding is dominant but carbon oxygen interaction is absent, see Fig. S3 in Supporting information. In [RmAzp][NTF2], we can see the C\\O interaction - but less hydrogen bonding occurs and the S-N-S angle doesn't decrease, see Fig. S4 in Supporting information. 2.2. Possible origin of the interactions The overall mobility of the ions, the observed conductivities of the corresponding ionic liquids and the ionic liquid structures is highly affected by charge transfer between ionic liquid or oppositely charged ions [29]. Thus microscopic parameters, such as origin of the interactions, aid in understanding ionic liquid properties and explaining unexpected trends in the aforesaid properties. Particularly interesting parameters are HOMO-LUMO gap and the nature of interaction in interacting
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L. Maftoon-Azad, F. Nazari / Journal of Molecular Liquids 242 (2017) 1228–1235 Table 2 Global surface optimums (kcal/mol), ESP surface.
Fig. 2. AIM view for [NTF2]− Li. Oxygens are shown with red spheres, nitrogen with blue, sulfurs with yellow, Lithium with dark grey, carbons with black, hydrogens with light grey and flours with golden spheres.
species. HOMO-LUMO gap for cations, anion and ion pairs are of the order of 10− 1 a.u. but Li doping of the species cause to decrease the HOMO-LUMO gap down to the order of 10−2 a.u., Table 1. Interaction of [NTF2]− with cations increases both HOMO and LUMO energies and thus doesn't affect the gap. Among Li doped species the lowest gap belongs to [RmPip] [NTF2] Li which is the least polar species, although the highest gap is seen in [RmPip][NTF2] compared to the other ion pairs. Li doping especially increases HOMO energy and decreases the LUMO slightly. While ion pairs have landa maxes in deep UV region, in Li doped ion pairs landa-max shifts to near IR region. It can be deduced from the elevation of the HOMO energy besides ESP surface, NBO and AIM analyses that electron is transferred from Li into the cation, anion and ion pairs and increases the softness of the system. The global maximum in ESP surface (the most positive part of the electrostatic potential surface) is due to Li atom and again, it is maximized in [RmPip][NTF2]Li (101.08 kcal/mol), Fig. S5 in Supporting information and Table 2. Also NBO analysis confirms relatively large positive charges of Li, the largest of which belongs to [RmPip][NTF2] Li with the lowest gap and lowest polarity among other doped ionic liquids, Fig. 3, Table 1 and table S3 in Supporting information. From the FMO (frontier molecular orbital) point of view, the HOMO energy of anion is very close to the LUMO of cation and it shows good electrophile-nucleophile interactions. This helps the charge transfer between the ions and thus the ion pairing. Li doping increases the HUMO of [NTF2]− from −0.16 to 0.016 a.u. and doesn't change the LUMO of the cation so much. Thus, Li weakens the interactions. In another
Doped ion pair
Maximum
Minimum
[RmPyrr][NTF2]Li [RmPip][NTF2]Li [RmAzp][NTF2]Li
97.68 101.68 55.77
−43.50 −32.10 −35.78
description, in charge transfer (CT) phenomenon, electrons (charges) can be partly transferred from one molecule or ion to another via a weak covalent bond like a hydrogen bond. We can see many hydrogen bondings between cation and anion in AIM scheme. In the doped ion pair, since the oxygens are interacting with lithium, the number of hydrogen bondings between the two ions and thus, the charge transfer and ion pairing declines. Therefore we expect better conductivities of ionic liquids as electrolytes of lithium metal batteries. This is also confirmed by means of ion pair binding energies (IPBE's) and dispersion forces. As Table 3 shows, in the presence of Li atom, the absolute values of IPBE decrease by about 10 kcal/mol for [Rmpyrr][NTF2] and about 5 kcal/mol in [Rmpip][NTF2]. Thus cation and anion are less bind to each other and exhibit better mobilities. Consequently, better conductivities are achieved. The case is the reverse for [RmAzp][NTF2] and the dispersion force is more in the presence of Li by about 3 kcals of absolute IPBE. The later can be understood by a quick look at the AIM Schemes of the free and doped ion pairs. Here, not only the lithium atom doesn't break the hydrogen interactions but also an oxygen-oxygen interaction becomes favorable in the vicinity of lithium, see Fig. S6 in Supporting information. 2.3. Details of the non-bonding interactions from AIM and NBO analyses H\\H bonding is the common feature of the studied systems. These interactions have been formed in all studied cations. In [RmPyrr]+, bond paths linking two bonded hydrogen atoms that bear similar charges are found between the hydrogen on the ether branch (H10, Fig. 4) and that on the ring (H17,Fig. 4). This non-conventional hydrogen bonding aids to form a six membered ring that involves the C6–C7 (Fig. 4) bond. This has forced this bond to deviate from the line of center by 5.6 about 1° more than other cases where there is no H\\H bonding which is evident from NBO analysis. Bending towards the rings interior shows electron depletion in all kinds of species. In the presence of Li atom, again, all studied cations show the non-conventional hydrogen bonding. In the presence of anion this bonding doesn't exist in [RmPyrr]anion ion pair and it is replaced by a network (or cage) constructed by medium C\\H…O and bifurcated three center hydrogen bonding. However, in [RmPip][anion] ion pair again we see non-conventional hydrogen- hydrogen bonding. In [Rmazp][NTF2] the situation for this bond
Table 1 The energy gap in atomic units between HOMO and LUMO orbitals for each species. Species
EG +
[RmPyrr] [RmPyrr]+ Li [RmPyrr][NTF2] [RmPyrr][NTF2]Li [NTF2]− EG = Energy gap.
0.2546 0.0594 0.225 0.083 0.272
Species
EG +
[RmPip] [RmPip]+ Li [RmPip][NTF2] [RmPip][NTF2]Li [NTF2]− Li
0.258 0.059 0.234 0.028 0.046
Species
EG +
[RmAzp] [RmAzp]+ Li [RmAzp][NTF2] [RmAzp][NTF2]Li
0.260 0.059 0.218 0.040
Fig. 3. HOMO-LUMO gap vs. polarity for different species.
L. Maftoon-Azad, F. Nazari / Journal of Molecular Liquids 242 (2017) 1228–1235 Table 3 Basis set superposition errors, zero point correction energy and IPBE's for different complexes. Species
BSSE energy (a.u.)
ZPE(a.u.)
IPBE (kcal/mol)
[RmPyrr]+ Li [RmPyrr][NTF2] [RmPyrr][NTF2]Li [RmPip]+ Li [RmPip][NTF2] [RmPip][NTF2]Li [RmAzp]+ Li [RmAzp][NTF2] [RmAzp][NTF2] Li [NTF2]− Li
0.001127 0.004955 0.007637 0.001082 0.006472 0.008959 0.001057 0.005597 0.008217 0.002994
0.2618 0.3149 0.3155 0.2909 0.3442 0.3442 0.3199 0.3689 0.3740 0.0537
−3.6272 −75.5645 −65.6557 −2.5594 −73.4505 −62.7023 −2.2651 −69.6268 −72.2141 −17.2964
path formation is provided by the hydrogens which are located on the branches (H14 and H18, Fig. S4 in Supporting information). These hydrogens also interact with F′s on the CF3. In Li doped ion pair, [ion pair][Li], complexes, ion pair including RmPyrr doesn't show, RmPip shows and RmAzp here shows a H\\Li weak interaction instead, see Fig. S6 in Supporting information. The latter was accepted as a weak hydrogen bond path due to the 2:83A_ length of Li\\H, the angle 106.53°
_ among Li, H and C and the Li\\C distance which matches 3:31A. Table S3 in Supplementary information, represents the values of the inter-nuclear separation (BL) and the bond critical point data for nonconventional H\\H interactions in all species. Bond lengths are very small about 4.0 a.u. which is less than the twice of van der Waals radius of hydrogen atom, 4.5 a. u [30]. The bond paths for all non-conventional H\\H interactions are curved. The bond path length (BPL), which exceeds the bond length by as much as 0.7 a.u. confirms this claim, Table S3 in Supporting information. The correlations exhibit the characteristics of closed shell interactions due to their low value of the density (ρb), relatively small positive values of the Laplacian (∇2ρb) and a positive value of the energy density Hb that is close to zero. Density and the internuclear distances were approximately constant in all non-conventional H\\H bonding. Figs. S6–S8 in Supporting information show the AIM 3D view for three Li doped ion pairs in the presence of Li atom. In the species
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containing [RmPyrr] + and [RmAzp]+, Li correlates with the ionpair through the oxygen on the ether functional group (density, 0.23, Laplacian, − 0.04), one of the oxygens in [NTF 2] − (density, 0.31 and Laplacian, − 0.062) and a hydrogen connected to the carbon adjacent to nitrogen on the ring (density, 0.006 and Laplacian, 0.0006). In [RmPip][NTF2]Li the interaction sites are the same oxygens and the nitrogen on [NTF 2] − (density, 0.029 and Laplacian, 0.048) which totally form the diamond of a kite (see Fig. S9 in Supporting information). Here the hydrogen is not available for Li due to stereo-chemical matters. Another interesting matter demonstrated in Fig. S6 in Supplementary information is an unconventional interaction between O2 on the anion and O1 atom on the etheric functional of the cation (with density of 0.01, Laplacian 0.035, kinetic energy density at the bond critical point (Gb), 0.009 and energy density (Hb), 0.026). Here the lithium atom is interacting with the two oxygens and thus they are located at proper position to correlate to each other. Another thing is about florins. In all three species a F of CF3 correlates with an oxygen in [NTF2]− with a low density of 0.006 but in [Rmpyrr][NTF2]Li it correlates to a hydrogen of the pyrrolidinium ring, too. Evidently, the latter is a weak normal hydrogen bonding with low density of 0.005 and a Laplacian of −0.006.
2.4. Reactivity of ion pairs and Li complexes Obtained results for Fukui functions are depicted in Fig. S9 in Supporting information. These data show that in cations the etheric oxygen and hydrogens connected to its adjacent carbons are the best sites for electrophilic attack. For anion, the electron density is distributed on nitrogen, oxygens, carbons and F6 and F13. These hydrogens and florins perform hydrogen bonding in ion pair which are presented in AIM view with density, 0.004 and Laplacian, − 0.004 (Figs. S1, S3 and S4 in Supporting information). In ion pair electrons are mostly found on nitrogen and oxygens of the anion and etheric oxygen in the cation. In the presence of the Li all of the reactive sites disappear. We deduce that Li is chelated by reacting to these sites. Performing the dual descriptor analysis, Fig. S10 in Supporting information shows that carbons adjacent to pyrrolidinium nitrogen
Fig. 4. The same as Fig. 2 for [RmPyrr]+ cation.
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in cation are shown to be positive and activated to nucleophilic attack. In the past sections we deduced that lithium atom injects electrons into the cation in the presence of anion. Thus, we expect a dipolar interaction. Here we see that this atom inserts electrons to electrophile sites while interacts electrophillically to other nucleophile sites in the ion pair. In the other words lithium which is become partly positive by sending its electrons to the ion pair attracts to the electron rich sites. This time we speculate polar covalent bonding between Li and reactive atoms. 3. Conclusion Here in the attempt is to utilize the electronic structure theories to understand the physical properties of three alicyclic ammonium based ionic liquids already hoped to be used as electrolytes in lithium metal batteries. [RmPyrr][NTF2], [RmPiP][NTF2] and [Rmazp][NTF2] with R = MeOCH2CH− 2 ionic liquids, their isolated ions and lithium doped ions and ion pairs were studied using ab initio calculations. The S\\N length and ∠ SNS angle in the anion, the extension of hydrogen bonding, HOMO-LUMO gaps, IPBE, FF, Δf and ESP were the efficient descriptors for apriori approximation of the properties of ionic liquids. In summary, Li often interacts with one of the S\\O oxygens in anion and the etheric oxygen in cation. The distance between the nitrogen atom and the sulfur close to Li is more strained and also longer in [NTF2 ]Li and [RmPiP][NTF 2 ]Li due to four membered ring constructions. ∠ SNS angle declines in ion pairs with respect to the free anion due to cross interactions and this depression is maximized in [RmPyrr][NTF2]. Four kinds of hydrogen bonding are detected in the species by means of atom in molecule utility: 1. Medium hydrogen bonding between C\\H and O. They are formed between cation and anion atoms and most of them break in the presence of Li. These interactions play central role in anion-cation charge transfer. 2. Medium hydrogen bonds between H and F. 3. Non-conventional H\\H bonding. This kind of bonded hydrogens was seen in the free cations, in two of ion pairs and also in one of the doped ion pairs. 4. H– Li weak interactions in doped [Rmazp][NTF2] ion pair. Also non-conventional O\\O bonding was detected in this ionic liquid in vicinity of Li. [RmPiP] [NTF2] shows the minimum dipolarity of all ion pairs in the presence of Li. It also shows the least HOMO-LUMO gap among other species. HOMO- LUMO gaps decrease in the presence of Li by an order of magnitude which is due to increase in HOMO energies in the ion pair. This is related to the charge transfer from Li into ion pair. This phenomenon is confirmed by ESP diagram and NBO charges which both show great positive electrostatic charges on Li. This elevation of HOMO energy causes a decrease in the power of electrophile-nucleophile interactions of cation and anion which is evident from ion pair binding energies. This in turn causes to decline of charge transfer and ion pairing in the presence of Li which implies lower viscosity and higher ion conductivity for the ionic liquid. This is noteworthy that Li is conjugated to the etheric oxygen. The improvement of ionic conductivity by introducing ether linkages in the side chains of the cations is experimentally exhibited before [1,31]. While Li inserts electrons into ion pairs and becomes positive, it seeks for electron rich sites on the ion pair. This is demonstrated by FF and Δf diagrams where all reactive sites have been disappeared in the presence of Li. Thus, we suggest that Li tends to form a solid-electrolyte interphase (SEI) to prevent explosive hazards. Also the electrolyte may be stable in the presence of Li against air and moisture reactions. Acknowledgement The authors thank the support of Research Committees of both Persian Gulf University and the Institute of Advanced Studies in Basic Sciences.
Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.molliq.2017.07.121.
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Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Anion-cation, anion-lithium, cation-lithium and ion pair-lithium interactions in alicyclic ammonium based ionic liquids as electrolytes of lithium metal batteries L. Maftoon-Azad a,⁎, F. Nazari b a b
Department of Chemistry, Faculty of Sciences, Persian Gulf University, Bushehr 75168, Iran Faculty of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), 5195-1159, Zanjan 45195, Iran
a r t i c l e
i n f o
Article history: Received 28 March 2017 Received in revised form 19 July 2017 Accepted 28 July 2017 Available online 30 July 2017
a b s t r a c t To shed light on the performances of the ionic-liquids (ILs) as electrolyte in lithium metal batteries, we have studied 1-R-1-methyl pyrrolidinium [RmPyrr]+, 1-R-1-methylpiperidinium [RmPiP]+, and 1-R-1-methylazepanium [RmAzp]+ as cations with R = MeOCH2CH2- and [NTF2]- as anion, ion pairs as pure IL's and ion pair plus Li as doped IL's by ab-initio methods. Obtained structural parameters and electronic properties from natural bond orbital, atom in molecule and density of states analyses reveal that in the presence of Li all active electrophile and nucleophile sites of the ion pair vanish. The ion pair binding energy reduces in the doped ion pairs which suggests the elevation of the ILs ionic conductivity in a quantum structure property relationship regime. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Among several anodic redox pairs typically found in commercial batteries, those of Li+/Li and Li+/LixCy Pairs are the most negative. Thus, very high voltages (around twice as high as those of other common batteries) can be produced. In lithium metal batteries, the reaction which occurs at the lithium metal anode is Li(0) ↔ Li+ + e− while the cathode may be composed of a large variety of materials based on diverse chemistries. Li ions are released from the anode, diffuse through the electrolyte and intercalate in the cathode. The electrolyte is most of the time a solution of a lithium salt (such as Li[PF6], Li[ClO4] or Li[TFSI]) in an organic solvent. This solvent might be a cyclic or acyclic hydrocarbon. In spite of the reversibility of Li+/Li, lithium metal batteries are not rechargeable due to high reactivity of pure lithium metal. During charge-discharge cycles, this fact might cause the decomposition of the electrolyte, temperature elevation and fire or explosion hazards. This defect can be removed by utilizing Li ion batteries with intercalated anodes. Here, a protective layer is formed by decomposition of electrolyte in contact with the electrode. This passivating shield, known as solid electrolyte interface (SEI), allows hundreds of charge-discharge cycles to be safely performed [1]. Nevertheless, fire hazards still exist due to over heating or short circuits. Also the charge density will decrease in comparison to Li metal batteries due to heavier anodes. Due to the above facts, electrolytes more resistant to lithium are intensely sought. Among the other alternatives, the attraction to ionic ⁎ Corresponding author. E-mail address: [email protected] (L. Maftoon-Azad).
http://dx.doi.org/10.1016/j.molliq.2017.07.121 0167-7322/© 2017 Elsevier B.V. All rights reserved.
liquids (ILs) is very considerable. ILs are non-flammable and non-volatile. They are at liquid state around or below 100°C (and even for some cases below 0 °C) [2]. Owing to their ionic nature, they have inherent ionic conductivities. Also high electrochemical stability and wide electrochemical windows are of their properties. These characters, make ILs very potential candidates to be used as electrolyte, in many areas such as metal electro-deposition [1,3], fuel cells [1,4], batteries [1,5] or super capacitors [1,6]. Very strong reducing and oxidizing agents can be used as anode and cathode, yielding high cell voltages (if desired) and high energy densities [7]. Given these features, the manufacture of safely rechargeable lithium metal batteries, with higher energy densities than commercial lithium ion cells may still be a demanding goal [1,7]. These materials can be used as standalone electrolytes, additives, plasticizers in gel polymer electrolytes and binders. The most popular ionic liquids used in lithium batteries are those based on fluorinated anions [1] due to their low viscosities and thus, − high conductivities. BF− 4 (bore tetra-fluoride) and PF6 (phosphor hexafluoride) are stable with respect to water but undergo slow hydrolysis in moist air [7]. [NTF2]− (bis(trifluoromethylsulfonyl)amide N(SO2CF3)2]−) is completely hydrolytically stable and a better case [1, 7]. Cation structures also exert influences. Imidazolium based systems tend to be better ionic conductors than pyrrolidinium or piperidinium counterparts but lower resistant to reduction at negative potentials. In fact, due to their aromaticity, their acidic hydrogen atoms and vacant π orbitals, they are prone to be reduced [1,4,8]. Alicyclic ammonium liquid salts are generally better conductors [1,6] and more resistant to
L. Maftoon-Azad, F. Nazari / Journal of Molecular Liquids 242 (2017) 1228–1235
reduction [1,9] than non-cyclic counterparts. The functionalized analogues of these ionic liquids are frequently chosen for battery applications due to their increased conductivities. By this brief view, our focus will be on [RmPyrr]+ (1-R-1-methyl pyrrolidinium), [RmPiP]+ (1-R-1-methylpiperidinium), and [Rmazp]+ (1-R-1-methylazepanium) cations, with R = MeOCH2CH− 2 and [NTF2]− anion. We tend to utilize the ab initio approach to elucidate the ion–ion and ion-metal interactions and ionic structures of the mentioned ionic liquids. These interactions control the transport properties (viscosity, ionic conductivity, and self diffusion coefficients of ions) and stabilities of ionic liquids. The shape and size of ions are important for diffusivity of ions. The magnitude of attraction between cation and anion substantially controls the diffusion of ions in ionic liquids [10]. The hydrogen bonding between counter ions enhances the ionic association of the ILs, the fact that affects the conduction properties in reverse direction. Interactions between the ions of the electrolyte and Li(0) would affect the stability and the electromotive force of the cell. Obviously, this approach will be effective in rational design of new ionic liquids as electrolytes in lithium batteries. 1.1. Computational details The Gaussian09 program [11] is used for first-principle calculations. Geometry optimization for neutral, cationic and anionic species and complexes is carried out in the framework of density functional theory
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[12] by using the hybrid exchange-correlation functionals (B3LYP) [13] and popular 6-311++G (d, p) basis set [14]. The optimized geometries have been characterized by harmonic analysis, and the nature of the stationary points is determined according to the number of negative eigenvalues of the Hessian matrix. Since we were investigating just a trend in the properties of the three ionic liquids and the Li effects, we roughly used one similar geometry for each case on the potential energy surface. The only constraint was to choose the structures to be at local minima. Counterpoise correction [15] has also been applied in determination of ion pair or ion metal binding energies (generally called IPBE here) to avoid basis set superposition error (BSSE) although its contribution to the interaction energy for an ionic salt has been already reported to be insignificant [16–19]. The following expression was used for the IPBE calculation: IPBE ¼ ECP ðC…AÞ−Emin ðC Þ−Emin ðAÞ þ ΔZPVE
ð1Þ
where ECP(C… A) is the counter-poise corrected electronic energy of the ion pair or ion metal complex and Emin(C) and Emin(A) are the electronic energies of the cation, anion or lithium in their minimum energy geometries. To avoid some limitations inherent to other DFT functionals, ion pair binding energies were calculated using M05-2x [20,21]. ΔZPVE is the zeropoint vibrational energy and is calculated here using scaled B3LYP/6-311++G (d, p) vibrational frequencies.
Fig. 1. Optimized structures for [RmPyrr]+, (I); [RmPyrr]Li, (II); [RmPyrr][NTF2]−, (III); [RmPyrr][NTF2]Li, (IV); [NTF2]−, (V); [NTF2]− Li (VI); [RmPip]+, (VII); [RmPip]+ Li, (VIII); [RmPip][NTF2], (IX); [RmPip][NTF2]Li, (X); [RmAzp]+, (XI); [RmAzp]+ Li, (XII); [RmAzp][NTF2], (XIII) and [RmAzp][NTF2] Li, (XIV). Figures are drawn by AVOGADRO [32].
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Fig. 1 (continued).
The bonding features of all the studied species are analyzed by means of the natural bond orbital (NBO) and the natural population analyses (NPA) [22]. We also have analyzed the nature of the bonding by using the atoms in molecules (AIM) [23] approach using AIM 2000 software. Reactivities of different atoms in studied species are investigated by using molecular electrostatic potential (MESP) [24], Fukui functions (FF) [25] and dual descriptors (Δf) [26]. 2. Results and discussion The picture that emerges from what has been reported so far in the literature is that ionic liquids are very complex media, characterized by the simultaneous presence of van der Waals, coulomb, dipole, and in some cases H-bonding interactions. In the following first sections, the geometries and bond characteristics of the ionic liquids are investigated. In the second and third parts we will discuss the dominant contribution of each interaction for the studied cations, anion, ion pairs and their Li doped counterpart. Finally, the most reactive sites will be considered in Sections 2.3 and 2.4. We will report the role of these parameters in structural and electronic properties of candidate ionic liquids and their lithium doped counterparts. 2.1. Structures and bonding features In all optimized cases, Fig. 1, internal coordinates of the isolated anion and cations are prone to change in weak and strong interacting environments. The dihedral angles are the most affected internal coordinates due to the presence of lithium. Lithium is always located close to the etheric oxygen in isolate cations. In the [NTF2]− Li complex lithium is located in the bridge positions above the two sulfur atoms. In [RmPyrr][NTF2]Li and [RmAzp][NTF2]Li complexes Li atom is located very close to the oxygen on [NTF2]− moiety with a close distance of 1:8 9A that can be regarded as a strong interaction. In [RmPip][NTF2]Li, Li
interacts with etheric oxygen in the cation in addition to nitrogen and one of the oxygens of the anion. Compared with [RmPyrr]+ and [RmAzp]+ cations, the presence of [RmPip]+ causes to a kite shaped appearance, see Fig. 2. We should emphasize that the tendency of Li+ ion to be coordinated to the oxygens of [NTF2]− moiety has been approved before by the XPS experiment and calculated DOS spectra [27]. Also Shimizu et al. [28] by means of the Raman spectroscopy and solvation number results have inferred that Li+ ion tends to be coordinated to etheric oxygens of an ether functionalized [RmPip] + based ionic liquid. The changes in two N\\S bonds length in [NTF2]− moiety can be regarded as an indicator of the changes of the interaction in the studied species too. The NBO analysis shows that both N\\S bonds are under strain in all kinds of species but in [NTF2]− Li pair this distortion is maximized for N- S2, Table S1 in Supporting information. This is probably due to the four membered ring formation between Li, N, S2 and O3 atoms which is illustrated in AIM 3D view, Fig. 2. Thus, it is longer than N\\S2 bond in other species. The bond stretches to relief from the tension. This strain is decreased in N\\S1 bond in comparison with N\\S2 since it is in a six member ring between N, S1, O1, F6, C2, S2 atoms. The cation affects this length and shortens the distance by the following trend: [RmPip]+ N [RmAzp]+ N [RmPyrr]+, see Table S1 in Supporting information. Also the S\\N bond which is close to Li is shorter than the further one. In [RmPip][NTF2]Li, again, this bond is larger than the two other Li doped species which decrease the stress of the four member ring which is formed due to the weak interactions. Investigating the role of covalent, van der Waals interactions or hydrogen bonds, the ∠SNS angle in [NTF2]− is also an appropriate index. The ∠ SNSangle in [NTF2]− is pushed inward in the presence of Li atom due to the weak Li- sulfur interactions. Also in [RmPyrr] [NTF2] and [RmPip] [NTF2] ion-pairs this angle is reduced, see Table S2 in Supporting information.
L. Maftoon-Azad, F. Nazari / Journal of Molecular Liquids 242 (2017) 1228–1235
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Fig. 1 (continued).
This reduction is maximized in [RmPyrr][NTF2] where the bonded oxygen to one sulfur (S2) interacts with fluors of bonded CF3 to the other sulfur (S1). This is evident from AIM calculations demonstrated in Fig. S1 in Supporting information. This also can be regarded to the bonded O1\\H8, O3\\H5, O3\\H14, O4\\H7 and O4\\H8 and the strong C6\\O1 interaction. C6 is in the pyrrolidinium cycle and adjacent to the nitrogen atom as illustrated in 3D view of AIM picture of the system in Fig. S1 in Supporting information. The NBO calculations show that in C6-N2 bond 67.63% and 32.37% of the electron density distribution is towards the nitrogen and carbon atoms, respectively. Thus an electron depletion on C6 can cause to C\\O interaction. In the presence of Li, a triangular ring among O3, H28 and Li 29 is seen instead of the C\\O bonding with bent paths which show that the bonds are under stress, see Fig. S2 in Supporting information. Therefore, the angle in doped ion pairs is close to that of [NTF2]−, Table S2 in Supporting information.
In [RmPip][NTF2] again hydrogen bonding is dominant but carbon oxygen interaction is absent, see Fig. S3 in Supporting information. In [RmAzp][NTF2], we can see the C\\O interaction - but less hydrogen bonding occurs and the S-N-S angle doesn't decrease, see Fig. S4 in Supporting information. 2.2. Possible origin of the interactions The overall mobility of the ions, the observed conductivities of the corresponding ionic liquids and the ionic liquid structures is highly affected by charge transfer between ionic liquid or oppositely charged ions [29]. Thus microscopic parameters, such as origin of the interactions, aid in understanding ionic liquid properties and explaining unexpected trends in the aforesaid properties. Particularly interesting parameters are HOMO-LUMO gap and the nature of interaction in interacting
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L. Maftoon-Azad, F. Nazari / Journal of Molecular Liquids 242 (2017) 1228–1235 Table 2 Global surface optimums (kcal/mol), ESP surface.
Fig. 2. AIM view for [NTF2]− Li. Oxygens are shown with red spheres, nitrogen with blue, sulfurs with yellow, Lithium with dark grey, carbons with black, hydrogens with light grey and flours with golden spheres.
species. HOMO-LUMO gap for cations, anion and ion pairs are of the order of 10− 1 a.u. but Li doping of the species cause to decrease the HOMO-LUMO gap down to the order of 10−2 a.u., Table 1. Interaction of [NTF2]− with cations increases both HOMO and LUMO energies and thus doesn't affect the gap. Among Li doped species the lowest gap belongs to [RmPip] [NTF2] Li which is the least polar species, although the highest gap is seen in [RmPip][NTF2] compared to the other ion pairs. Li doping especially increases HOMO energy and decreases the LUMO slightly. While ion pairs have landa maxes in deep UV region, in Li doped ion pairs landa-max shifts to near IR region. It can be deduced from the elevation of the HOMO energy besides ESP surface, NBO and AIM analyses that electron is transferred from Li into the cation, anion and ion pairs and increases the softness of the system. The global maximum in ESP surface (the most positive part of the electrostatic potential surface) is due to Li atom and again, it is maximized in [RmPip][NTF2]Li (101.08 kcal/mol), Fig. S5 in Supporting information and Table 2. Also NBO analysis confirms relatively large positive charges of Li, the largest of which belongs to [RmPip][NTF2] Li with the lowest gap and lowest polarity among other doped ionic liquids, Fig. 3, Table 1 and table S3 in Supporting information. From the FMO (frontier molecular orbital) point of view, the HOMO energy of anion is very close to the LUMO of cation and it shows good electrophile-nucleophile interactions. This helps the charge transfer between the ions and thus the ion pairing. Li doping increases the HUMO of [NTF2]− from −0.16 to 0.016 a.u. and doesn't change the LUMO of the cation so much. Thus, Li weakens the interactions. In another
Doped ion pair
Maximum
Minimum
[RmPyrr][NTF2]Li [RmPip][NTF2]Li [RmAzp][NTF2]Li
97.68 101.68 55.77
−43.50 −32.10 −35.78
description, in charge transfer (CT) phenomenon, electrons (charges) can be partly transferred from one molecule or ion to another via a weak covalent bond like a hydrogen bond. We can see many hydrogen bondings between cation and anion in AIM scheme. In the doped ion pair, since the oxygens are interacting with lithium, the number of hydrogen bondings between the two ions and thus, the charge transfer and ion pairing declines. Therefore we expect better conductivities of ionic liquids as electrolytes of lithium metal batteries. This is also confirmed by means of ion pair binding energies (IPBE's) and dispersion forces. As Table 3 shows, in the presence of Li atom, the absolute values of IPBE decrease by about 10 kcal/mol for [Rmpyrr][NTF2] and about 5 kcal/mol in [Rmpip][NTF2]. Thus cation and anion are less bind to each other and exhibit better mobilities. Consequently, better conductivities are achieved. The case is the reverse for [RmAzp][NTF2] and the dispersion force is more in the presence of Li by about 3 kcals of absolute IPBE. The later can be understood by a quick look at the AIM Schemes of the free and doped ion pairs. Here, not only the lithium atom doesn't break the hydrogen interactions but also an oxygen-oxygen interaction becomes favorable in the vicinity of lithium, see Fig. S6 in Supporting information. 2.3. Details of the non-bonding interactions from AIM and NBO analyses H\\H bonding is the common feature of the studied systems. These interactions have been formed in all studied cations. In [RmPyrr]+, bond paths linking two bonded hydrogen atoms that bear similar charges are found between the hydrogen on the ether branch (H10, Fig. 4) and that on the ring (H17,Fig. 4). This non-conventional hydrogen bonding aids to form a six membered ring that involves the C6–C7 (Fig. 4) bond. This has forced this bond to deviate from the line of center by 5.6 about 1° more than other cases where there is no H\\H bonding which is evident from NBO analysis. Bending towards the rings interior shows electron depletion in all kinds of species. In the presence of Li atom, again, all studied cations show the non-conventional hydrogen bonding. In the presence of anion this bonding doesn't exist in [RmPyrr]anion ion pair and it is replaced by a network (or cage) constructed by medium C\\H…O and bifurcated three center hydrogen bonding. However, in [RmPip][anion] ion pair again we see non-conventional hydrogen- hydrogen bonding. In [Rmazp][NTF2] the situation for this bond
Table 1 The energy gap in atomic units between HOMO and LUMO orbitals for each species. Species
EG +
[RmPyrr] [RmPyrr]+ Li [RmPyrr][NTF2] [RmPyrr][NTF2]Li [NTF2]− EG = Energy gap.
0.2546 0.0594 0.225 0.083 0.272
Species
EG +
[RmPip] [RmPip]+ Li [RmPip][NTF2] [RmPip][NTF2]Li [NTF2]− Li
0.258 0.059 0.234 0.028 0.046
Species
EG +
[RmAzp] [RmAzp]+ Li [RmAzp][NTF2] [RmAzp][NTF2]Li
0.260 0.059 0.218 0.040
Fig. 3. HOMO-LUMO gap vs. polarity for different species.
L. Maftoon-Azad, F. Nazari / Journal of Molecular Liquids 242 (2017) 1228–1235 Table 3 Basis set superposition errors, zero point correction energy and IPBE's for different complexes. Species
BSSE energy (a.u.)
ZPE(a.u.)
IPBE (kcal/mol)
[RmPyrr]+ Li [RmPyrr][NTF2] [RmPyrr][NTF2]Li [RmPip]+ Li [RmPip][NTF2] [RmPip][NTF2]Li [RmAzp]+ Li [RmAzp][NTF2] [RmAzp][NTF2] Li [NTF2]− Li
0.001127 0.004955 0.007637 0.001082 0.006472 0.008959 0.001057 0.005597 0.008217 0.002994
0.2618 0.3149 0.3155 0.2909 0.3442 0.3442 0.3199 0.3689 0.3740 0.0537
−3.6272 −75.5645 −65.6557 −2.5594 −73.4505 −62.7023 −2.2651 −69.6268 −72.2141 −17.2964
path formation is provided by the hydrogens which are located on the branches (H14 and H18, Fig. S4 in Supporting information). These hydrogens also interact with F′s on the CF3. In Li doped ion pair, [ion pair][Li], complexes, ion pair including RmPyrr doesn't show, RmPip shows and RmAzp here shows a H\\Li weak interaction instead, see Fig. S6 in Supporting information. The latter was accepted as a weak hydrogen bond path due to the 2:83A_ length of Li\\H, the angle 106.53°
_ among Li, H and C and the Li\\C distance which matches 3:31A. Table S3 in Supplementary information, represents the values of the inter-nuclear separation (BL) and the bond critical point data for nonconventional H\\H interactions in all species. Bond lengths are very small about 4.0 a.u. which is less than the twice of van der Waals radius of hydrogen atom, 4.5 a. u [30]. The bond paths for all non-conventional H\\H interactions are curved. The bond path length (BPL), which exceeds the bond length by as much as 0.7 a.u. confirms this claim, Table S3 in Supporting information. The correlations exhibit the characteristics of closed shell interactions due to their low value of the density (ρb), relatively small positive values of the Laplacian (∇2ρb) and a positive value of the energy density Hb that is close to zero. Density and the internuclear distances were approximately constant in all non-conventional H\\H bonding. Figs. S6–S8 in Supporting information show the AIM 3D view for three Li doped ion pairs in the presence of Li atom. In the species
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containing [RmPyrr] + and [RmAzp]+, Li correlates with the ionpair through the oxygen on the ether functional group (density, 0.23, Laplacian, − 0.04), one of the oxygens in [NTF 2] − (density, 0.31 and Laplacian, − 0.062) and a hydrogen connected to the carbon adjacent to nitrogen on the ring (density, 0.006 and Laplacian, 0.0006). In [RmPip][NTF2]Li the interaction sites are the same oxygens and the nitrogen on [NTF 2] − (density, 0.029 and Laplacian, 0.048) which totally form the diamond of a kite (see Fig. S9 in Supporting information). Here the hydrogen is not available for Li due to stereo-chemical matters. Another interesting matter demonstrated in Fig. S6 in Supplementary information is an unconventional interaction between O2 on the anion and O1 atom on the etheric functional of the cation (with density of 0.01, Laplacian 0.035, kinetic energy density at the bond critical point (Gb), 0.009 and energy density (Hb), 0.026). Here the lithium atom is interacting with the two oxygens and thus they are located at proper position to correlate to each other. Another thing is about florins. In all three species a F of CF3 correlates with an oxygen in [NTF2]− with a low density of 0.006 but in [Rmpyrr][NTF2]Li it correlates to a hydrogen of the pyrrolidinium ring, too. Evidently, the latter is a weak normal hydrogen bonding with low density of 0.005 and a Laplacian of −0.006.
2.4. Reactivity of ion pairs and Li complexes Obtained results for Fukui functions are depicted in Fig. S9 in Supporting information. These data show that in cations the etheric oxygen and hydrogens connected to its adjacent carbons are the best sites for electrophilic attack. For anion, the electron density is distributed on nitrogen, oxygens, carbons and F6 and F13. These hydrogens and florins perform hydrogen bonding in ion pair which are presented in AIM view with density, 0.004 and Laplacian, − 0.004 (Figs. S1, S3 and S4 in Supporting information). In ion pair electrons are mostly found on nitrogen and oxygens of the anion and etheric oxygen in the cation. In the presence of the Li all of the reactive sites disappear. We deduce that Li is chelated by reacting to these sites. Performing the dual descriptor analysis, Fig. S10 in Supporting information shows that carbons adjacent to pyrrolidinium nitrogen
Fig. 4. The same as Fig. 2 for [RmPyrr]+ cation.
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in cation are shown to be positive and activated to nucleophilic attack. In the past sections we deduced that lithium atom injects electrons into the cation in the presence of anion. Thus, we expect a dipolar interaction. Here we see that this atom inserts electrons to electrophile sites while interacts electrophillically to other nucleophile sites in the ion pair. In the other words lithium which is become partly positive by sending its electrons to the ion pair attracts to the electron rich sites. This time we speculate polar covalent bonding between Li and reactive atoms. 3. Conclusion Here in the attempt is to utilize the electronic structure theories to understand the physical properties of three alicyclic ammonium based ionic liquids already hoped to be used as electrolytes in lithium metal batteries. [RmPyrr][NTF2], [RmPiP][NTF2] and [Rmazp][NTF2] with R = MeOCH2CH− 2 ionic liquids, their isolated ions and lithium doped ions and ion pairs were studied using ab initio calculations. The S\\N length and ∠ SNS angle in the anion, the extension of hydrogen bonding, HOMO-LUMO gaps, IPBE, FF, Δf and ESP were the efficient descriptors for apriori approximation of the properties of ionic liquids. In summary, Li often interacts with one of the S\\O oxygens in anion and the etheric oxygen in cation. The distance between the nitrogen atom and the sulfur close to Li is more strained and also longer in [NTF2 ]Li and [RmPiP][NTF 2 ]Li due to four membered ring constructions. ∠ SNS angle declines in ion pairs with respect to the free anion due to cross interactions and this depression is maximized in [RmPyrr][NTF2]. Four kinds of hydrogen bonding are detected in the species by means of atom in molecule utility: 1. Medium hydrogen bonding between C\\H and O. They are formed between cation and anion atoms and most of them break in the presence of Li. These interactions play central role in anion-cation charge transfer. 2. Medium hydrogen bonds between H and F. 3. Non-conventional H\\H bonding. This kind of bonded hydrogens was seen in the free cations, in two of ion pairs and also in one of the doped ion pairs. 4. H– Li weak interactions in doped [Rmazp][NTF2] ion pair. Also non-conventional O\\O bonding was detected in this ionic liquid in vicinity of Li. [RmPiP] [NTF2] shows the minimum dipolarity of all ion pairs in the presence of Li. It also shows the least HOMO-LUMO gap among other species. HOMO- LUMO gaps decrease in the presence of Li by an order of magnitude which is due to increase in HOMO energies in the ion pair. This is related to the charge transfer from Li into ion pair. This phenomenon is confirmed by ESP diagram and NBO charges which both show great positive electrostatic charges on Li. This elevation of HOMO energy causes a decrease in the power of electrophile-nucleophile interactions of cation and anion which is evident from ion pair binding energies. This in turn causes to decline of charge transfer and ion pairing in the presence of Li which implies lower viscosity and higher ion conductivity for the ionic liquid. This is noteworthy that Li is conjugated to the etheric oxygen. The improvement of ionic conductivity by introducing ether linkages in the side chains of the cations is experimentally exhibited before [1,31]. While Li inserts electrons into ion pairs and becomes positive, it seeks for electron rich sites on the ion pair. This is demonstrated by FF and Δf diagrams where all reactive sites have been disappeared in the presence of Li. Thus, we suggest that Li tends to form a solid-electrolyte interphase (SEI) to prevent explosive hazards. Also the electrolyte may be stable in the presence of Li against air and moisture reactions. Acknowledgement The authors thank the support of Research Committees of both Persian Gulf University and the Institute of Advanced Studies in Basic Sciences.
Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.molliq.2017.07.121.
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