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Li Local Structure in Hydrofluoroether Diluted Li-Glyme Solvate Ionic Liquid Soshi Saito, Hikari Watanabe, Kazuhide Ueno, Toshihiko Mandai, Shiro Seki, Seiji Tsuzuki, Yasuo Kameda, Kaoru Dokko, Masayoshi Watanabe, and Yasuhiro Umebayashi J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.5b12354 • Publication Date (Web): 09 Mar 2016 Downloaded from http://pubs.acs.org on March 18, 2016

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The Journal of Physical Chemistry

J. Phys. Chem.: Article for Special Issue

Li+ Local Structure in Hydrofluoroether Diluted LiGlyme Solvate Ionic Liquid Soshi Saitoa, Hikari Watanabea, Kazuhide Uenob, Toshihiko Mandaic, Shiro Sekid, Seiji Tsuzukie, Yasuo Kamedaf, Kaoru Dokkoc, Masayoshi Watanabec, Yasuhiro Umebayashia*

a

Graduate School of Science and Technology, Niigata University, 8050 Ikarashi, 2-no-cho, Nishi-ku, Niigata City, 950-2181, Japan

b

Graduate School of Medicine, Yamaguchi University, 2-16-1 Tokiwadai, Ube City, Yamaguchi 7558611, Japan c

Department of Chemistry and Biotechnology, Yokohama National University,79-5 Tokiwadai, Hodogaya-ku, Yokohama City, Kanagawa 240-8501, Japan

d

Materials Science Research Laboratory, Central Research Institute of Electric Power Industry (CRIEPI), 2-6-1 Nagasaka, Yokosuka City, Kanagawa 240-0196, Japan

e

Research Center for Computational Design of Advanced Functional Materials (CD-FMat), National

Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan f

Department of Material and Biological Chemistry, Faculty of Science, Yamagata University, 1-4-12, Kojirakawa-machi, Yamagata City, Yamagata 990-8560, Japan 1 Environment ACS Paragon Plus


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CORRESPONDING AUTHOR FOOTNOTE: To whom correspondence should be addressed. Telephone/Fax: +81-25-262-6265. E-mail: [email protected]

ABSTRACT

Hydrofluoroethers are recently used as the diluent to a lithium battery electrolyte solution to increase and decrease the ionic conductivity and the solution viscosity, respectively. In order to clarify the Li+ local structure in the 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (HFE) diluted [Li(G4)][TFSA] (G4: tetraglyme, TFSA: bis-(trifluoromethanesulfonyl)amide) solvate ionic liquid, Raman spectroscopic study has been done with the DFT calculations. It is turned out that the HFE never coordinates to the Li+ directly, and that the solvent (G4) shared ion pair of Li+ with TFSA anion (SSIP) and the contact ion pair between Li+ and TFSA anion (CIP) are found in the neat and the HFE diluted [Li(G4)][TFSA] solvate ionic liquid. It is also revealed that the two kinds of the CIP in which TFSA anion coordinates to the Li+ in mono-dentate and bi-dentate manners (Hereafter, we call them the monodentate CIP and the bi-dentate CIP, respectively.) exist with the SSIP of predominant [Li(G4)]+ ion-pair species in the neat [Li(G4)][TFSA] solvate ionic liquid, and that the mono-dentate CIP decrease as diluting with the HFE. To obtain further insight, X-ray total scattering experiments (HEXTS) were carried out with the aid of MD simulations, where the inter-molecular force fields parameters, mainly partial atomic charges, have been newly proposed for the HFE and glymes. New peak appeared at around 0.6 –0.7 Å–1 in X-ray structure factors, which was ascribed to the correlation between the [Li(G4)][TFSA] ion pairs. Furthermore, MD simulations were in good agreement with the experiments, from which it is suggested that the terminal oxygen atoms of the G4 in [Li(G4)]+ solvated cation frequently repeats coordinating/un-coordinating to the Li+, although the almost all of the G4 coordinate to the Li+ to form [Li(G4)]+ solvated cation in the neat and the HFE diluted [Li(G4)][TFSA] solvate ionic liquid.

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KEYWORDS: Li+ local structure, solvate ionic liquids, Raman, X-ray scattering, MD simulation, 1. Introduction Room-temperature ionic liquids consist of solely ions, so that they have environmentally and ecologically favorable properties such as practical inflammability due to negligible vapor pressure.1, 2 In addition, their ion conductive nature has attracted much attention as the electrolyte for the next generation batteries, supercapacitors and fuel cells.3 Recently, Angell et al. proposed that ionic liquids can be classified into 4 groups; 1) protic, 2) aprotic, 3) inorganic and 4) solvate ionic liquids, respectively. Solvate ionic liquids may be well known, for instance a hydrate melt Ca(NO3)2·4H2O whose melting point is about 40 ºC, is composed of a hydrated cation [Ca(H2O)]2+ and NO3– anions.4 Hence, strong Coulombic interaction between positive and negative charges on the respective cation and anion should be reduced due to the solvated molecules of electrically neutral to yield low melting point molten salts. On the other hand, vapor pressure arising from the solvated molecules should be decreased because of their strong coordination to the cation. Therefore, such low melting point molten salts; solvate ionic liquids show ionic liquids like behavior. Solvate ionic liquids as well as ordinary aprotic ones are expected as the electrolyte solvent for the new lithium secondary batteries. Among the next generation lithium secondary batteries, Li – sulfur battery is one of the promised ones owing to their potential high energy density.5-8 Watanabe et al. proposed Li – sulfur batteries composed of the Liglymes solvate ionic liquids [Li(glymes)][TFSA] (glymes; triglyme CH3-(OCH2CH2)3-OCH3 G3 or tetraglyme CH3-(OCH2CH2)4-OCH3 G4, TFSA; bis-(trifluoromethanesulfonyl)amide (CF3SO2)2N–, cf. Chart 1) as the electrolyte.9-14

Chart 1. Molecular structures of 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (HFE), tetraglyme (G4) and bis-(trifluoromethanesulfonyl)amide (TFSA). 3 Environment ACS Paragon Plus


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It is important to understand the Li+ solvation (structure) in the electrolyte solution for the lithium secondary batteries, because it should play a key role both in the solvation/de-solvation accompanied by the Li oxidation/Li+ reduction at the electrolyte/electrode interface and in the Li+ ionic conduction in the bulk electrolyte solution. In addition, it could be crucial for Li-sulfur batteries in terms of the restriction of lithium polysulfides Li2Sn dissolution into the electrolyte solution. Hence, the Li+ solvation structure has been well studied not only that in aqueous15 but also in non-aqueous solvent16-21 solutions. We have so far investigated the Li+ solvation structure in ordinary aprotic ionic liquids.22-27 According to accumulated studies on the Li+ solvation structure in the aqueous, the non-aqueous solvents and ordinary aprotic ionic liquids solutions, it is well established that the Li+ in solutions prefers 4-coordinated solvation structure. More recently, the Li+ solvation and its transport properties were studied in the Li-glymes solvate ionic liquids by Raman spectroscopy, the PGSE-NMR selfdiffusion measurements and theoretical calculations.28-38 Among these studies, in the Li-G3/G4 solvate ionic liquids, it is revealed that the almost all of the Li+ in the solvate ionic liquids are coordinated by G3/G4, and that interactions between Li+ and TFSA anion in the solvate ionic liquids is weak relative to those of the Li+ – G3/G4 on the basis of the fact that TFSA anion partially forms the solvent shared ion pair (SSIP) or the contact one (CIP) with the Li+ in the solvate ionic liquids. Thus, the lowered free G3/G4 concentration in the LiTFSA and G3/G4 equimolar (or slightly salt excess) mixtures yields essential nature as solvate ionic liquids, not ordinary concentrated salt solutions. To achieve more efficient Li secondary batteries, in general, larger ionic conductivity/lower viscosity is necessary, so that low viscous molecular solvents like acyclic carbonates are often added to the electrolyte solutions. To this purpose, 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether CF2HCF2-O-CH2CF2CF2H has been tested to enhance the performance of various kinds of Li secondary batteries.39-46 (hereafter, the abbreviation HFE is used for 1,1,2,2-tetrafluoroethyl 2,2,3,3tetrafluoropropyl ether throughout this paper.) Watanabe et al. demonstrated that addition of the HFE to the [Li(G3)][TFSA] solvate ionic liquid is effective to the Li(Li+)-S battery performance.47 They also studied 1 mol dm–3 LiTFSA in G3-HFE mixed solvent solutions by the PGSE NMR and Raman 4 Environment ACS Paragon Plus

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spectroscopy to elucidate speciation of the Li+ in the electrolyte solutions. NMR and Raman studies exhibited that G3 solvated cation of [Li(G3)]+ and TFSA anion form the contact ion pair in the HFE diluted

[Li(G3)][TFSA]

solvate

ionic

liquid

(LiTFSA:G3:HFE

=

1:1:4.46).

Originally,

hydrofluoroethers have been developed as the alternatives to the hydrofluorocarbon, the hydrochlorofluorocarbons, or the chlorofluorocarbons,48 and are used as a vapor decreasing solvent, a refrigerant and a heat transfer fluid or an anhydrous fluid cleaner. However, little has been reported on the physical chemistry of the HFE as a solvent for the electrolyte solutions.49-52 In this paper, we described the local structure of the Li+ in the neat and the HFE diluted [Li(G4)][TFSA] solvate ionic liquid by means of Raman spectroscopy and high-energy X-ray total scattering (HEXTS) experiments with the aid of DFT calculations and MD simulations. It is revealed that the HFE never coordinates to the Li+ directly, and that the SSIP and the CIP are found in the neat and the HFE diluted [Li(G4)][TFSA] solvate ionic liquid. Raman spectroscopic study indicates that the CIP, in which the mono-dentate TFSA anion is contained, (the modno-dentate CIP) changes to the SSIP but the CIP containing bi-dentate TFSA anion (the bi-dentate CIP) remains when diluted with the HFE. In addition, a peak at around 0.6 –0.7 Å–1 newly appeared in the experimental X-ray structure factor, which was assigned to the correlation between the [Li(G4)][TFSA] ion pairs. Detailed Li+ local structure was discussed in the neat and the HFE diluted [Li(G4)][TFSA] solvate ionic liquid with the aid of MD simulations.

2. EXPERIMENTAL SECTION Materials. Purified LiTFSA and G4 were kindly supplied by Solvay and Nippon Nyukazai Co., respectively. The HFE was purchased from Daikin Industries and used without further purification. LiTFSA was dried under vacuum at 120 °C for a few days, and then stored in an Ar glovebox (the water content was kept less than 1 ppm) before use. The test solutions, [Li(G4)][TFSA]x[HFE](1 – x) (x: the mole fraction of [Li(G4)][TFSA] solvate ionic liquid, x = 0.0, 0.172, 0.333, 0.667 and 1.0), were 5 Environment ACS Paragon Plus


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prepared by mixing [Li(G4)][TFSA] and the HFE in different molar ratios and were stirred overnight at ambient temperature to obtain transparent solutions. The water content of each test solution was measured by the Karl Fischer titration to be less than 100 ppm. Raman spectra. Raman spectra were measured by using a JASCO RMP-510 Raman spectrometer equipped with a single monochrometer and a CCD detector at the optical resolution of 2.8 cm–1. For excitation, Coherent Inova 70 Ar+ laser was used and was operated at about 500 mW to obtain enough S/N ratio, and the 514.5 nm line was used. Test solution was inserted into the hermetically sealed 5 mmφ Pyrex® glass tube. The HEXTS experiments. The HEXTS measurements were carried out at 298 K using the BL04B2 beam-line of SPring-8 at the Japan Synchrotron Radiation Research Institute (JASRI).54,

55

Sample

solution was set in a cell consisted of 2 mm thickness a polyetheretherketone plate as a body with Kapton® films as an X-ray window hermetically sealed with Kalrez® O rings, and stainless steel cover plates. Monochrome 61.6 keV X-rays were obtained using a Si (220) monochromator. The observed Xray intensity was corrected for absorption55 and polarization. Incoherent scatterings 56 were subtracted to obtain coherent scatterings, Icoh(Q). The X-ray structure factor SHEXTS(Q) and X-ray radial distribution function GHEXTS(r) per stoichiometric volume were respectively obtained according to

S HEXTS (Q ) =

G HEXTS ( r ) − 1 =

I coh (Q ) − ∑ ni f i (Q ) 2 ( ∑ ni f i (Q )) 2

1

Q max

2π rρ 0 2

∫0

+1 (2),

Q{ S (Q ) − 1} sin(Qr ) exp(− BQ 2 )dQ (3),

where ni and fi(Q) denote the number and the atomic scattering factor of atom i,57 respectively, ρ0 is the number density, and B is the damping factor. All data treatment was carried out using the program KURVLR.58


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Molecular orbital calculations. For theoretical Raman spectra of the HFE, the B3LYP/6-311+G(d,p) level of theory was adopted. For partial atomic charge evaluation, the MP2/cc-pvTZ(-f)// HF/6-31G(d) levels of theory was used. The Gaussian 03 program59 was used throughout this study. Molecular dynamics simulations. All MD simulations in this study are based on the OPLS-AA manner60 except a combination rule; the Lorentz-Berthelot rule (arithmetic and geometric mean for Lennard-Jones parameters σ and ε, respectively) was used as well as the CLaP force fields61-65 to keep consistency with our previous work.22, 24 Lennard-Jones parameters proposed by Soetens et al. for the lithium ion in non-aqueous cyclic/acyclic carbonate solutions was employed.66 The CLaP force fields were used for TFSA anion. Lennard-Jones parameters both for the G4 and the HFE were employed from the OPLS-AA (compatible) force fields.67, 68 The atomic partial charges used here are listed in Table S2 for the HFE and Table S3 for G4, respectively. The intra-molecular force fields parameters for the G4 were taken from the OPLS-AA compatible.67 With regard to the HFE, the rigid model was used in our simulations to reduce the computing cost, though the flexible one may sometime give better results. Details are discussed in Results and Discussion. Here, we put only a brief explanation; LennardJones parameters and atomic partial charges are also shown in Table S2, and Cartesian coordinates and selected intramolecular coordinates of the rigid model HFE are shown in Table S1. In our simulations, Gear’s predictor-corrector algorithm69, 70 was employed for integration of the equations of motion with 0.5 fs time steps. The systems contained 256 ion pair/molecules as the sum of the [Li(G4)][TFSA] ion pairs and the HFE molecules under NTP ensemble conditions controlled by the Nose’s thermostat71, 72 and the Parrinello-Rahman barostat.73, 75 The latter was always set to atmospheric pressure. Long-range interactions were estimated using the Ewald summation method with real-space cutoff distances of 11 Å. The simulation runs typically consisted of a 2.5-20 ns equilibration periods followed by 0.5 ns production runs, whose trajectories were then analyzed. All simulations were carried out using Materials Explorer 5.0 MD program package with a graphical user interface (Fujitsu). The X-ray structure factor SMD(Q) was calculated as:



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  MD S (Q) =     S MD (Q) =   

∑∑ {n (n i

i

j

− 1) f i (Q ) f j (Q ) / N ( N − 1)}

∫

j

2

  ∑ (nk f k (Q ) / N ) k  ∑∑ (2ni n j f i (Q) f j (Q) / N 2 ) i

j

  ∑ (nk f k (Q ) / N ) k 

2

∫

r 0

r 0

4 πr 2 ρ0 ( g MD (r ) − 1) ij

4 πr 2 ρ0 ( g ijMD (r ) − 1)

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sin(Qr ) dr + 1 Qr

sin(Qr ) dr + 1 Qr

(i = j )

(i ≠ j )

(4), where ρ0 denotes the ensemble average of the number density, and the total number of atoms in the simulation box N is given by N = ∑ nk k

(5).

The X-ray radial distribution function GMD(r) was obtained from SMD(Q) by a Fourier transform procedure similar to that of SHEXTS(Q).

3. RESULTS AND DISCUSSION Raman spectra. Before discussing the Li+ local structure in the neat and the HFE diluted [Li(G4)][TFSA] solvate ionic liquid, it may be valuable to discuss Raman spectrum for the neat liquid HFE briefly. Observed Raman spectrum for the neat HFE is shown in Figure S1(a) at the frequency range of 180 – 1600 cm–1. To assign the Raman bands, the DFT calculations performed to obtain one candidate among the potential equilibrium conformers. Theoretical Raman spectrum for thus obtained conformer at the optimized geometry is displayed in Figure S1(b) at the same frequency range with the experiment. The optimized structure is also shown in Figure S1(b) as the inset. As clearly shown in these figures, the experimentally observed Raman bands were partially reproduced with the one conformer, suggesting that the neat liquid HFE contains other conformers. As expected, it is necessary to identify predominant equilibrium conformers in the neat liquid HFE for complete Raman bands assignments. However, we go ahead toward our purpose in this study; to reveal the Li+ local structure in the neat and the HFE diluted 8 Environment ACS Paragon Plus

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[Li(G4)][TFSA] solvate ionic liquid. Raman spectra are shown in Figure S1(c) at the whole frequency range of 180 – 1600 cm–1 examined in this study. It should be noted that no significant peak variation from the neat HFE was found for the Raman bands ascribable to the HFE molecule in the HFE diluted [Li(G4)][TFSA] solvate ionic liquids. It indicates that the HFE never directly coordinates to the Li+ in the mixtures of the HFE and [Li(G4)][TFSA] SIL. This is supported by MD simulations described in the later section.

Figure 1. Raman spectra for neat and the HFE diluted [Li(G4)][TFSA] solvate ionic liquid at the frequency range of 720 – 770 cm–1 at 298 K. Black, red, blue, purple and green lines are Raman spectrum for [Li(G4)][TFSA] solvate ionic liquid mole fraction x = 0.0, 0.172, 0.333, 0.667 and 1.0, respectively.

Figure 1 shows Raman spectra for the neat and the HFE diluted [Li(G4)][TFSA] solvate ionic liquid of the [Li(G4)][TFSA] mole fraction x = 0.0, 0.172, 0.333, 0.667 and 1.0 at the frequency range of 720 –770 cm–1, where the most intense Raman band ascribable to the S−N symmetric stretching vibration coupled with the CF3 bending arising from a TFSA anion can be found. A weak Raman band of 743 cm–1 can be found for the neat liquid HFE, which should be superposed with the intense TFSA Raman band. According to the aforementioned DFT calculations, the 743 cm–1 band could be assigned to the vibration mode corresponding to theoretically predicted band of 718 cm–1. Comparing the predicted Raman activity of 0.88 Å4/amu for the 718 cm–1 band from the HFE with 26.3 Å4/amu for the overlapped one from the TFSA anion75, the interference from the HFE can be estimated to be at most 9 Environment ACS Paragon Plus


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about 10 % in a total Raman band area at the x = 0.172. Therefore, the overlapping with the Raman band from the HFE can be negligible in the following discussion. In addition, the relative Raman band intensity for the Raman spectra shown in Figure 1 was normalized by taking the predicted Raman activity and the mole fraction into account. Raman spectrum for the neat [Li(G4)][TFSA] solvate ionic liquid was consistent with that previously reported for the [Li(G3)][TFSA] solvate ionic liquid29, 33, 47, 76; the intense peak appears at around 742 cm–1 accompanied by the small shoulder of the higher frequency side. The higher frequency shoulder can be mainly attributed to the TFSA anion directly coordinating to the Li+; the CIP. The corresponding theoretical Raman bands were reported for those in the gas phase by us27 and also by Lassègues et al. 77 Theoretical Raman bands were calculated for the SSIP and the CIP in the gas phase32 to confirm the assignments. (Figure S2) When diluted with the HFE, the Raman band intensity decreased, and the band shape approached to be more symmetric. In order to discuss more quantitatively, the observed Raman bands were analyzed with a curve fitting technique by using the pseudo-Voigt peak functions. A typical non-linear least square fitting result is shown in Figure S2, in which it is clearly shown that the intense Raman band at around 742 cm–1 arising from the TFSA anion can be well reproduced with two peak functions except that at around 765 cm–1. Similarly, two pseudo-Voigt functions were in good accordance with the Raman bands for the other solutions examined in this study. It is worth mentioning that three peak functions fitting always gave a more wrong result or failed to convergence. In the neat [Li(G4)][TFSA] solvate ionic liquid, at least two or more ion pair species of the TFSA anion should exist; i.e., two distinguishable the SSIP and the CIP have potentially plural isomers. This indicates that it is difficult to distinguish the TFSA species among the respective SSIP/CIP isomer by Raman spectroscopy. The relative Raman band area for the higher shoulder, attributable to the CIP, to the total area (a) and the peak positions (b) and the half width at the half maximum (c) for the respective deconvoluted Raman band components are shown in Figure 2, respectively. As the increase of the HFE content in the solution, the relative Raman band area for the higher shoulder to the total area significantly decreased with small peak position shifting toward the 10 Environment ACS Paragon Plus

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higher frequency and with remarkable sharpening. Peak position for the lower Raman band component (the SSIP) also slightly shifted to the higher frequency with barely narrowing as the increase the HFE content.

Figure 2. Relative Raman band area in percent for the higher shoulder (the CIP) to the total area, the peak positions and the half width at the half maximum (HWHM) for the TFSA anions in the SSIP or the CIP, respectively, as the function of [Li(G4)][TFSA] solvate ionic liquid mole fraction.

The observed Raman bands variations suggest that the higher shoulder is composed of two or more CIP species in the neat solvate ionic liquid, and the CIP that has Raman band of the lower peak position should change to the other species having much lower peak position in the HFE diluted [Li(G4)][TFSA] solvate ionic liquid. This another ion pair species should increase by diluting with the HFE, leading to the relative intensity increase of the deconvoluted lower Raman band component originally arising from the SSIP in the neat solvate ionic liquid. One possible explanation can be made for the higher shoulder CIP species in the neat [Li(G4)][TFSA] solvate ionic liquid; the CIP containing the bi-dentate and the mono-dentate TFSA anion directly coordinating to the Li+. According to our preliminary dielectric relaxation measurements, the neat HFE and the neat [Li(G4)][TFSA] solvate ionic liquid have the static relative permittivity of about 7 and 17, respectively. Hence, the inter-molecular Coulombic interaction not only between Li+ and TFSA but also between Li+ and G4 should become stronger when diluted with the HFE. Taking it into consideration, it is plausible that the mono-dentate CIP changes to the SSIP and the bi-dentate CIP remains as the increase of the HFE content. 11 Environment ACS Paragon Plus


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The HEXTS experiments. To yield further insight, the HEXTS experiments were carried out. X-ray structure factors SHEXTS(Q) of 0 < Q < 2.5 Å–1 and the corresponding X-ray radial distribution functions as the form of r2{GHEXTS(r) –1} up to 15 Å for the neat and the HFE diluted [Li(G4)][TFSA] solvate ionic liquid are shown in Figure 3(a) and 3(b), respectively. SHEXTS(Q) data for the neat [L(G4)][TFSA] is taken from Ref. 29; two weak peaks appears at 0.9 and 1.5 Å–1. To the best of our knowledge, SHEXTS(Q) for the HFE was reported for the first time here. The SHEXTS(Q) for the neat HFE is a typical one for the non-aqueous solvents; there is one intense peak at around 1.6 Å–1. When diluting the [Li(G4)][TFSA] solvate ionic liquid with the HFE, the peak at 0.9 Å–1 decreased in intensity with the peak position shift toward the higher Q. On the other hand, the peak at 1.5 Å–1 was intensified with shifting its peak position to the lower Q. Interestingly, a small but significant new peak can be found at around 0.6 – 0.7 Å–1 whose peak position shifted lower Q as the HFE dilution, which indicates that the longer correlation that is never found in the respective neat liquid appears only in the HFE – [Li(G4)][TFSA] mixtures.

Figure 3. Experimental X-ray structure factors at the Q range of 0 – 2.5 Å–1 SHEXTS(Q) (a) and radial distribution functions as the form of r2{GHEXTS(r) – 1} at the r range of 0 – 15 Å (b) for neat and the HFE diluted [Li(G4)][TFSA] solvate ionic liquid at 298 K. The corresponding MD derived ones are shown as (c) and (d), respectively. The line coloring is the same in Figure 2. 12 Environment ACS Paragon Plus

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It is more easily to clarify a long range correlation in solutions with the radial distribution function as the form of r2{GHEXTS(r) – 1}. In all of the r2{GHEXTS(r) – 1} evaluated here, sharp and distinct peaks appeared at the r range of 0 < r/Å < 4, which can be mainly assigned to the intra-molecular atom-atom correlations. However, these intra-molecular atom-atom correlations are not discussed in detail in this paper. With regard to the neat HFE liquid, the first and the second broad peaks can be clearly observed at around 5 and 9 Å, respectively, which indicates that the nearest neighboring and the further neighboring HFE molecules surround a HFE molecule locate at the respective length scale. On the other hand, with the neat [Li(G4)][TFSA] solvate ionic liquid, the first and the second broad peaks appeared at around 5 and 8 Å, respectively. These two peaks can be ascribed to the closest [Li(G4)]+ solvated cation – TFSA anion correlation and the further correlation between the ions of the same sign. With the dilution with the HFE, the broad first peak increased in intensity, while the intensity in the r2{GHEXTS(r) – 1} decreased around 7 – 8 Å. However, it is difficult to discuss in detail only from the experimental radial distribution functions. In the following section, we attempted to discuss with the aid of MD simulations. It is worth discussing newly appeared peak at around 0.6 – 0.7 Å–1 in SHEXTS(Q) for the HFE – [Li(G4)][TFSA] mixtures. Though rough, the correlation length L = 2π/Qpeak is convenient because it gives real space information about the peak in S(Q). The L values are plotted against the [Li(G4)][TFSA] mole fraction x in Figure 4. The L of about 8 Å at x = 0.667 lengthened up to about 11 Å at x = 0.172 with the dilution with the HFE. Interestingly, this length scale agrees well with the average distance between the ion pairs of the [Li(G4)]+ solvated cation and the TFSA anion in the mixtures. The average distance between the ion pairs estimated from the molar volume is drawn as a solid line in Figure 4 for comparison. As shown in the figure, the solid line agrees with the experimental plots. Hence, the newly appeared peak at around 0.6 – 0.7 Å–1 in SHEXTS(Q) for the HFE – [Li(G4)][TFSA] mixtures could be attributed to the correlation between the ion pairs of the [Li(G4)]+ solvated cation and the TFSA anion, though the other possibilities can not be rejected at all . 13 Environment ACS Paragon Plus


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Figure 4. Correlations length L of the newly appeared peak in the X-ray structure factors for the HFE diluted [Li(G4)][TFSA] solvate ionic liquid except neat HFE and neat [Li(G4)][TFSA] solvate ionic liquid as the function of mole fraction of [Li(G4)][TFSA]. Solid line shows average distance among the [Li(G4)][TFSA] ion pair in solutions.

MD simulations. Aforementioned, we attempted to obtain more clear picture of the local structure of the Li+ in the neat and the HFE diluted [Li(G4)][TFSA] solvate ionic liquid, so that the MD simulations for these systems were performed. With the neat HFE, MD simulations have never been reported until now. Therefore, the inter-molecular force fields parameters for the HFE were newly constructed in this study. It should be noted that, according to the Raman result, the HFE never coordinates to the Li+ directly in its diluting [Li(G4)][TFSA] solvate ionic liquid. Hence, we employed a rigid model for the solvent HFE in this study. Cartesian and selected intra-molecular coordinates of the rigid model are listed in Table S1. First, we attempted to evaluate atomic partial charges on the HFE, particularly, that on the ether oxygen to clarify the electron pair donating ability of the HFE.


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Figure 5. Electrostatic potential for the HFE (a) and ethylpropylether (b) evaluated by the ChelpG method at the HF/6-31G(d)//MP2/cc-pvTZ(-f) level of theory.

Figure 5 shows the electrostatic potential around the isolated HFE in a gas phase accompanied by that of an ethyl-propyl-ether as the reference; the values are listed in Table S2. As clearly shown in this figure, the atomic partial charge on the ether oxygen of the HFE is similar with that on the fluorine of the molecule and is much smaller than that of the corresponding dialkylether, suggesting that the electron donating ability of the HFE should be weak due to electron withdrawing by the fluorine atoms. Though rough comparison, this is consistent with the calculated stabilization energy; that of -164 kJ mol–1 for [Li(HFE)]+ is much smaller than those of -450.6 and -574.0 kJ mol–1 for [Li(G4)]+ and [Li(TFSA)], respectively, at the MP2/6-311G(d,p)//HF/6-311G(d,p) level of theory. Taking the transferability into consideration, the inter-molecular force fields parameters were made and are listed in Table S2. To hold consistency, the atomic partial charges for the glymes were also evaluated and the atomic partial charges as the force fields parameters were revisited. According to the literature, the atomic partial charges have been estimated by using monoglyme (dimethoxyethane) as a model molecule67. Therefore, the OPLS-AA compatible charge on the ether oxygen for polyethyleneoxide is independent from the ether oxygen position in the polyethers. This may be appropriate approximation for such a large polymer. For the glymes of oligoethers, we calculated their atomic partial charges to check the validity of this approximation. The ChelpG atomic partial charges at the MP2 density for glymes are shown in Figure S4 and Table S3. As shown in the figure and the table, the atomic partial 15 Environment ACS Paragon Plus


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charge on the ether oxygen significantly depends on the oxyethylene chain length and its intramolecular position. Taking it into account, newly proposed atomic partial charges for glymes are also shown in the figure and the table. It is worth mentioning that the atomic partial charge on the glymes oxygen becomes larger from the terminal to the center, and the trend is clearer for the longer glyme. This trend indicates that the glymes coordination ability to the Li+ could changes not only in entropy but also in enthalpy as lengthening the ethylene oxide chain. Before detailed discussion, we checked the accordance with the experiments; the solution densities and the X-ray structure factors/radial distribution functions. Comparison in detail with the experiments is given in Figure S3. X-ray structure factors SMD(Q) and radial distribution functions r2{GMD(r) – 1} obtained from MD simulations are shown in Figure 3(c) and 3(d), respectively for comparison. As can be seen in these figures, MD simulations well reproduced the experimental data. Hence, to discuss the Li+ local structure in the neat and the HFE diluted [Li(G4)][TFSA] solvate ionic liquid, the Li – X (X = C, H, N, O, F and S) atom-atom pair correlation functions gLi-X(r) were evaluated and are displayed in Figure S5. It is valuable to discuss the Li – N (TFSA) atom-atom pair correlation functions gLi-N(r) for the neat and the HFE diluted [Li(G4)][TFSA] solvate ionic liquid. The gLi-N(r) gave two shorter and longer peaks at r < 5 Å; the former and the latter can be assigned to the bi-dentate and the mono-dentate TFSA anion coordinated to the Li+, respectively.22, 24, 25

Figure 6. Gibbs free energy change ∆Gº for the reaction from the mono-dentate CIP to the bi-dentate one TFSA (a), peak positions (b) and coordination numbers (c) of the first peak in the Li – O (G4, TFSA anion and HFE, respectively) pair correlation functions for neat and the HFE diluted [Li(G4)][TFSA] solvate ionic liquid. 16 Environment ACS Paragon Plus

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The Gibbs free energy change ∆Gº for the reaction from the mono-dentate TFSA CIP to the bidentate one estimated with the peak intensity ratio in gLi-N(r) for the neat and the HFE diluted [Li(G4)][TFSA] solvate ionic liquid is plotted against the solvate ionic liquid mole fraction x in Figure 6(a). The positive ∆Gº value for the neat solvate ionic liquid turned to the negative for the dilution with the HFE, which clearly shows the mono-dentate CIP decreased with the HFE dilution in the simulations. Coordination numbers and the first peak positions of the Li – O atom-atom pair correlation functions gLi-O(r) are shown in Figure 6(b) and 6(c), respectively. A coordination number and the first peak

position of gLi-O(r) for the neat [Li(G4)][TFSA] solvate ionic liquid are in good agreement with those from our previous MD simulations.28-30 As clearly shown in the figure, the HFE oxygen never coordinates to the Li+ in the HFE diluted [Li(G4)][TFSA] solvate ionic liquid. Additionally, the HFE coordination at the F atom to the Li+ is also rejected from the Li – F pair correlation functions for the HFE diluted [Li(G4)][TFSA] solvate ionic liquid shown in Figure S5. These are in good accordance with the Raman experiments. On the other hand, the coordination number and the first peak position of gLi-O(r) for the G4 oxygen in the HFE diluted [Li(G4)][TFSA] solvate ionic liquid slightly increased and

lengthened, respectively, which suggests that the G4 solvation to the Li+ is strengthened when the solvate ionic liquid is diluted with the HFE. In addition, small but significant decrease can be found in the coordination number of the TFSA oxygen without practical bond length change, which implies that the mono-dentate CIP decrease with the HFE dilution. Typical Li+ local structures found in the MD snapshot are drawn in Figure 7, which is consistent with the Raman spectra interpretation.



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Figure 7. Typical structure of the [Li(G4)][TFSA] SIP (a), the mono-dentate CIP (b) and the bidentate CIP (c) in the MD snapshot for neat and the HFE diluted [Li(G4)][TFSA] solvate ionic liquid.

G4 has five oxygen atoms; two of the terminal Ot, two of the middle Om and one of the center oxygen Oc. Figure 8(a) and 8(b) shows the coordination numbers and the peak positions, respectively, in each Li – OX (X = t, m and c) pair correlation function gLi-OX(r) for the neat and the HFE diluted [Li(G4)][TFSA] solvate ionic liquid. The coordination numbers for the Om and the Oc were two and one, respectively, and they are independent from the solution composition. This suggests that the middle and the center three oxygen atoms rigidly coordinate to the Li+ in the [Li(G4)]+ solvated cation. On the other hand, the coordination number for the terminal oxygen was about 1.4 that is clearly smaller than actual two terminal oxygen atoms in G4, indicating that the terminal oxygen frequently repeats coordinating to and un-coordinating from the Li+ in these solutions. In addition, the Li+ – Ot bond length were considerably longer than those of the others, which suggests that the coordination structure of [Li(G4)]+ solvated cation is rather distorted in the solutions.


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Figure 8. Li – O (G4) pair correlation functions for neat and the HFE diluted [Li(G4)][TFSA] solvate ionic liquid. Closed circles, triangles and squares represent the center Oc and the middle Om and the terminal ether oxygen atoms Ot in G4, respectively. Apart from the Li+ local structure, newly appeared peak at the lower Q region of 0.6 –0.7 Å–1 in the SHEXTS(Q) is discussed from the MD simulations. As mentioned above, this new peak could be mainly

attributed to the correlations between the [Li(G4)][TFSA] ion pairs in the solutions. Therefore, we attempted to evaluate potential mean force P.M.F. = –RT lngLi-Li(r) for the Li – Li correlation in MD simulations was evaluated and is shown as Figure 9, where gLi-Li(r) represents the Li-Li pair correlation in the neat and the HFE diluted [Li(G4)][TFSA] solvate ionic liquid. For the neat [Li(G4)][TFSA] solvate ionic liquid, a significant minimum can be found at around 9.5 Å. When diluted with the HFE, the intensity at around 7.5 and 9.5 Å in the P.M.F. increased, whilst that of around 13 Å decreased. In the P.M.F. for the most diluted solution of x = 0.172, there is a minimum at around 13 Å. This suggests the average distance between the [Li(G4)][TFSA] ion pairs is lengthened as the [Li(G4)][TFSA] solvate ionic liquid is diluted with the HFE. This scale length found in the MD simulations is in agreement with the experimental correlation length variation.



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Figure 9. Potential mean force for the Li – Li pair correlation for neat and the HFE diluted [Li(G4)][TFSA] solvate ionic liquid. The line coloring is the same in Figure 2. As the final remarks, though rough, it is worth pointing out that the Li+ exchange reactions relevant to the Li+ conduction in the solutions. One may consider that the Li+ conduction should be much restricted in the solutions, in which the SSIP and the CIP are strongly formed. However, the P.M.F. for the Li – Li correlation shows significant valleys at r < 7 Å, though scattered. This suggests the [Li(G4)][TFSA] ion pairs aggregation, which could contribute the Li+ inter-exchange between the [Li(G4)]+ solvate cations. In fact, according to our preliminary SAXS (small angle X-ray scattering) experiments, SAXS intensity increased as Q approaches to zero when [Li(G4)][TFSA] solvate ionic liquid is diluted with the HFE. In addition, Figure S6 shows P.M.F. for the Li –N (TFSA) correlation in the neat [Li(G4)][TFSA] solvate ionic liquid, in which two minima at around 4 Å can be attributed to two kinds of the CIP and also two valleys at around 6 – 8 Å can be ascribed to two sort of the SSIP. As shown in the figure, the barrier at around 5 Å between the CIP and the SSIP is rather small in the free energy scale, implying that the TFSA anion exchange could be also labile. Hence, our simulations suggest that ion exchange reaction could play a key role in ionic conduction in such low permittivity solutions containing a concentrated salt.


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4. CONCLUSIONS To reveal the Li+ local structure in the neat and the HFE diluted [Li(G4)][TFSA] solvate ionic liquid, Raman spectra were recorded and the HEXTS experiments were performed. DFT calculations at the B3LYP/6-311+G(d,p) level of theory were carried out to predict a neat HFE Raman spectrum. It is elucidated that the predicted Raman bands at one of the optimized geometries partially agree well with the experimental Raman spectrum, and thus the neat HFE liquid has several conformers. With regard to the HFE diluted [Li(G4)][TFSA] solvate ionic liquid, no significant Raman band can be found attributable to the HFE coordinated directly to the Li+. In addition, the intense Raman band at about 742 cm–1 was observed with the higher frequency shoulder in the Raman spectra for the neat and the HFE diluted solvate ionic liquid. The relative Raman band area for the higher shoulder to the total area decreased with the HFE dilution, which suggests that the mono-dentate CIP or the bi-dentate one exists in the neat [Li(G4)]TFSA] solvate ionic liquid together with the SSIP, and that the mono-dentate CIP decreased but the SSIP increased when diluted with the HFE. New peak appeared at around 0.6 – 0.7 Å– 1

in the X-ray structure factors SHEXTS(Q) with the HEXTS experiments for the HFE diluted

[Li(G4)][TFSA] solvate ionic liquid, which was ascribed to the correlation between the ion pairs of [Li(G4)][TFSA]. The inter-molecular force fields parameters, particularly atomic partial charges, have been newly developed for the HFE and glymes based on the ab initio calculations at the MP2/cc-pvTZ(f)//HF/6-31G(d) level of theory with the ChelpG method. MD simulations were well reproduced experimental solution density and SHEXTS(Q), and were consistent with the interpretations of Raman and X-ray scattering experiments. Furthermore, MD simulations suggests that the G4 strongly solvates to the Li+ at the three oxygen atoms of the center and the middle positions, but the terminal oxygen atoms frequently repeat coordinating/un-coordinating in the neat and the HFE diluted [Li(G4)][TFSA] solvate ionic liquid.



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ACKNOWLEDGMENT: This study was supported in part by the Grants-in-Aid for Scientific Research Nos. 23350033 and 24655142 from the Japan Society for the Promotion of Science (JSPS), and by the Advanced Low Carbon Technology Research and Development Program (ALCA) of the Japan Science and Technology Agency (JST), and by the Technology Research Grant Program of the New Energy and Industrial Technology Development Organization (NEDO) of Japan. The synchrotron radiation experiment was carried out with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2011A1373, 2012A1571, 2012A1669, 2012A1682 and 2012B1709).

Supporting Information: Additional data of Raman spectra, the HEXTS experiments and MD simulations, these analyzed one and force fields parameters are giving as Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

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