Local Structure of Li+ in Concentrated Ethylene Carbonate Solutions

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Local Structure of Li in Concentrated Ethylene Carbonate Solutions Studied by Low-frequency Raman Scattering and Neutron Diffraction with Li/Li Isotopic Substitution Methods 6

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Shunya Maeda, Yasuo Kameda, Yuko Amo, Takeshi Usuki, Kazutaka Ikeda, Toshiya Otomo, Maho Yanagisawa, Shiro Seki, Nana Arai, Hikari Watanabe, and Yasuhiro Umebayashi J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b10933 • Publication Date (Web): 14 Nov 2017 Downloaded from http://pubs.acs.org on November 16, 2017

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The Journal of Physical Chemistry

Isotropic Raman scattering and time-of-flight neutron diffraction measurements were carried out for concentrated LiTFSA-EC solutions in order to obtain structural insight on solvated Li+ as well as the structure of contact ion pair, Li+…TFSA-, formed in highly concentrated EC solutions. Symmetrical stretching vibrational mode of solvated Li+ and solvated Li+…TFSA- ion pair were observed at ν = 168~177 and 202~224 cm-1, respectively. Detailed structural properties of solvated Li+ and Li+…TFSA- contact ion pair were derived from the least squares fitting analysis of first-order difference function, ∆Li(Q), between neutron scattering cross sections observed for

6

Li/7Li

isotopically substituted 10 and 25 mol% *LiTFSA-ECd4 solutions. It has been revealed that Li+ in the 10 mol% LiTFSA solution is fully solvated by ca. 4 EC molecules. The nearest neighbor Li+…O(EC) distance and Li+…O(EC)=C(EC) bond angle are determined to be 1.90 ± 0.01 Å and 141 ± 1º, respectively. In highly concentrated 25 mol% LiTFSA-EC solution, the average solvation number of Li+ decreases to ca. 3 and ca. 1.5 TFSA- are directly contacted to Li+. These results agree well with the results of band decomposition analyses of isotropic Raman spectra for intramolecular vibrational modes of both EC and TFSA-.

INTRODUCTION Ethylene carbonate (EC) has widely been used as solvent of electrolyte solution for lithium ion secondary batteries because of its electrochemical stability and high solubility of lithium salts.1 Although typical solute concentration of electrolyte solutions employed for practical lithium ion 2

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batteries, ca. 1M (= mol dm–3), recently, highly concentrated solutions receive much attention relating to a possibility to realize higher voltage batteries.2,3 In order to understand physico-chemical properties of these solutions, information on the microscopic structure of solvated Li+ is therefore indispensable. Solvation structure of Li+ in the EC solutions has been investigated by means of infrared (IR)4-6 and Raman spectra.5-10 IR spectra of LiClO4-EC solutions have indicated that ring-bending (710 cm-1) and C=O stretching (1800 cm-1) modes of EC molecule shift toward higher frequency side with increasing LiClO4 content.5,7 The solvation number of Li+ in EC solutions has been estimated from the band decomposition analysis of the ring-bending mode of EC observed from Raman spectra of LiClO4-EC solutions.6 The solvation number of Li+ has been determined to 5.9 for 1 wt% LiClO4 solution and decreased to 2.3 for highly concentrated 45.4 wt% LiClO4 solution.6 Earlier Raman and 13

C NMR studies indicated that solvation number of Li+ is ca. 2 in concentrated

LiClO4-EC-propylene carbonate (PC) mixed solvent solutions.8 Solvation number 3.8 has been reported for 1M LiClO4-EC solution from band decomposition analysis of Raman ring-breathing mode of EC appearing at ~900 cm-1.9 More recent Raman study has indicated that the solvation number of Li+ increases with increasing temperature.10 The concentration dependence of the Li+…EC coordination has extensively been examined by Raman spectra of the EC vibrational band in the 895-905 cm-1 region.11 The Li+…TFSA- coordination in various crystalline solvate involving LiTFSA has been investigated by Raman band of TFSA- observed at ν ≈ 740 cm-1.12 3


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The solvation number of Li+ obtained from the 1H pulsed-gradient spin-echo nuclear magnetic resonance (PGSE-NMR) measurements is ca. 7 for 9 mol% LiClO4 and decreases to ca. 3 for 25 mol%

LiClO4-EC

solutions.10

Environmental

structure

of

Li+

in

EMI+TFSA-

(EMI+:

1-ethyl-3-methylimidazolium cation) ionic liquid has been investigated by Raman spectra.13 The ion pair, Li+(TFSA-)2, in which each TFSA- coordinates two oxygen atoms to central Li+, is formed in the ionic liquid.13 The results of

17

O NMR measurements for LiTFSA-EC solutions have indicated

that the Li+ is coordinated by 4 oxygen atoms of solvent molecules of EC and TFSA-.14 On the other hand, the electrospray ionization-mass spectroscopic study has shown that Li+(EC)3 cluster is stable in the gaseous state.15 More detailed structural properties have been obtained from ab initio calculation and molecular dynamics (MD) simulation studies. Klassen et al. indicated that the optimized Li+…O(EC) distance in isolated Li+(EC)4 cluster is 1.93 Å by means of ab initio calculation.16 On the other hand, the value, r(Li+…O(EC)) = 1.749 Å, has been reported for Li+EC and Li+(EC)2 clusters.17 Recent ab initio studies have revealed that Li+…O(EC) distance gradually elongated form 1.81118 and 1.80019Å for isolated Li+(EC)2 to 1.96518 and 1.94719 Å for isolated Li+(EC)4 clusters. The Li+…O(EC)=C(EC) bond angle is calculated to be 162º.19 More recent DFT calculation employing the B3LYP/6-311+G(d) level of theory indicated that Li+…O(TFSA-) distance in Li+(TFSA-)2 cluster is 1.951 Å.13 The C1 (cis) conformer of TFSA- is found to be more preferred to the C2 (trans) conformer in the first coordination shell of Li+.13 Information on the environmental structure of Li+ in the liquid 4



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state has been reported by means of MD simulation studies. Soetens et al. indicated that the solvation number of Li+ is four in the system involving one LiBF4 and 214 EC molecules.20 The nearest neighbor Li+…O(EC) distance has been determined to 1.78 Å.20 The most probable value of the bond angle, Li+…O(EC)=C(EC), is estimated to be 160º.18 Recent MD simulation study by Borodin and Smith has indicated that Li+ is surrounded by ca. 3 oxygen atoms of EC and ca. 1 oxygen atom of TFSA- from simulations for both systems involving 20 LiTFSA and 400 EC and 20 LiTFSA and 200 EC molecules.21 The nearest neighbor Li+…O(EC) distance is determined to be 1.95 Å21 which is significantly longer than that previous study by Soetens et al. The average bond angle, Li+…O(EC)=C(EC), is obtained to 140º.21 First-principle MD study on a system containing one LiPF6 and 63 EC molecules by Ong et al. has shown that the nearest neighbor Li+…O(EC) distance is 1.9 Å with the Li+…O(EC)=C(EC) bond angle of ca. 140º,22 which agrees well with those predicted by classical MD study by Borodin and Smith, however, direct experimental determination of solvation structure of Li+ in EC solutions in the liquid state has not yet been reported at present. Recently, the coordination structure of the EC molecules around Li+ in crystalline state has widely been studied by Henderson et al.11,12,23 In the crystalline (EC)3.LiTFSA and (EC).LiClO4, Li+ is coordinated with 3 EC molecules and 1 anion, the nearest neighbor Li+…O(EC) distance is reported to 1.933-1.93611 and 1.893-1.939

Å23

for

(EC)3.LiTFSA

and

(EC).LiClO4,

respectively.

The

bond

angle,

∠Li+…O(EC)=C(EC), was determined to be 137-15111 and 138-143°,23 for (EC)3.LiTFSA and (EC).LiClO4, respectively. These results indicate that the coordination structure of Li+ in the 5


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crystalline state might be very similar to that in the solution. Neutron diffraction with 6Li/7Li isotopically substitution method is one of the most effective experimental techniques to obtain direct information on environmental structure around Li+ in the solution.24 Main difficulty in applying the 6Li/7Li substitution method arises from extremely large absorption cross section of 6Li nucleus (940 barn).25 Since the absorption cross section of atom is proportional to the neutron wavelength, time-of-flight neutron diffraction employing short incident wavelength with small scattering angles is preferable. Moreover, the inelasticity effect is expected to be small in these experimental conditions. Another difficulty emerges from relatively small atomic fraction of Li in the nonaqueous solutions because of larger number of atoms involved in the solvent molecule. To overcome these difficulties, the use of high-performance neutron spectrometer with next-generation high-intensity pulsed neutron source is necessary. In this work, to obtain further insight into the local structure of the Li+ in the liquid state, we describe results of neutron diffraction measurements on 6Li/7Li substituted concentrated 10 and 25 mol% *LiTFSA-ECd4 solutions to yield direct information on the structure of both the solvated Li+ and Li+…TFSA- ion pair formed in highly concentrated EC solutions. Neutron diffraction was carried out using high-performance total scattering spectrometer installed at the high-intensity pulsed neutron source in the J-PARC, Japan. Furthermore, the low-frequency isotropic Raman spectral measurements have been conducted to obtain information on the intermolecular vibrational modes of both the solvated Li+ and Li+…TFSA- ion pair, which may support data analysis of neutron diffraction 6



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data. The low-frequency isotropic Raman spectra can extract intermolecular vibrational modes with higher symmetry out of a number of other intermolecular modes involved in the solution.26-28

EXPERIMENTAL SECTION Materials. 6Li-enriched LiTFSA was prepared by reacting 6Li2CO3 (95.5 % 6Li) with aqueous HTFSA solution. The dehydration of the product solution was achieved by heating at 80 ºC under vacuum for more than a day. The weighted amounts of anhydrous 6Li-enriched and natural (92.5% 7

Li) *LiTFSA were dissolved into anhydrous EC-d4 (98% D) to prepare 10 and 25 mol% *LiTFSA

solutions with different 6Li/7Li isotopic compositions and used for neutron diffraction measurements. Natural abundance LiTFSA-EC solutions were employed for Raman scattering measurements. Raman Scattering Measurements. The sample solution was filtered by 45µm Teflon® Millipore filter before introducing in a Pyrex Raman cell (10 × 10 mm, 40 mmH) to remove dust particles. Raman spectra was recorded at 25 ºC in the frequency range of 30 ≤ ν ≤ 1000 cm-1 using a JASCO NR-1100 double grating monochromator with 532 nm excitation line of Spectra-Physics EXLSR-532-300C-KE laser operated at 300 mW. Raman scattering intensities were measured at 1 cm-1 intervals in both the parallel and perpendicular polarization modes with a spectral slit width of 5 cm-1. Observed parallel and perpendicular spectra were converted to the reduced intensity, which is proportional to the square of the polarizability change during the vibration.29-31

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Icorrected(ν) = (ν0 - ν)-4ν [1 – exp(-hν/kT)]Iobs(ν)

(1)

where, ν and ν0 denote the Stokes Raman shift and wavenumber of the incident light, respectively. h and k stand for the Planck constant and the Boltzmann constant, respectively. The absolute temperature is denoted by T. The isotropic Raman intensity was obtained by

Iiso(ν) = I//(ν) – 4/3τI⊥(ν)

(2)

where, I//(ν) and I⊥(ν) are the corrected parallel and perpendicular intensities, respectively. The calibration factor, τ, was carefully determined through measurements of ν1, ν2, and ν4 bands of liquid CCl4. Since the corrected isotropic Raman spectra only involve contributions from the square of the diagonal components in the polarizability tensor for Raman scattering,29 difficulties arising from band overlapping with depolarized components can be significantly reduced. The peak decomposition of the observed isotropic intensity was performed using the least-squares fitting procedures by the SALS program.32 The Voigt peak shape function33,34 was employed with straight background. In the present analysis, the Gaussian width parameter in the Voigt function was fixed at the optical resolution of the spectrometer (5 cm-1). The ratio of integrated intensities observed for “Li+-bound” TFSA-, IbTFSA, and “free” TFSA-, IfTFSA, can be written as the following equation employing the mole fraction of the “Li+-bound” 8



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TFSA-, fbTFSA,

IbTFSA/IfTFSA = (JbTFSA/JfTFSA)×[fbTFSA/(1 - fbTFSA)]

(3)

where, JbTFSA and JfTFSA denote the Raman scattering coefficients of the observed vibrational mode for the “bound” and “free” TFSA-, respectively. In the present analysis, vibrational peak appearing at 740 cm-1 was used for evaluation of fbTFSA. The mole fraction of the EC molecule “bound” with Li+, fbEC, can be estimated from the ratio of Raman integrated intensities observed for “Li+-bound” and “free” EC bands, IbEC and IfEC, respectively.

IbEC/IfEC = (JbEC/JfEC)×[fbEC/(1 - fbEC)]

(4)

where, JbEC and JfEC denote the Raman scattering coefficients for “Li+-bound” and “free” EC molecules, respectively. The average coordination number of EC molecule around Li+, nLiO, is evaluated by

nLiO = (1 – x) fbEC /x

(5)

9


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where, x is the mole fraction of LiTFSA. In the present study, the ring-breathing vibrational band of EC molecule9 located at ν ≈ 900 cm-1 was analyzed to evaluate the nLiO.

Neutron Diffraction Measurements. The sample solution was introduced into thin walled cylindrical vanadium cell (10.0 mmφ in inner diameter and 0.1 mm in wall thickness) and sealed by an indium seal. The sample parameters are listed in Table S1. Neutron diffraction measurements were carried out at 25 ºC using the NOVA total scattering spectrometer35 installed at BL21 of the MLF pulsed neutron source in J-PARC, Tokai, Japan. The incident beam power of the proton accelerator was 150 kW. Scattered neutron (neutron waveband of 0.1 ≤ λ ≤ 8.7 Å) were detected by 900 of 20 atm 3He position sensitive proportional counters (1/2 in.

φ, 800 mm in active length with 5 mm in positional resolution) installed at 20º (13.1-27.9º), 45º (33.6-54.9º), 90º (72.7-107.4º), and back scattering (136.5-169.0º) detector banks. The data accumulation time was ca. 4 h for each sample. Measurements were made in advance for 8.1 mmφ in diameter vanadium rod, empty cell, and instrumental background. Observed scattering intensities for sample solutions were corrected for instrumental background, absorption of sample and cell,36 and multiple37 and incoherent scatterings. The coherent scattering lengths, scattering and absorption cross sections for the constituent nuclei were referred to those tabulated by Sears.25 The wavelength dependence of the total scattering cross sections for H and D nuclei was estimated from the observed total cross sections for liquid H2O and D2O, respectively.38 10



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The inelasticity correction was adopted using the self-scattering intensity observed for the null-H2O.39

RESULTS AND DISCUSSION Raman Spectra.

Concentration dependence of low-frequency isotropic Raman spectra observed

for (LiTFSA)x(EC)1-x, x = 0.1, 0.15, 0.2, and 0.25, is indicated in Figure 1. A peak appearing at ν ≈ 120 cm-1 is attributable to SNS twisting mode of TFSA-.40 Intramolecular CF wagging combined with SO2 wagging vibrational modes of TFSA- is observed at ν ≈ 280 cm-1.40 In the isotropic spectra for 10 mol% LiTFSA solution, another vibrational peak is found at ν ≈ 170 cm-1, which cannot be assigned to any intramolecular vibrational mode of TFSA-

40

nor EC.9 This peak is therefore

attributed to intermolecular vibrational mode with higher symmetry. Symmetrical stretching modes of solvated Li+ in aqueous solutions has been observed at ν ≈ 170

27

and 250 cm-1.27,41 In the

methanolic LiBr solutions, symmetrical stretching mode of solvated Li+, Li+(CH3OH)4, has been reported in the isotropic spectra at ν ≈ 150 cm-1.42 Symmetrical stretching vibrational mode of solvated Li+ in the LiCl-tetrahydrofuran (THF) solution has been found at ν = 184 cm-1.28 The present vibrational peak at ν ≈ 170 cm-1 may be attributable to symmetrical stretching mode of solvated Li+, Li+(EC)n. The solvation number, n, will be determined from the neutron diffraction data which is described in the latter section. In the isotropic spectra for highly concentrated 25 mol% LiTFSA solution, additional broadened peak can be observed at around ν ≈ 220 cm-1. Corresponding 11


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vibrational peak can be identified in the spectra for 20 mol% LiTFSA solution. No intramolecular vibrational mode is present for TFSA- and EC molecules in this frequency region of isotropic spectra. Therefore, this peak may be assigned to symmetrical stretching vibrational mode of the solvated ion pair, Li+TFSA-(EC)m. Preliminary analysis has shown that at least four Voigt peaks are necessary to reproduce the present isotropic Raman spectra in the range of 60 ≤ ν ≤ 300 cm-1. Then, peak decomposition analyses have been carried out employing four Voigt functions with a straight background. The results of least-squares fitting procedure are summarized in Table S4. The symmetrical stretching mode of solvated ion pair in non-aqueous solutions has been found for concentrated LiBr-methanol (ν = 375~383 cm-1),42 LiBr-acetone (ν = 365~370 cm-1),43 and LiCl-THF (ν = 435 cm-1).28 The present value of 220 cm-1 observed for vibrational frequency of Li+TFSA-(EC)m structural unit is significantly lower, which implies that Li+…TFSA- interaction is weaker than that for Li+…Br- and Li+…Cl-. It is well known that intramolecular vibrational frequencies of both TFSA- and EC molecules are sensitive to their environment, i. e. electrostatic interaction between Li+. The Li+…TFSA- contact ion pair produces higher frequency component of 740 cm-1 peak which is assigned to the combination of SNS symmetrical stretching and CF3 umbrella vibrational mode of TFSA-.40 Indeed, the Li+ bound peak is clearly observed in the present isotropic Raman spectra of frequency region of 690 ≤ ν ≤ 770 cm-1 as shown in Figure 2. In the isotropic spectra for 10 and 15 mol% LiTFSA solutions can successfully be reproduced by three Voigt functions (ring bending mode of “free” and “bound” EC,4 12



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and “free” TFSA-), while, additional peak corresponding to Li+…TFSA- ion pair is found to be necessary for 20 and 25 mol% LiTFSA solutions. In order to reduce uncertainties in peak decomposition analysis, the peak width parameter for the “free” and “Li+-bound” components of TFSA- was assumed to be identical in the fitting procedure. The parameters of peak components determined through the least-squares fitting procedures are indicated in Table S5. The absence of the “Li+-bound” peak in the isotropic spectra for 10 and 15 mol% LiTFSA solutions is consistent with those found in the low-frequency isotropic Raman spectra as mentioned above. If we assume the ratio of Raman scattering coefficients, JbTFSA/JfTFSA, is 0.9 as determined for LiTFSA solutions in EMI+TFSA- ionic liquid,13 the mole fraction of TFSA- bound with Li+ is roughly estimated to 38 ± 4 and 52 ± 7 % for the 20 and 25 mol% LiTFSA solutions, respectively. In the 25 mol% LiTFSA solution, approximately half of TFSA- is bound to Li+. The solvation number of Li+ can be estimated from the ratio of integrated intensity observed for “Li+-bound” and “free” components of the ring-breathing vibrational mode of EC molecule located at ν ≈ 900 cm-1.9 In the present study, the least-square fitting analysis was carried out for observed isotropic Raman spectra in the frequency range of 870 ≤ ν ≤ 930 cm-1. Voigt function was employed for peak shape function. The Lorentz width parameter was assumed to be identical for the “bound” and “free” components. The results are indicated in Figure 3. The peak parameters are summarized in Table S6. According to earlier work by Hyodo and Okabayashi, the ratio of the Raman scattering coefficients for “Li+-bound” and “free” components is determined to be JbEC/JfEC = 1.09 for the 13


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ring-breathing mode of EC molecule.9 The solvation number of Li+ in the 10 mol% LiTFSA solution was evaluated to 3.8 ± 0.3, implying 4-coodinated Li+. In highly concentrated 25 mol% LiTFSA solution, the solvation number decreases to 2.6 ± 0.1, which implies that the contact ion pair, Li+…TFSA- should be formed. The information on the intramolecular conformation of TFSA- can be obtained from intramolecular OSNS scissor combined with CF3 twisting vibrational mode appearing at ν ≈ 400 cm-1.44 Fujii et al. have indicated that the vibrational frequencies arising from C1 (cis) and C2 (trans) conformers fall at ν = 407 and 398 cm-1, respectively.44 Isotropic Raman spectra observed for LiTFSA-EC solutions are depicted in Figure. 4. The vibrational peak seems to be a single component centered at ν ≈ 400 cm-1. In the present study, the least-squares fitting analysis was performed to obtain peak parameters for the Voigt peak-shape function. In the present analysis, observed isotropic spectra can be reproduced by a single peak component. The results of the least-squares fitting analyses are summarized in Table S7. The peak position determined to be 400 cm-1 indicates the C2 conformer of the TFSA- in the present solutions. Neutron Diffraction.

Scattering cross sections for (6LiTFSA)0.25(EC-d4)0.75 solution observed at

20º (2θ = 13.06 ~ 26.85º) and 45º (33.62 ~ 54.90º) detector banks are shown in Figure. S1. In spite of large magnitude of absorption correction, corrected scattering cross sections agree well each other within the statistical uncertainties. This implies that the present data correction and normalization procedures have been performed adequately. The overall normalization factor of the interference 14



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Page 14 of 40

term observed for each sample solution was determined from the least-squares fit of the observed interference term combined with Q-interval of 0.1 Å-1 to the following intramolecular interference term, iintra(Q).

iintra(Q) = xiintra(Q)(for TFSA-) + (1-x)iintra(Q)(for EC-d4)

(6)

where,

iintra(Q) = ΣΣbibjexp(-lij2Q2/2)sin(Qrij)/(Qrij).

(7)

i≠j

The amplitude of intermolecular interference term is known to diminish rapidly at relatively high-Q region (typically Q > 10 Å-1), interference features appearing at higher-Q region can be regarded as the intramolecular interference term. In the present study, the fitting procedure was carried out in the range of 14 ≤ Q ≤ 40 Å-1 as indicated in Figure. 5. The normalization factors for intramolecular interference term were determined to 1.00 ± 0.01 and 1.02 ± 0.01 for 10 mol% 6LiTFSA and nat

LiTFSA solutions, respectively. For 25 mol% 6LiTFSA and

nat

LiTFSA solutions, normalization

factors 1.02 ± 0.09, and 1.03 ± 0.01 were respectively obtained. Closeness of these normalization factors to the unity indicates that the data correction and normalization procedures have been successfully adopted. The first-order difference function, ∆Li(Q), was obtained from numerical difference between 15


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scattering cross sections observed for 6Li-enriched and

nat

Li sample solutions. Since ∆Li(Q)’s

observed for scattering range of 13.06 ≤ 2θ ≤ 54.9º agree well each other within the statistical uncertainties, they were combined in the Q-interval of 0.1 Å-1 and used for subsequent analyses. The ∆Li(Q)’s observed for 10 and 25 mol% *LiTFSA-EC-d4 solutions are indicated in Figure 6. The diffraction peak at Q ≈ 1.5 Å-1 and oscillatory features extending to higher-Q region are obviously observed. The distribution function around Li+, GLi(r), was derived from the Fourier transform of the observed ∆Li(Q) as shown in Figure 7. GLi(r) for the 10 mol% *LiTFSA solution exhibits a sharp first peak located at r ≈ 2 Å, which is attributable to the nearest neighbor Li+…O(EC) interaction. Structural features appearing 3 < r < 6 Å in the present GLi(r) should involve non-bonding contribution from the other atoms within EC molecules in the first solvation shell of Li+. Structural parameters concerning the first solvation shell of Li+ were determined form the least-squares fitting analysis of the observed ∆Li(Q). In the fitting procedure, contribution from the first neighbor interactions between Li+ and EC was taken into account in the model function. The nearest neighbor Li+…TFSA- contribution was also involved in the model function to consider possibility of the formation of Li+…TFSA- ion pair. The conformation of TFSA- was fixed to C2 (trans) form as determined from isotropic Raman spectra mentioned in the previous section. The result of the fit is represented in Figure 6. The observed ∆Li(Q) is successfully reproduced by the model function in the whole Q-range employed in the fitting procedure. An agreement between observed and calculated GLi(r) is also satisfactory. Structural parameters obtained 16



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are summarized in Table 1. The present nearest neighbor Li+…O(EC) distance for the 10 mol% LiTFSA solution, 1.90 ± 0.01 Å is in good agreement with that predicted by ab initio molecular dynamics study (rLiO = 1.9 Å),22 and that reported for the crystalline (EC)3.LiTFSA, 1.933-1.936 Å,11 while, significantly shorter than that observed in 10 mol% LiPF6-PC (PC: propylene carbonate, rLiO = 2.04 ± 0.04 Å)45 and that in 9.6 mol% LiPF6-DMC (DMC: dimethyl carbonate, rLiO = 2.08 ± 0.02 Å).46 The present rLiO value is slightly shorter than that reported in aqueous (1.94 - 2.00 Å)47-59 and methanolic solutions (1.93 – 1.97 Å).42,60 The shorter rLiO distance in the EC solution implies the interaction between Li+ and oxygen atom of the EC molecule is stronger than that in the PC and DMC solutions. Li+ is surrounded by 3.8 ± 0.2 oxygen atoms of EC molecules within the first solvation shell. This nLiO value agrees well with that obtained from the present isotropic Raman spectra, 3.8 ± 0.3. Although nLiO value for the nearest neighbor Li+…TFSA- is determined to 3.2 ± 0.7 for the 10 mol% LiTFSA solution, the shortest Li+…O(TFSA-) distance evaluated from optimized dx, dy and dz values exceeds 4.3 Å implying that no TFSA- is involved in the first solvation sphere of Li+. A broadened distribution of TFSA- around Li+ is located at around r ≈ 4.5 Å. The bond angle, ∠Li+…O(EC)=C(EC), was determined to be 141 ± 1º. The value is in good agreement with that predicted from ab initio MD study22 and that reported for the crystalline (EC)3.LiTFSA, 138-147°.11 The present bond angle agrees well with the value, 138 ± 2º reported for 10 mol% LiPF6-PC-d6 solutions,45 implying the similarity of packing geometry of EC and PC molecules in the first solvation shell of Li+. 17


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The difference function, ∆Li(Q), and corresponding distribution function, GLi(r), observed for 25 mol% LiTFSA-EC solution are shown in Figures 6 and 7, respectively. The first peak at r ~ 2 Å in the present GLi(r) seems much broadened compared with that observed for 10 mol% LiTFSA-EC solution. This implies that the first peak involves both contributions from Li+…O(EC) and Li+…O(TFSA-) interactions. Structural parameters were obtained through the least-squares fitting analysis of the observed ∆Li(Q). Satisfactory fit was obtained in the range of 0.1 ≤ Q ≤ 20 Å-1. The Fourier transform of the model function successfully reproduces major structural features in the observed GLi(r). Results of the least-squares fitting are summarized in Table 1. The nearest neighbor Li+…O(EC) distance and coordination number are determined to be 1.853 ± 0.006 Å and 2.87 ± 0.02, respectively. The Li+…O(EC) distance for 25 mol% LiTFSA-EC solution is slightly shorter than that observed in the 10 mol% LiTFSA-EC solution. The coordination number of EC molecules around Li+ obtained from the present neutron diffraction data agrees well with the value, 2.6 ± 0.1, estimated from peak decomposition analysis of ring-breathing vibrational band of EC molecule observed in the isotropic Raman spectra. The nearest neighbor Li+…O(TFSA-) distances are evaluated to be 2.18 ± 0.01 and 1.98 ± 0.01Å, respectively. These values agree well with Li+…O(TFSA-) distances reported for dilithium (N-mrthyl-N-butylpyrrolidinium)tris(bis(triflourosulfonyl)imide in the crystalline state (1.911 – 2.163 Å)61, where Li+ is 5-coordinated with neighboring TFSA anions.

In the 25 mol%

LiTFSA solution, the contact ion pair, Li+…TFSA- is formed and TFSA- coordinates in a bidentate manner in the first solvation shell of Li+. The bidentate coordination of TFSA- to Li+ has been 18



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reported in LiTFSA solutions in EMI+TFSA- ionic liquid.13 The present value of the Li+…O(EC)=C(EC) bond angle, 148.7 ± 0.5º is slightly larger than that observed in 10 mol% LiTFSA-EC solution. The conductivity of EC and PC solutions of various Li salts exhibits the maximum at the salt concentration of ca. 1M, corresponding to ca. 10 mol% Li salt.3, 62, 63 The present neutron diffraction results indicates that the formation of the Li+…anion ion pair may be suppressed up to the solute concentration up to ca. 10 mol% and lower conductivity in highly concentrated solutions should be caused from the formation of Li+…anion contact ion pair.

CONCLUSIONS Information on the microscopic structure of the first solvation shell of Li+ in concentrated LiTFSA-EC solutions was successfully obtained from isotropic Raman spectra and neutron diffraction measurements. It has been revealed that Li+ is fully solvated by ca. 4 EC molecules with the nearest neighbor Li+…O(EC) distance of 1.90 ± 0.01 Å in the 10 mol% LiTFSA-EC solution. The bond angle, ∠Li+…O(EC)=C(EC) is determined to be 141 ± 1º. On the other hand, in the highly concentrated 25 mol% LiTFSA-EC solution, on the average, 3 EC and 1.5 TFSA- are involved in the first solvation shell of Li+ with the Li+…O(EC) distance of 1.853 ± 0.006 Å, which is significantly shorter than that reported in concentrated aqueous and alcoholic solutions, indicating stronger interaction between Li+ and EC molecule in the first solvation shell of Li+. In the 25 mol% LiTFSA 19


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solution, TFSA- exhibit bidentate coordination to form Li+…TFSA- contact ion pair.

ACKNOWLEDGEMENTS The authors thank members of the NOVA group for their help during neutron diffraction measurements. Neutron scattering experiments were approved by the Neutron Scattering Program Advisory Committee of IMSS, KEK (Proposal No. 2014S09 and No. 2015A0121). All calculations were carried out at Yamagata University Networking and Computing Service Center. This work was partially supported by Grant-in-Aid Scientific Research (C) (Nos. 2639110 and 16K05508), from the Ministry of Education, Culture, Sports, and Technology, Japan. This study was supported in part by the Advanced Low Carbon Technology Research and Development Program (ALCA-SPRING) of the Japan Science and Technology Agency (JST).

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Figure Captions Figure 1. Low-frequency isotropic Raman spectra observed for (LiTFSA)x(EC)1-x, x = 0.1, 0.15, 0.2 and 0.25 (dots). Calculated spectra using four Voigt functions are indicated by black line. Contributions from symmetrical stretching vibrational modes of Li+(EC)n and Li+TFSA-(EC)m units are denoted by blue and red lines, respectively.

Figure 2. Isotropic Raman spectra observed for (LiTFSA)x(EC)1-x, x = 0.1, 0.15, 0.2 and 0.25 (dots) in the range of 690 ≤ ν ≤ 770 cm-1. Peak components of the ring-bending mode of EC are denoted by green lines. The SNS symmetrical stretching mode combined with CF3 umbrella vibrational mode of “free” and “bound” TFSA- is indicated by red and blue lines, respectively.

Figure 3. Isotropic Raman spectra observed for (LiTFSA)x(EC)1-x, x = 0.1, 0.15, 0.2 and 0.25 (dots) in the range of 870 ≤ ν ≤ 930 cm-1. Peak components from the “free” and “bound” EC are denoted by red and blue lines, respectively.

Figure 4. Isotropic Raman spectra observed for (LiTFSA)x(EC)1-x, x = 0.1, 0.15, 0.2 and 0.25 (dots) in the range of 370 ≤ ν ≤ 460 cm-1. Calculated Voigt peak functions are denoted by red lines.

Figure 5. Total interference term observed for 6LiTFSA- and natLiTFSA-ECd4 solutions are indicated 30



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by red and blue dots, respectively. The best-fit of the calculated iintra(Q) is shown by black solid line. The residual functions, δ(Q), are indicated below.

Figure 6. ∆Li(Q) observed for (LiTFSA)x(EC-d4)1-x, x = 0.1 and 0.25 (dots), and the best-fit of the calculated ∆Limodel(Q) (solid line). The residual functions, δ(Q), are indicated below.

Figure 7. Distribution function around Li+, GLi(r), observed for (LiTFSA)x(EC-d4)1-x, x = 0.1 and 0.25 (dots). Fourier transform of the calculated ∆Limodel(Q) is shown by black line. The short-range Li+…EC and Li+…TFSA- contributions are denoted by red and blue lines, respectively.

31


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Table 1. Results of the Least-Squares Fitting Analyses of Neutron First-Order Difference Functions Observed for 10 and 25 mol% *LiTFSA-EC-d4 Solutionsa interaction +…

Li

+…

Li

EC

TFSA

-

Long-range

a

parameter

(LiFSA)0.1(EC-d4)0.9

(LiFSA)0.25(EC-d4)0.75

rLiO/Å lLiO/Å l’ /Å

1.90(1) 0.07(2) 0.09(3)

1.853(6) 0.116(4) 0.016(5)

nLiO α/º

3.8(2) 141(1)

2.87(2) 148.7(5)

β/º

38(7)

38(1)

dx/Å dy/Å dz/Å lLiO/Å

7.1(3)

1.617(8)

-0.5(7) -1.1(8) 0.04(3)

-1.452(5) 0.19(3) 0.082(7)

l’ /Å nLiO

0.4(1) 3.2(7)

0.01(1) 1.51(1)

r0/Å

6.1(3)

5.918(7)

l0/Å

1.9(1)

2.10(1)

Estimated standard deviations are given in parentheses.

32



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Figure 1. Maeda et al.

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Table of Contents

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Local Structure of Li+ in Concentrated Ethylene Carbonate Solutions

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