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Solvation Structure of Li in Methanol and 2- Propanol Solutions Studied by ATR-IR and Neutron Diffraction With Li/Li Isotopic Substitution Methods 6

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Yasuo Kameda, Koichi Sato, Ryo Hasebe, Yuko Amo, Takeshi Usuki, Yasuhiro Umebayashi, Kazutaka Ikeda, and Toshiya Otomo J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.9b03477 • Publication Date (Web): 16 May 2019 Downloaded from http://pubs.acs.org on May 18, 2019

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The Journal of Physical Chemistry

Solvation Structure of Li+ in Methanol and 2propanol Solutions Studied by ATR-IR and Neutron Diffraction with 6Li/7Li Isotopic Substitution Methods

Yasuo Kameda,*,† Koichi Sato,† Ryo Hasebe, † Yuko Amo,† Takeshi Usuki,† Yasuhiro Umebayashi,‡ Kazutaka Ikeda,§ and Toshiya Otomo§

†Department

of Material and Biological Chemistry, Faculty of Science, Yamagata University,

Yamagata, Yamagata 990-8560, Japan ‡Graduate

School of Science and Technology, Niigata University, 8050 Ikarashi, 2-no-cho,

Nishi-ku, Niigata City, 950-2181, Japan §Institute

of Material Structure Science, KEK, Tsukuba, Ibaraki 305-080, Japan

ACS Paragon Plus Environment

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ABSTRACT: Neutron diffraction measurements have been carried out for 10 mol% LiTFSA (TFSA: bis(trifluoromethylsulfonil)amide) solutions in methanol-d4 and 2-propanol-d8 to obtain information on the solvation structure of Li+. Detailed coordination structure of solvent molecules within the first solvation shell of Li+ was determined through the least squares fitting analysis of the difference function between normalized scattering cross sections observed for 6Li/7Li

isotopically substituted sample solutions. The nearest neighbor Li+…O distance and

coordination number determined for 10 mol% LiTFSA-methanol-d4 solution are rLiO = 1.98 ± 0.02 Å, and nLiO = 3.8 ± 0.6, respectively. In the 2-propanol-d8 solution, it has been revealed that 2-propanol-d8 molecules within the first solvation shell of Li+ take at least two different coordination geometries with the intermolecular nearest neighbor Li+…O distance of rLiO = 1.93 ± 0.04 Å. Li+…O coordination number, nLiO = 3.3 ± 0.3, is determined. Ion pair formation in the LiTFSA-methanol and -2-propanol solutions has been investigated by ATR-IR spectroscopic method. Mole fractions of free, Li+-bound, and aggregated TFSA- are derived from the peak deconvolution analysis of vibrational bands observed for TFSA-.

INTRODUCTION Solvation structure of lithium ion in various organic solvents plays an important role in extensive fields of both fundamental sciences and industrial processes. In recent years, considerable attention has been received for organic solutions involving lithium salts as electrolyte solutions for next-generation high performance lithium ion batteries.1-4 Although coordination structure of solvated lithium ion with organic ligands in crystalline state has widely been investigated,5-8 limited number of structural studies concerning the solvation structure of


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lithium ion in the liquid state have been reported. Recently, theoretical9-13 and spectroscopic studies14-15 have been reported for concentrated organic solutions involving Li+, however, direct experimental information on the solvation structure of Li+ derived from diffraction measurements has not yet extensively been carried out. Solvation structure of Li+ in highly concentrated 25 mol% LiBr and 33 mol% LiI methanolic solutions has been investigated by neutron diffraction with

6Li/7Li

isotopic substitution

method.16,17 In these solutions, Li+ takes four-fold coordination with oxygen atoms of methanol molecules and with Br- or I- anions to make the contact ion pair. The nearest neighbor distance between Li+ and alcoholic oxygen and coordination number of O atom around the Li+ in the LiBr and LiI solutions have been determined to be rLiO = 1.97 ± 0.06 Å, nLiO = 3.0 ± 0.5, and rLiO = 1.93 ± 0.06 Å, nLiO = 1.8 ± 0.5, respectively. On the other hand, X-ray diffraction studies on more diluted LiCl- and LiI-methanol solutions have indicated that the first solvation shell of Li+ involves ca. 6 methanol molecules,18 which is much larger than that predicted by recent theoretical studies.19,20 The coordination structure of Li+ with carbonyl oxygen atoms in 6 mol % LiBr-acetone solutions has been studied by means of neutron diffraction with 6Li/7Li isotopic substitution method.21 The first solvation shell of Li+ involves 3.2 oxygen atoms of acetone molecule with interatomic distance of 2.24 ± 0.01 Å. Shorter interatomic distances between carbonyl oxygen atoms and Li+ have been reported for 10 mol% propylene carbonate (PC), rLiO = 2.04 ± 0.04 Å,22 ethylene carbonate (EC), rLiO = 1.90 ± 0.01 Å (10 mol% LiTFSA solution) and rLiO = 1.853 ± 0.006 Å (25 mol% LiTFSA solution),23 and dimethyl carbonate (DMC), rLiO = 2.08 ± 0.02 Å,24 solutions. Four-fold coordination of Li+ has been found in these solutions. Coordination structure of Li+ with ether oxygen atoms has been reported for tetrahydrofuran (THF)25 and tetraglyme


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(G4)26 solutions. In the 4 mol% LiCl-tetrahydrofuran solution, the first solvation shell of Li+ consists of 2.7 THF with the Li+…O(THF) distance of 2.21 ± 0.01Å.25 The Li+…O(THF) distance of 2.07 ± 0.01Å is obtained for the 10 mol% LiClO4-THF solution.25 The contact ion pair formation is found in these THF solutions. In the equimolar LiTFSA-G4 solution, Li+ takes a deficient five-coordination with ether-oxygen atoms of the G4 molecule with the Li+…O(G4) distances of 1.93 – 2.24 Å.26 The nearest neighbor Li+…O distance in poly(ethylene oxide) determined by neutron diffraction has been reported to be 2.07-2.10 Å.27-29 The solvation structure of Li+ in dimethyl sulfoxide (DMSO) solutions has been investigated by both X-ray and neutron diffraction experiments.30 The nearest neighbor Li+…O(DMSO) distance has been reported to be 2.02 ± 0.02Å (X-ray) and 2.01 ± 0.02Å (neutron), respectively. The Li+…O distance reported for these organic solutions exhibits much wider variation ranging from 1.90 to 2.24 Å than that observed in aqueous solutions (rLiO = 1.94 - 2.01 Å).31,32 These results suggest that a wide variation of the coordination structure occurs in the interaction between Li+ and oxygen atoms of the solvent molecule in organic solutions. The solvation structure of Li+ should strongly be affected by the formation of contact ion pair with counter anions as well as the steric hindrance occurring among solvent molecules in the first solvation shell. In the present paper, we describe the results of neutron diffraction measurements on 6Li/7Li isotopically substituted 10 mol% *LiTFSA solutions in methanol-d4 and 2-propanol-d8 solutions to obtain direct structural insight on the solvation structure of Li+ in these organic solutions. Neutron diffraction measurements were carried out using high-performance total scattering spectrometer installed at the high-intensity pulsed neutron source in the J-PARC, Japan. The observed difference function between normalized scattering cross sections observed for 6Li-


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emriched and natural abundance sample solutions, ΔLi(Q),33 was analyzed by the least squares fitting procedure to obtain detailed geometry of solvent molecules in the first solvation shell of Li+. Attenuated total reflection infrared (ATR-IR) spectra of LiTFSA-methanol and 2-propanol solutions were measured in advance to obtain information on the Li+…TFSA- contact ion pair formation in these solutions.

EXPERIMENTAL SECTION Materials.

6Li

enriched LiTFSA was prepared by reacting 6Li2CO3 (95.5% 6Li) with

aqueous HTFSA solution. The weighted amounts of anhydrous 6Li-enriched and natural (92.5% 7Li)

*LiTFSA were dissolved in anhydrous fully deuterated methanol-d4 (99.5% D) and 2-

propanol-d8 (99.5% D) to prepare 10 mol% LiTFSA solutions with different 6Li/7Li isotopic ratios and used for neutron diffraction measurements. Natural abundance LiTFSA solutions were employed for ATR-IR measurements. ATR-IR Measurements. The ATR-IR spectra for (LiTFSA)x(CH3OH)1-x, x = 0.05, 0.08, 0.10,

0.12,

0.15,

0.20,

0.25,

0.28,

0.30

and

0.33

(natural

abundance),

and

(LiTFSA)x[(CH3)2CHOH]1-x, x = 0.02, 0.05, 0.08, 0.10, 0.12, 0.14, 0.16, 0.18, 0.20, 0.22, 0.25, 0.28, 0.30, 0.32, 0.33, 0.25 and 0.40 (natural abundance) were measured at 25 ºC using JASCO FT-IR-410 spectrometer with ATR-500/M attachment with a Ge-prism (incident angle: 45º). Five-reflections ATR-IR intensities recorded at 1 cm-1 intervals in the range of 400    4000 cm-1 with a resolution of 4 cm-1 were accumulated 100 times to keep sufficient statistical accuracies of observed data.


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The observed ATR absorbance spectra were corrected for wavelength dependence of the penetrating depth of the IR radiation by the following equation. Icorrected() = Iobs()(/0)

(1)

In the present analysis, 0 was set to be 2000 cm-1. For deconvolution analyses of the S-N stretching, C-S stretching, and CF3 bending mode of TFSA- appearing at   740 cm-1,34 and the C-S stretching and CF3 bending mode observed at   790 cm-1,34 the least squares fitting procedures were performed using the SALS program.35 The Voigt peak shape function36,37 was employed with a straight background. In the present analysis, the Gaussian width of the Voigt function was fixed at the resolution of the spectrometer (4 cm-1). Neutron Diffraction Measurements. The sample solution was introduced in a thin-walled cylindrical vanadium cell (10.0 mm in inner diameter and 0.1mm in wall thickness) and sealed by a PTFE O-ring. Sample parameters are listed in Table 1. Neutron diffraction measurements were carried out at 25 ºC using the NOVA total scattering spectrometer39 installed at BL21 of the MLF high intensity pulsed neutron source in J-PARC, Tokai, Japan. The data accumulation time was 3 and 1.5 h for 6Li-enriched and natural samples, respectively. Details of the neutron diffraction measurements and data correction and normalizing procedures have been given elsewhere.31 The first-order difference function, ΔLi(Q),33 is derived from the numerical difference between scattering cross sections observed for two solutions that are identical except for the scattering length of Li. The ΔLi(Q) normalized for stoichiometric units, (*LiTFSA)x(X)1-x, (X: CD3OD or


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(CD3)2CDOD) can be written as linear combination of partial structure factors, aLij(Q), involving correlations form the Li-j pair:

ΔLi(Q) = ΣALij[aLij(Q) -1] + B[aLiLi(Q) -1]

(2)

where ALij = 2cLicj(b6Li – bnatLi)bj and B = cLi2(b6Li2 – bnatLi2). cj denotes the number of atom j involved in the stoichiometric unit. Numerical values of coefficients in Eq. 2, ALij and B, are summarized in Table 2. Since the observed ΔLi(Q) from forward angle detector pixels located at 13.1  2  54.9° agrees well within the statistical uncertainties, they were combined at the Qinterval of 0.1 Å-1 and used for subsequent analyses. The distribution function around Li+, GLi(r), is deduced from the Fourier transform of ΔLi(Q):

Qmax

∫

GLi(r) = 1 + (ΣALij + B)-1(2π2ρ0r)-1 QΔLi(Q) sin(Qr) dQ 0

= [ΣALijgLij(r) + BgLiLi(r)] (ΣALij + B)-1.

(3)

The upper limit of the integral was set to 20 Å-1 in the present study.


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Structural parameters concerning the solvation shell of Li+ were obtained through the leastsquares fitting procedure of the following model function:40-42

ΔLimodel(Q) = Σ2cLinLijbj(b6Li – bnatLi)exp(-lLij2Q2/2)sin(QrLij)/(QrLij) + 4πρ0(ΣALij + B)exp(-l02Q2/2)[Qr0cos(Qr0) – sin(Qr0)]Q-3,

(4)

where, cLi and nLij are the number of Li+ in the stoichiometric unit and the coordination number of the j atom around Li+, respectively. Parameters, lLij and rLij denote the root mean square amplitude and internuclear distance of Li+…j pair, respectively. The long-range parameter, r0, is the distance beyond which continuous distribution of atoms around Li+ can be assumed. The parameter, l0, describes the sharpness of the boundary at r0. Structural parameters, nLij, lLij, rLij, r0, l0, were determined from the least squares fit to the observed ΔLi(Q). The fitting procedure was performed in the range of 0.1  Q  20 Å-1 with the SALS program,35 assuming that statistical uncertainties are distributed uniformly. In the least squares fitting calculation, the Marquardt method was employed. The dynamical bi-weight procedure was used for weight adjustment of the observed data.28 Prior to the fitting analysis, correction for the low-frequency systematic error involved in the observed ΔLi(Q) was adopted.43

RESULTS AND DISCUSSION


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ATR-IR

Spectra.

Concentration

dependence

of

ATR-IR

spectra

observed

for

(LiTFSA)x(CH3OH)1-x is indicated in Figure 1a. According to previous Raman spectroscopic studies, the vibrational peak at   740 cm-1 of TFSA- can be employed to estimate fractions of free and Li+-bonded TFSA- in various solutions.4,23,44-56 In the present ATR-IR spectra, integrated intensities of absorption bands located at   740 and 790 cm-1 both increase almost linearly against molar concentration of the solute, implying that the molar absorption coefficients for free and Li+-bonded TFSA- is approximately identical. In the preliminary analysis, it has been found that the observed peak shape in the whole composition range can successfully be reproduced by the sum of peak components centered at  = 741.5, 745.0, and 747.5 cm-1 for the absorption band at 740 cm-1. The peak shape of the 790 cm-1 band can also be well described by the sum of three components centered at  = 791.25, 796.4, and 801.4 cm-1. Since these peak components are considerably overlapped each other, therefore, peak positions of these components were fixed in the present least squares fitting analyses. The Lorentzian width parameter of the Voigt peak shape function for these peak components was assumed to be identical in the fitting procedure to reduce number of independent parameters. Results of the least squares fitting analyses of the 740 and 790 cm-1 absorption bands are summarized in Tables S1 and S2, respectively. In the preliminary analysis, the fitting procedure has been carried out in the condition with the Lorentzian width parameters being allowed to vary independently. The results for the 740 cm-1 band for the methanol solutions (Table S1 and Figure S1b) indicate very similar molar fraction diagram except for experimental uncertainties as obtained from the fitting procedure with the Lorentzian width parameters being fixed to be identical. Previous Raman study indicated that vibrational frequencies of free- and Li+-bound TFSA- in ethylene carbonate solutions fall at  = 741 and 746 cm-1, respectively.23 In highly concentrated LiTFSA-acetonitrile


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solution, higher frequency Raman component at  = 750 cm-1 has been observed and assigned to TFSA- coordinating to two or more Li+s (aggregate species).4,46 The present peak components observed at  = 741.5, 745.0, and 747.5 cm-1 are then attributed to free, Li+-bound, and aggregate TFSA-, respectively. If we assume the molar absorption coefficient of these species are almost identical, mole fraction of each species can be estimated from the ratio of the peak area of these peak components to the total integrated absorbance of the 740 cm-1 band. Estimated concentration dependence of the mole fractions of the free, Li+-bound and aggregate species of TFSA- is shown in Figure 2a. The mole fraction of Li+-bound TFSA- increases with increasing solute concentration up to x  0.25, while the fraction of the Li+-bound TFSA- decreases with increasing the solute content in the composition range of x > 0.25. Mole fraction of Li+-bound TFSA- at x = 0.1 is roughly estimated to be ca. 0.3. The aggregate species emerges at concentration range of x > 0.25. Approximately half of TFSA- is present as aggregated species at highly concentrated solution, x = 0.33. The similar band decomposition analyses were carried out for the absorption band observed at  = 790 cm-1. Results of the least squares fitting analyses are indicated in Figure 1b. Parameters of the peak components are summarized in Table S2. We tentatively attributed the peak components centered at  = 791.25, 796.4, and 801.4 cm-1 to free, Li+-bound and aggregated TFSA-, respectively. Concentration dependence of mole fraction of these species derived from the deconvolution analyses of the 790 cm-1 band (shown in Figure 2b) is remarkably similar to that estimated from the 740 cm-1 band. This implies that the above assignment of peak components is valid. The mole fraction of Li+-bound TFSA- at x = 0.1 is evaluated to be ca. 0.3, which agrees well with that obtained from the analysis of the 740 cm-1 band as described above. This indicates that the average coordination number of TFSA- around Li+ is ca. 0.3 in the 10 mol% LiTFSA-methanol solution.


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Figures 3a and 3b represent concentration dependence of ATR-IR band at  = 740 and 790 cm-1 observed for (LiTFSA)x[(CH)2CHOH]1-x solutions, respectively. The same fitting procedure as used for band deconvolution analyses for LiTFSA-methanol solutions, was adopted for these absorption bands. A significant peak observed at  = 816 cm-1 is attributable to the C-C and C-O stretching vibrational mode of 2-propanol.47 In the band fitting analysis of 790 cm-1 band, the peak position, Lorentzian width and peak height parameters of the  = 816 cm-1 peak component were treated as independent parameters. Mole fractions of free, Li+-bound, and aggregated TFSA-, were estimated from the ratio to the integrated intensities of these peak components. Results of the least squares fitting analyses for the 740 and 790 cm-1 absorption bands are summarized in Tables S3 and S4, respectively. Figures 4a and 4b indicate the mole fraction of these species estimated from the band deconvolution analyses of absorption bands observed at  = 740 and 790 cm-1 bands, respectively. Mole fractions of free, Li+-bonded and aggregate forms of TFSA- deduced from analyses of the 740 and 790 cm-1 absorption bands agree well with each other. The mole fraction of Li+-bound TFSA- at 10 mol% LiTFSA-2-propanol solution is estimated to be 0~0.1. Neutron Diffraction. Total interference terms observed for 10 mol% *LiTFSA-methanol-d4 and *LiTFSA-2-propanol-d8 are indicated in Figure 5. Prior to take the difference between 6Li and

natLi

sample solutions, the overall normalization factor, , of each solution was determined

by the least squares fitting procedure of observed total interference term combined with Qinterval of 0.1 Å-1 to the following intramolecular interference term, iintra(Q).

iintra(Q) =  [xiintra(Q)(for TFSA-) + (1-x)iintra(Q)(for merthanol-d4 or 2-propanol-d8)]

(5)


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

(6)

ij

Intramolecular parameters, lij and rij were taken from the literature determined from gas phase electron diffraction,48 and rotational spectroscopic49 studies for methanol, and from gas phase electron diffraction50,51 works for 2-propanol, and from single crystal X-ray diffraction52,53 studies for TFSA-. The amplitude of the 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 procedures were carried out in the range of 12 < Q < 40 Å-1 and 10 < Q < 40 Å-1 for 10 mol% *LiTFSA-methanold4 and 2-propanol-d8 solutions, respectively. Results of the least squares fitting analyses are indicated in Figure 5. The normalization factors, , for intramolecular interference term were determined to be 1.02  0.01 and 1.03  0.02 for 10 mol% 6LiTFSA- and natLiTFSA-methanol-d4 solutions, respectively. For 10 mol%

6LiTFSA-

and

natLiTFSA-2-propanol-d 8

solutions,

normalization factors,  = 0.978  0.007 and 0.981  0.007 were respectively obtained. Closeness of the normalization factor to the unity indicates that the data correction and normalization procedures have been adequately adopted. The first-order difference function, ΔLi(Q), was obtained from numerical difference between scattering cross sections observed for 6Li and

natLi

solutions. Because ΔLi(Q)’s observed for


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scattering angles of 13.06  2  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 ΔLi(Q)s observed for 10 mol% *LiTFSA-methanol-d4 and *LiTFSA-2-propanol-d8 solutions are shown in Figure 6. The negative diffraction peak at Q  1.5 Å-1 and oscillatory features extending to higher Q region are obviously observed. The distribution function, GLi(r), around Li+ was derived from the Fourier transform of the observed ΔLi(Q), as depicted in Figure 7. GLi(r) for both 10mol% *LiTFSA-methanol-d4 and -2-propanol-d8 solutions indicates a sharp peak located at r  2 Å, which is attributable to the nearest neighbor Li+…O interaction. Structural features appearing at 2.5 < r < 5 Å should involve non-bonding contributions from the other atoms within both alcohol molecules and possibly TFSA- belonging to the first solvation sphere of Li+. Structural parameters concerning the first solvation shell of Li+ were determined from the least squares fitting analysis of the observed ΔLi(Q). In the fitting procedure, contribution from the first neighbor interaction between Li+ and methanol (or 2-propanol) molecules was taken into account in the model function. The model function ΔLimodel(Q) was evaluated on the basis of the following assumptions. a) For the nearest neighbor Li+…methanol (or 2-propanol) interaction, structural parameters, rLiO, lLiO, nLiO, the bond angle  (= Li+…O-C), and the dihedral angle between plane involving atoms Li+…O-C and the plane involving Li+…O-D atoms, β, are treated as independent parameters. In the present analysis, the cartesian coordinates of atoms within the methanol (or 2-propanol) molecule are fixed so that the oxygen atom is placed at the origin, and the O-C bond goes along the x-axis. The alcoholic hydrogen atom was placed within the x-y plane. The x, y, and z coordinates of Li+ are expressed as Li+(x, y, z) = (rLiOcos(-), rLiOsin(-)cos, rLiOsin(-)sin). The root-


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mean-square displacements for non-bonding interactions between Li+…methanol (or 2-propanol) molecule except for the first nearest Li+…O pair, were approximated through the following equation:40

lLij = (lLiO + l*)  (rLij/rLiO)1/2

(7)

The modification factor for the r.m.s displacement, l*, was introduced to take into account for the geometrical fluctuation of the Li+…methanol interaction within the first solvation shell. b) Structural parameters for the nearest neighbor Li+…TFSA- interaction, rLiO, lLiO, nLiO, the bond angle  (= Li+…O-S), and the dihedral angle between plane involving atoms Li+…O-S and the plane involving O-S-O’ atoms, β, were treated as independent parameters. The r.m.s. displacement for Li+…O pair and the modification factor for the r.m.s. displacement were treated as independent parameters as the same manner as the Li+…methanol interaction. c) Structural parameters for long-range random contribution of atoms, r0 and l0 were allowed to vary independently. In the preliminary analysis, it has been found that the double peaks located at r  2.5 and 3 Å in the GLi(r) for the 10 mol% *LiTFSA-2-propanol solution, cannot be reproduced by a single type of geometry of the nearest neighbor Li+…2-propanol interaction. Therefore, two different geometries of the nearest neighbor Li+…2-propanol molecules within the first solvation shell of Li+ were introduced in the present fitting procedure. Structural parameters, rLiO, lLiO and l*, for two types of the nearest neighbor Li+…2-propanol interactions, were assumed to be identical in


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the present fitting analysis. On the other hand, the observed ΔLi(Q) and GLi(r) functions for the 10 mol% LiTFSA-methanol solutions can be reproduced satisfactory by a single geometry model for the nearest neighbor Li+…methanol interaction. The result of the fit is represented in Figure 6. The observed ΔLi(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 are summarized in Table 3. The present values of rLiO and  (Li+…O-C) for 10 mol% *LiTFSA-methanol-d4 solution, rLiO = 1.98  0.02 Å and 119  4º, agree well with that reported for highly concentrated 25 mol% LiBr-methanol solution (1.97 ± 0.06 Å,  = 128  12º)17 and slightly longer than that calculated values for Li+(CH3OH)4 cluster (1.936 – 1.957 Å).20 The coordination number nLiO is determined to be 3.8  0.6, implying that Li+ takes four-fold coordination structure. The number of TFSA- in the first solvation shell of Li+ is determined to be 0.28  0.07, in good agreement with the value ca. 0.3 deduced form the ATR-IR data. The nearest neighbor Li+…O(TFSA-) distance is determined to be 2.4  0.4 Å. In the 10 mol% *LiTFSA-2-propanol solution, the nearest neighbor Li+…O distance is determined to be 1.93  0.04 Å, which is slightly shorter than that observed for 10 mol% *LiTFSA-methanol solution. The observed ΔLi(Q) is reproduced satisfactory and the double peaks appearing in the observed GLi(r) at r  2.5 and 3 Å are also well reproduced by the model function employing two geometries of 2-propanol molecules in the first solvation shell of Li+. The total Li+…O coordination number, nLiO = 3.3  0.3, is slightly smaller than that for the methanol solution. Although the nearest neighbor Li+…TFSA- coordination number is quite large (nLiO(TFSA-) = 2.4  0.6), the determined Li+…O (TFSA-) distance (rLiO = 3.7  0.2 Å) indicates


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that no TFSA- is present in the first solvation shell of Li+. The bulky propyl group of 2-propanol molecule might prevent four-fold coordination around Li+. According to the result from the present ATR-IR data, mole fraction of TFSA- bound to Li+ is very small (0 ~ 0.1), which is due to weaker interaction between Li+ and TFSA- in the 10 mol% LiTFSA-2-propanol solution. As shown in the TOC graphic, some voids are present in the first solvation shell of Li+ in the 2propanol solution. More detailed structural information on the first solvation shell will be provided by the 6Li/7Li substitution experiments combined with H/D (alcoholic- and methylhydrogen atoms) substituted sample solutions. These will be future subjects.

CONCLUSIONS Information on the microscopic structure of the first solvation shell of Li+ in 10 mol% LiTFSAmethanol and -2-propanol solutions was successfully obtained from neutron diffraction and ATR-IR spectroscopic methods. It has been revealed that Li+ forms, on the average, Li+(CD3OD)4 in 10 mol% LiTFSA-CD3OD, and Li+[(CD3)2CDOD]3 in 10 mol% LiTFSA(CD3)2CDOD solutions, respectively. The nearest neighbor Li+…O distance in the methanol and 2-propanol solutions was determined to 1.98  0.02 and 1.93  0.04 Å, respectively. The structure of the first coordination sphere of Li+ is strongly affected by the bulkiness of the solvent molecules.

ASSOCIATED CONTENTS Supporting Information


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The supporting information is available free of charge on the ACS Publications website at DOI: 10.1021 acs.jpcb.XXXXXXX. Results of the least squares fitting analyses of IR bands observed for (LiTFSA)x(CH3OH)1-x and (LiTFSA)x(2-PrOH)1-x observed in 720    760 cm-1 and in 775    820 cm-1, Concentration dependence of mole fraction of free, Li+-bound and aggregated TFSA- in methanol solution estimated from ATR-IR bands observed at   740 cm-1 in the fitting condition of a) Lorentzian peak width parameters are kept identical b) the Lorentzian peak widths of peak components are treated as independent parameters. (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Y. Kameda) Tel: +81-23-628-4581

ORCID Yasuo Kameda: 0000-0002-4534-4013 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS


17


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The authors thank members of the NOVA experimental group for their help during neutron diffraction measurements. Neutron diffraction experiments were approved by the Neutron Scattering Program Advisory Committee of IMSS, KEK (Proposal No. 2014S06). All calculations were performed at Yamagata University Networking and Computing Service Center. This work was partially supported by Grant-in-Aid for Scientific Research (B) (No. 18H01994) and (C) (Nos. 26390110 and 16K05508) from the Ministry of Education, Sports, Culture, Science and Technology, Japan.

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Figure Captions Figure 1. Concentration dependence of ATR-IR bands of TFSA- observed at   a) 740 and b) 790 cm-1 in methanol solutions. Peak components corresponding to free, Li+-bound and aggregated species are indicated by blue, red, and green lines, respectively. Figure 2. Concentration dependence of mole fraction of free (red), Li+-bound (blue) and aggregated (green) TFSA- in methanol solution estimated from ATR-IR bands observed at   a) 740 and b) 790 cm-1, respectively. Figure 3. The same notations as Figure 1 except for 2-propanol solutions. Figure 4. The same notations as Figure 2 except for 2-propanol solutions. Figure 5. Total interference term observed for 6LiTFSA- and

natLiTFSA-methanol-d

4

and -2-

propanol-d8 solutions are indicated by red and blue dots, respectively. The best-fit of the calculated iintra(Q) is shown by the black line. The residual functions are indicated below. Figure 6. ΔLi(Q) observed for (*LiTFSA)0.1(methanol-d4)0.9 and (*LiTFSA)0.1(2-propanol-d8)0.9 (dots) and the best fit of the calculated ΔmodelLi(Q) (solid line). The residual functions are indicated below. Figure 7. Distribution function around Li+, GLi(r), observed for (*LiTFSA)0.1(methanol-d4)0.9 and (*LiTFSA)0.1(2-propanol-d8)0.9 (dots). Fourier transform of the calculated ΔmodelLi(Q) is shown by the blue line. The short-range Li+…alcohol and Li+…TFSA- interactions are indicated by brown (or green) and black broken lines, respectively.


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Table 1. Isotopic composition, average neutron scattering length of Li, total cross section and number density of sample solutions scaled in the stoichiometric units, (*LiTFSA)0.1(X)0.9 , (X = methanol-d4, 2-propanol-d8), σt and ρ0, respectively

σt/barnsb

ρ0/Å-3

0.181

82.129

0.011841

92.5

-0.190

36.161

95.5

4.5

0.181

109.135

7.5

92.5

-0.190

63.427

Sample

6Li/%

7Li/%

(6LiTFSA)0.1(methanol-d4)0.9

95.5

4.5

(natLiTFSA)0.1(methanol-d4)0.9

7.5

(6LiTFSA)0.1(2-propanol-d8)0.9 (natLiTFSA)0.1(2-propanol-d8)0.9 a

Taken from Ref. 38.

b

For incident neutron wavelength of 1.0 Å.

bLi/10-12 cma

0.007200

Table 2. Numerical Values of Coefficients ALij and B in Eq. 2 Sample

ALiO

ALiC

ALiD

ALiS

ALiN

ALiF

B

/barns

/barns

/barns

/barns

/barns

/barns

/barns

(*LiTFSA)0.1(methanol-d4)0.9

0.0560

0.0542

0.1768

0.0042

0.0070

0.0252

0.00003

(*LiTFSA)0.1(2-propanol-d8)0.9

0.0560

0.1430

0.3536

0.0042

0.0070

0.0252

0.00003


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Table 3. Results of the Least Squares Fitting Analyses of Neutron First-Order Difference Functions Observed for 10 mol% *LiTFSA- methanol-d4 and 2-propanol-d8 solutionsa (*LiTFSA)0.1(CD3OD)0.9 (*LiTFSA)0.1[(CD3)2CDOD]0.9

parameter rLiO/Å lLiO/Å l*/Å Li+…Solvent(I) /ºc β/ºd nLiO rLiO/Å lLiO/Å l*/Å Li+…Solvent(II) /ºc β/ºd nLiO rLiO/Å lLiO/Å l*/Å Li+…TFSA/ºe β/ºf nLiO Long-range r0/Å l0/Å aEstimated standard deviations are

1.98(2) 0.14(1) 0.06(3) 119(4) -31(8) 3.8(6) 2.4(4) 0.1(3) 0.0(4) 107(19) 24(15) 0.28(7) 4.1(2) 0.9(2) given in parentheses.

1.93(4) 0.12(1) 0.03(2) 105(5) -19(3) 1.0(1) [1.93]b [0.12]b [0.03]b 136(2) 10(9) 2.3(3) 3.7(2) 0.7(15) -0.4(15) 107(5) 71(5) 2.4(6) 5.6(2) 1.6(2) bFixed at parameters for the

Li+…Solvent(I) interaction. cBond angle, Li+…O-C. dDihedral angle between plane involving Li+…O-C atoms and plane involving Li+…O-D atoms. eBond angle, Li+…O-S. fDihedral angle between plane involving Li+…O-S atoms and plane involving O-S-O’ atoms within TFSA-.


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Figure 1

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Figure 4

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Figure 5

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Figure 6. Kameda et al.


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Figure 7. Kameda et al.


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Table of Contents TOC Graphic


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Solvation Structure of Li+ in Methanol ... - ACS Publications

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