Solvation Structure and Dynamics of the Lithium ... - ACS Publications

Article Journal of Physical Solvation The Structure Chemistry C is published by the American and Dynamics of the Chemical Society. 1155 Sixteenth Street N.W., Lithium IonWashington, in Organic DC 20036 Published by by American Subscriber access provided CORNELL Chemical Society. UNIVERSITY LIBRARY Copyright © American Chemical Society.

Carbonate-Based Electrolytes: A TimeThe Journal of Physical Chemistry C is published by the American Dependent Infrared Chemical Society. 1155 Sixteenth Study Street N.W., Spectroscopy Washington, DC 20036

Published by by American Subscriber access provided CORNELL Chemical Society. UNIVERSITY LIBRARY Copyright © American Chemical Society.

Kristen D. Fulfer, and Daniel G Kuroda The Journal of Physical

J. Phys. Chem. C,Chemistry Just Accepted C is published by the 10.1021/ American Manuscript • DOI: Chemical Society. 1155 acs.jpcc.6b08607 • Publication Sixteenth Street N.W., Date (Web): 27 Sep 2016 Washington, DC 20036 Published by by American Subscriber access provided CORNELL Chemical Society. UNIVERSITY LIBRARY Copyright © American Chemical Society.

Downloaded from http:// pubs.acs.org on October 3, 2016 The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Street N.W., Just Accepted Sixteenth Washington, DC 20036 Published by by American Subscriber access provided CORNELL Chemical Society. UNIVERSITY LIBRARY Copyright © American Chemical Society.

“Just Accepted” manuscripts have been pe online prior to technical editing, formatting f The Journal of Physical is published as a f Society providesChemistry “Just CAccepted” the American dissemination of by scientific material as soon Chemical Society. 1155 appear in full in PDF format by Sixteenth Streetaccompanied N.W., Washington, DC 20036 fully peer reviewed, but should not be consid Published by by American Subscriber access provided CORNELL readers and citable by the Digital Object Ide Chemical Society. UNIVERSITY LIBRARY Copyright © American Chemical Society.

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The Journal of Physical Chemistry

Structures of (a) tetrahedrally solvated Li+ ion, (b) contact ion pair, (c) solvent separated ion pair, and the space-filled optimized geometries for Li+ tetrahedrally solvated by (d) DMC and (e) EC. Structures in (a-c) are shown with EC as the model organic carbonate. Scheme 2 100x162mm (300 x 300 DPI)

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Solvation Structure and Dynamics of the Lithium Ion in Organic Carbonate-Based Electrolytes: A Time-Dependent Infrared Spectroscopy Study Kristen D. Fulfer and Daniel G. Kuroda* Department of Chemistry, Louisiana State University, Baton Rouge, LA 70803 Keywords: infrared spectra, two-dimensional infrared spectroscopy, electrolytes, dimethyl carbonate, butylene carbonate ABSTRACT: The structure and dynamics of electrolytes composed of lithium hexafluorophosphate and either butylene carbonate or dimethyl carbonate were investigated using steady state and two dimensional infrared spectroscopies. This study was focused on lithium ion compositions similar to commercial batteries (i.e., X(Li+)=0.09) and higher. Experiments provide sufficient evidence to demonstrate that both organic carbonates form tetrahedral solvation complexes around the lithium ion. Ab-initio computations confirmed that the IR spectroscopic signatures derived from experiments correspond to a tetrahedral arrangement of carbonate molecules in the lithium ion solvation shell. Time resolved experiments further revealed that the solvation shell formed by cyclic carbonates is more rigid than that of its linear carbonate analogue. In addition, butylene carbonate was found to present a more organized “overall” solvent structure than dimethyl carbonate. At lithium salt concentrations beyond that of a conventional electrolyte, the electrolytes displayed changes in the dynamics and structure of their molecular components due to the presence of ion pairs. Cyclic and linear carbonates were found to preferentially form ion pairs with two distinct structures: contact and solvent separated ion pairs, respectively. The formation of distinct ion pairs by butylene carbonate and dimethyl carbonate is predicted to arise from the different solvation shells formed by the two carbonates; this was confirmed by ab-initio frequency calculations and FTIR. Results of this study shed some light on the characterization of the solvation structure of the lithium ion in the electrolyte which could help to rationalize the importance of the electrolyte composition on the performance of the lithium ion battery.

INTRODUCTION

(EC) or propylene carbonate (PC), and linear organic carbonates, such as dimethyl carbonate (DMC) or diethyl carbonate (DEC), with lithium hexafluorophosphate (LiPF6) salt (Scheme 1). The use of organic carbonates is based on both their high dielectric constants, which allow for full dissociation of the salt, as well as, their capacity for forming stable solid-electrolyte interphases on the negative electrode.3-4 In contrast, the selection of the lithium salt concentration is generally defined by the maximum of the conductivity curve.5

Lithium ion batteries have taken over the portable energy storage market in the last two decades. Despite the extended use of lithium-ion technology, lithium-ion batteries are far from their theoretical power density maximum.1 Thus, there has been an increased interest in developing new battery technologies to fill this technological/scientific gap.2 One of the vastly neglected areas in the development of better lithium-ion storage technologies is the electrolyte. Typical electrolytes of commercial lithium-ion batteries contain a mixture of cyclic organic carbonates, such as ethylene carbonate

Organic carbonates are now ubiquitous to lithium-ion storage, but this technology has problems associated with 1



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stability and safety. In order to overcome drawbacks of this battery technology, the development of next generation electrolytes has triggered an interest in the basic characterization of lithium-ion electrolytes.2 For example, it was recently demonstrated that further improvements on the safety of lithium ion battery technologies can be achieved by modifying the composition of the electrolyte.6-7 Thus, there is now a general consensus that in order to increase the efficiency, durability, and safety of a lithium-ion battery, it is not sufficient to only characterize the lithiation and delithiation processes occurring at the electrodes and their interfaces, because the electrolyte molecular properties directly affect the efficiency of these processes.2, 8

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absorption of the carbonyl stretch has limited the study of the Li+ solvation in pure carbonates using this particular vibrational mode. Most studies dedicated to the carbonyl stretch mode used either diluted solutions,43-44 or ATRFTIR (attenuated total reflectance FTIR) spectroscopy to study Li+ solvation in carbonate-based electrolytes.40-41, 4547 Other studies focused on the vibrational modes of the anion in order to overcome the absorption issue.17, 23, 48 In either case, the studies presented so far were limited to changes in the linear IR spectra as a function of salt concentration for the derivation of the structure of the Li+ solvation shell. In this study, a combination of linear infrared (FTIR) and two-dimensional infrared (2DIR) spectroscopies are used to obtain direct insights into the structure and dynamics of Li+ solvation in organic carbonates. To this end, vibrational photon echo (2DIR) offers unique advantages in time and frequency resolution for studying both the structure and dynamics of the carbonates in the Li+ solvation shell. It is important to note that the molecular properties derived from 2DIR experiments are not accessible to other traditional methodologies, such as NMR spectroscopy. Moreover, the enhanced frequency resolution of 2DIR compared to traditional FTIR techniques49 and the vibrational lifetime difference of different species50-51 allows for the resolution of peaks which may not be spectrally resolvable in the linear spectrum. In addition, the presence of other spectral features, such as cross peaks due to vibrational coupling processes, provides another spectroscopic signature for gaining further insight into the structure and dynamics of Li+ electrolytes.52-55 Our IR studies have focused on Li+ electrolytes composed of either a linear or a cyclic carbonate, since both are commonly found in current commercial lithium-ion battery technologies.2

Scheme 1. The structures of (a) EC, (b) PC, (c) 1,2butylene carbonate (BC), (d) DMC, (e) DEC, and (f) LiPF6.

Numerous experimental9-27 and theoretical 13, 28-37 methodologies have been used to investigate the lithium ion (Li+) solvation shell structure or the organic carbonate-based electrolytes. However, simple molecular structural characteristics remain controversial. For example, the coordination number of Li+ has been proposed to be between 2 and 6 in organic carbonates, with most studies suggesting a coordination number of either 4 or 6.9-21, 26-36 Presently, there is still no definitive answer to the nature of the Li+ coordination in carbonatebased electrolytes. Moreover, there is also not a clear understanding of how the structure of the carbonate affects the conductivity and viscosity of the electrolyte.2

EXPERIMENTAL METHODS Sample Preparation. LiPF6 98% pure from Acros Organics and 99+% from Strem Chemicals, DMC 99+% pure from Acros Organics, and 1,2-butylene carbonate (BC) >98% pure from TCI America, Inc. were used without further purification. BC was used to represent cyclic organic carbonates due to its similar structure and dielectric constant to both EC and PC (Scheme 1).2 Other cyclic organic carbonates, including EC, EC-d4, EC-13C3, PC, PC-d6, and fluoroethylene carbonate, were evaluated, but each contained effects from Fermi resonances in the carbonyl stretch region, and thus, only the BC sample has been considered in this study. Electrolyte solutions were prepared in a N2-filled glovebox.

It is well established that lithium salts dissociate in organic carbonates because Li+ coordinates with carbonates via the carbonyl oxygen atom of the molecule.10, 14, 22-25, 29, 37 Furthermore, the ion-dipole interaction with the carbonate molecules has been predicted to be ~50 kcal/mol due to the small size of Li+.28, 38-39 Consequently, solvent molecules in the solvation shell of Li+ can be easily observed by changes in the transition frequency of many vibrational modes either arising from the solvent or the anion.16-17, 40-42 Spectroscopic differentiation between the free and coordinated solvent molecules has been used to study the solvation of Li+ in organic carbonates. However, the strong infrared

Sample cells used for FTIR consisted of approximately 30 L of electrolyte in between two CaF2 windows without spacer. The sample cells used for 2DIR spectroscopy consisted of the electrolyte in between a CaF2 window and a CaF2 convex lens (1 m focal length) without spacer. For the 2DIR experiments two different lithium salt concentrations were used. In order to mimic real battery electrolytes, the lower salt concentration electrolytes had 2


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a mole fraction of X(Li+)=0.09, which is approximately the concentration of a “Generation 2” commercial lithium ion battery electrolyte.56 The high concentration solutions had mole fractions of X(Li+)~0.15.

the carbonyl stretch of the organic carbonates located in the 1700-1800 cm-1 region.59 The IR spectra as a function of Li+ concentration for the two organic carbonates (BC and DMC) are shown in FIGURE 1. Both studied carbonates present a single carbonyl stretch peak in the pure solvent (solid black lines in FIGURE 1). The addition of Li+ gives rise to a second absorption peak downshifted by 30 cm-1, which increases with increasing Li+ concentration. Due to the clear change of the carbonyl stretch peaks with salt concentration, the high and low frequency peaks have been previously assigned as the free and coordinated carbonyl stretches, respectively;40 the same convention is used to refer to these two bands throughout this manuscript. The data in FIGURE 1 is consistent with previous studies of LiPF6 in organic carbonates.41

Linear FTIR Spectroscopy. FTIR were collected on a Bruker Tensor 27 equipped with a liquid nitrogen cooled narrow band MCT detector, with a resolution of 0.5 cm-1. ATR-FTIR were collected on a Bruker 27 equipped with a Pike diamond/ZnSe ATR crystal and a DTGS detector, with a resolution of 4 cm-1. The linear spectra were modeled with Voigt profiles using OriginLab software. 2DIR Photon Echo Spectroscopy. 2DIR photon echo experiments have been performed with a setup similar to that previously described in the literature.49 Briefly, a Spectra Physics Spitfire Ace Ti:Sapphire amplifier with a 5 Khz repetition rate and OPA-800C were used to produce broadband infrared pulses with temporal width of circa 60 fs. These infrared pulses were split into three identical pulses and later focused on the sample with a boxcar configuration.57 The time intervals:  (between the first and second pulses), Tw (between the second and third pulses, the latter of which generates the third order non-linear response, or photon echo), and t (between the fourth pulse and the photon echo) were adjusted with four computer controlled translation stages (PI Micos). Photon echo generated in the –k1+k2+k3 phase matching direction was heterodyned with a fourth infrared pulse (local oscillator) and detected using a liquid nitrogen cooled 64 element MCT array (Infrared Systems Development) after being dispersed by a Triax monochromator (100 grooves/mm). Data were collected for Tw times of 0 to 5 ps in 0.5 ps steps. For each Tw time, the first time interval, , was scanned from -4 ps to +4 ps in 5 fs steps in order to collect the rephrasing and nonrephasing data. To obtain the 2DIR spectrum, the photon echo signal is transformed from (, Tw, t) to (, Tw, t) by a double Fourier transform along  and t, while Tw is kept as a parameter. It should be noted that the Tw=0 ps data point was excluded from some of the analysis due to possible distortions from pulse overlap.

Interestingly, the free and coordinated carbonyl stretch bands of the cyclic carbonate are significantly broader than those of the linear carbonate, which as it will be shown later is related to the ordering of the cyclic carbonate electrolyte. In addition, the cyclic carbonate shows the rise of a third peak on the low frequency side (1730 cm-1) for the highest investigated concentration which is not observed in the linear electrolyte. The vibrational transition appearing at 1730 cm-1 is likely related to the formation of ion pairs or aggregates. 12

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DFT Calculations. Density functional theory (DFT) calculations were performed using the Gaussian 09 software58 at the B3LYP level of theory using the 6311++G** basis set. Geometry optimizations and frequency calculations were performed in the gas phase. PC was used to represent the cyclic carbonates, while DMC was used to represent the linear carbonates. The DFT computed frequencies were corrected with a constant to match the experimental values. Tabulated DFT frequencies and IR intensities as well as the geometries used for each solvation complex can be found in the supplemental information.

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Wavenumbers (cm-1) FIGURE 1. FTIR of the carbonyl stretch region. Spectra are shown for the (a) BC and (b) DMC electrolytes, at various concentrations of Li+. Arrows indicate changes in the spectra with increasing salt concentration.

RESULTS First, the effect of lithium salt on the structure and dynamics of the solvent was investigated by steady-state IR spectroscopy. To this end, the experiments focused on

The structure and dynamics of the two electrolytes were further investigated, utilizing 2DIR spectroscopy. 2DIR spectra for the BC and DMC electrolytes at 3



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X(Li+)=0.09 are shown in FIGURE 2. Along the diagonal, the 2DIR spectra of each electrolyte show two pairs of peaks. As in the FTIR spectra (FIGURE 1), the pairs of peaks located at low and high frequencies in the 2DIR spectra (FIGURE 2) are assigned to the coordinated and free carbonyl stretch modes, respectively. The grey and purple contributions in each peak arise from different third order field interactions.57 Signal arising from interactions involving the ground and first excited vibrational states (i.e. bleach [𝜈 = 0 → 1] and photoinduced emission [𝜈 = 1 → 0] transitions) produces the grey peaks which appear close to the diagonal. The purple peaks, which are shifted off of the diagonal due to the anharmonicity of the carbonyl stretch, represent the signals yielded by interactions involving the first two excited states (i.e., excited state absorption [𝜈 = 1 → 2]). At zero waiting time (Tw = 0 ps), both pairs of 2DIR peaks (free and coordinated) exhibit a set of cross peaks occurring at (t, ) = (1770 cm-1, 1800 cm-1) and (1800 cm1, 1750 cm-1) for BC and ( ,  ) = (1715 cm-1, 1755 cm-1) and  t (1760 cm-1, 1725 cm-1) for DMC. These cross peaks are more visible for the linear carbonate electrolyte than for its cyclic analogue.

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Electrolytes with high Li+ concentration (X(Li+)~0.15) show comparable 2DIR spectra (FIGURE 2) to that of the low concentration electrolytes (X(Li+)=0.09). The most obvious difference between the low and high Li+ concentration 2DIR spectra is the intensity of the coordinated carbonyl stretch (low frequency diagonal peaks), which as expected has a higher intensity in the 2DIR spectra of electrolytes with higher salt concentration. It is important to note that higher salt concentration also enhances the 2DIR cross peaks between the free and coordinated diagonal peaks at Tw = 0 ps. The BC electrolyte with high Li+ concentration shows a substantially different 2DIR spectra at long waiting times compared to its corresponding low concentration spectra, as evidenced by the presence of an extra pair of diagonal peaks on the low frequency side of the coordinated carbonyl stretch ( = 1755 cm-1). This second transition is red shifted from the coordinated band by approximately 15 cm-1 and is not easily distinguishable at early Tw. However, the growth of cross peaks between the two BC coordinated transitions makes this second transition evident at longer waiting times. While the existence of an extra coordinated diagonal peak is clear in the BC electrolyte at high Li+ concentration (FIGURE 2), no evidence of this low frequency transition is observed in the 2DIR spectra at X(Li+)=0.09.

Analysis of the waiting time dependence of the 2DIR spectra (FIGURE 2) reveals that the peak shapes of the free and coordinated carbonyl stretch modes undergo a substantial change as waiting time increases. At Tw = 0 ps both peaks are elongated along the diagonal, and as waiting time increases they acquire a more upright and rounder shape, which evidences the occurrence of spectral diffusion.57 While the change in shape of the peaks is considerable for both electrolytes, a substantial difference is observed between the 2DIR spectra of the BC- and DMC-based electrolytes at various waiting times. In the latter case, the 2DIR peaks become almost upright, where as in the former, the peaks remain significantly elongated at Tw = 5 ps. This marked difference in the 2DIR spectral changes with waiting time exposes disparities in the dynamics of the molecular motions of the two electrolytes. The time evolution of the diagonal peaks is also accompanied by a substantial increase in the intensity of the cross peaks between the free and coordinated diagonal peaks, such that their signal magnitude is comparable to that of the diagonal peaks at Tw = 5 ps.

The splitting of the coordinated band in the 2DIR spectra is limited to the BC electrolyte as there is no spectroscopic evidence of this transition in the 2DIR spectra of DMC at any concentration. Moreover, the 2DIR spectra of the DMC electrolyte reveal a different cross peak at  = 1725 cm-1 arising from the coordinated peak which is more evident in the 2DIR at X(Li+)~0.15 and at longer waiting times. The new cross peak in the coordinated band of the DMC electrolytes evidences the existence of a low frequency transition located at ~1685 cm-1, which is down shifted 40 cm-1 from the carbonyl stretch band of carbonates in the solvation shell of Li+. The contrast between the number and location of carbonyl coordinated transitions in the 2DIR spectra of the two high salt concentration electrolytes is related to the formation of ion pairs with different structures.

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FIGURE 2. Absorptive 2DIR spectra of Li+ in organic carbonates. 2DIR spectra of LiPF6 in BC and DMC with X(Li+)=0.09 and X(Li+)~0.15 for waiting times Tw=0 ps, Tw=2 ps, and Tw=5 ps. Black line corresponds to the diagonal ( = t).

DISCUSSION

only valid if the parameters of the infrared transition, such as the transition dipole magnitude, are equal for the coordinated and free carbonyl stretch peaks. Moreover, this type of modeling only accounts for transitions arising exclusively from the free and coordinated carbonates and neglects the presence of other species, such as ion pairs.

Interpretation of FTIR data Modeling of the FTIR with two Voigt profiles (FIGURE 3 and TABLE S1 of the SI) shows that the fractional area of the coordinated carbonyl stretch band grows non-linearly as function of X(Li+) in both electrolytes. Moreover, the coordinated fractional area exhibits different growths for the two investigated electrolytes, with the fastest growth corresponding to the electrolyte composed of cyclic carbonates. The growth of the area ratio is well modeled + with an exponential of the form 𝑦0(1 − 𝑒 −(𝑋(𝐿𝑖 )⁄𝐶) ), where C represents the characteristic growth of the coordinated fractional area. Exponential modeling reveals that the fractional area of the coordinated peak grows 10% faster for the cyclic carbonate than for the linear carbonate (TABLE S2). The modeling results obtained here are in agreement with previously published data for DMC electrolytes,41 albeit small differences which might be caused by the use of FTIR-ATR in the previous study.60

The presence of multiple transitions in the high Li+ concentration 2DIR spectra demonstrate that the coordinated carbonyl stretch band cannot be assigned to a single transition. Furthermore, analysis of the fitting parameters used for obtaining the area ratio (FIGURE 3, bottom panel) shows that the full-width at half maximum (FWHM) of the coordinated carbonyl peak changes with increasing X(Li+) in the two electrolytes. The changes in the lineshape parameters as a function of Li+ concentration further support the idea that the observed spectroscopic features are related to the existence of ion pairs. Hence, the difference in the exponential growth of the fractional area of the coordinated peak with Li+ concentration only demonstrates that the interaction between Li+ and its counter ion is different in linear and cyclic carbonates, rather than revealing a change in coordination number as previously proposed.41

FTIR fractional areas have previously been used to show that different carbonates confer different solvation structures to the lithium ion.11, 41 However, the coordination number derived from the fractional area is 5



For each electrolyte, the 2DIR spectra (FIGURE 2) show the presence of cross peaks at Tw = 0 ps. The existence of cross peaks at Tw = 0 ps is further demonstrated by measuring the 2DIR spectra with the k1 and k2 pulses having a 45° polarization, which is a pulse sequence known to enhance the off-diagonal peak intensity with respect to the diagonal peaks (FIGURE S2).65 These cross peaks observed in the 2DIR spectra may be produced by two different mechanisms: chemical exchange or vibrational coupling. In this case, the chemical exchange mechanism would involve moving carbonates in and out of the Li+ solvation shell with an ultrafast time scale (exc < 70 fs, pulse width) such that cross peaks are observed in the 2DIR spectra at Tw = 0 ps. However, the chemical exchange mechanism is unlikely to have a such a fast time constant for this system since the Li+carbonate interaction is very strong; i.e., ~50 kcal/mol.28, 38-39 Thus, the 2DIR cross peaks between the free and coordinated carbonyl stretch bands at Tw = 0 ps are most likely the signature of vibrational coupling between vibrational modes.55

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The cross peaks in the 2DIR at Tw = 0 ps (FIGURE 2) evidence vibrational coupling between the coordinated peak and an underlying transition located at a transition frequency similar to that of the free carbonyl stretch. Since the free carbonyl stretch peak appears to only decrease with increasing Li+ concentration, this underlying transition must be weak in intensity compared to that of the free carbonate. Thus, the vibrational coupling of the high frequency carbonyl stretch transition to the coordinated carbonyl transition is strong evidence of the existence of an IR forbidden transition related to the specific arrangement of organic carbonates in the Li+ solvation shell.

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0.30

X(Li+) FIGURE 3. Modeling of the FTIR spectra. (a) Ratio of the peak areas for the Li+-coordinated carbonyl stretch to the sum of the free and coordinated carbonyl stretches as a function of LiPF6 concentration for BC (black squares) and DMC (green circles). Lines represent the exponential trend of the area ratios. FWHM of the coordinated carbonyl stretch as a function of Li+ concentration for (b) BC and (c) DMC.

While Li+ favors tetrahedral coordination in many solvents,66-68 the coordination number of Li+ in organic carbonates has been postulated to be between 2 and 6.9-21, 28-36 Considering all Li+-carbonate complexes with a Li+ coordination in this range, only solvation complexes with 2, 3, 4 and 6 coordinated molecules should have a single IR active vibrational transition,69 which discards 5 as a possible coordination number. In addition, a solvation structure containing 6 carbonates should have two Raman active transitions besides the single IR active mode, while those containing 2, 3 or 4 carbonates should only have one additional Raman active transition.69 Raman studies on DEC, PC, and EC confirmed that only two Raman bands (one for the coordinated and one for the free) are observed in organic carbonate solutions of Li+, demonstrating carbonates do not form octahedral coordination shells around Li+.15-16, 70 Thus, the only possible solvation numbers for Li+ in carbonate electrolytes are 2, 3, and 4.

Structure of the Li+ solvation shell Previous studies reported a Li+ solvation shell consisting of 2 to 6 carbonates in the first coordination sphere.9-21, 26-36 Since the interaction energy between Li+ and carbonates is 45 - 60 kcal/mol,28, 38-39 it is expected that Li+ will have a stable solvation shell with specific signatures in the IR spectrum. The FTIR spectra (FIGURE 1) show only two peaks for both electrolytes at X(Li+)>0. However, the presence of overlapping transitions from different species makes the determination of the infrared signatures very challenging by FTIR alone. In contrast, 2DIR spectroscopy can assess the spectroscopic signatures of a molecular system not only through its enhanced resolution61-62 which allows coupling constants to be obtained,63 but also through the occurrence of cross peaks which are spectral signatures of vibrationally coupled transitions.64

Theory has shown that Li+ favors a coordination number of four.28, 33, 39 DFT frequency calculations confirm that this is also the case for carbonate electrolytes. Similar to the predictions from simple symmetry arguments,69 the 6


Page 17 of 25

linear, trigonal planar, tetrahedral, contact ion pair, and solvent separated ion pair solvation complexes. Left and right columns correspond to the cyclic and linear organic carbonates, respectively. The free carbonyl stretch was calculated separately and is shown as a grey open square. The DFT frequencies were corrected by a constant of 0.950 for cyclic molecules and 0.965 for linear carbonates.

results of the DFT calculations for the cyclic (CC) and linear (LC) carbonate solvation complexes (FIGURE 4) show that the linear, trigonal planar, and tetrahedral complexes present 1, 2 or 3 low frequency asymmetric stretch transitions and a IR forbidden high frequency mode, which is symmetric. DFT also predicts that the IR experimental findings are best described by the frequency locations and intensities of the carbonyl stretch transitions for a Li+ coordination number of 4. The tetrahedral complex has four vibrational transitions, but effectively only one band should be seen in the linear IR spectrum since three transitions (the asymmetric carbonyl stretches) are very close in frequency and the other transition is forbidden (totally symmetric carbonyl stretch). Moreover, the totally symmetric stretch mode is computed to be downshifted only ~10 cm-1 from the carbonyl stretch of the free carbonates (FIGURE 4). Consequently, the cross peaks at Tw = 0 ps are due to the vibrational coupling between the three asymmetric stretch modes and the totally symmetric carbonyl stretch mode of the tetrahedral solvation arrangement. Although the totally symmetric carbonyl stretch mode should give rise to a non-discernible 2DIR cross peak due to its small transition dipole magnitude, the cross peak can be readily seen in 2DIR at Tw = 0 ps. Hence, the appearance of the cross peaks is likely to be related to a break in the symmetry of the tetrahedral carbonate complex which makes the symmetric transition weakly allowed. The mechanism of symmetry breaking could arise from solvent motions, conformers, or ion pairs. Overall, the appearance of cross peaks in the carbonyl stretch region, as illustrated in 2DIR spectra in FIGURE 2, confirms that the Li+ solvation shell consists of vibrationally coupled transitions arising from a tetrahedral arrangement of organic carbonates.

Intensity (103 km/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2 1 0

Composition of the Li+ solvation shell It is now evident that at low Li+ concentration the solvation shell of Li+ is composed exclusively of carbonates. However, at high Li+ concentrations the spectroscopic signatures of new species are observed in the linear and 2DIR spectra. The IR data of the DMC electrolyte (FIGURE 2) demonstrates that high Li+ concentrations not only enhance the cross peak intensity, but also result in the appearance of various cross peaks which are further evidenced in the broadening of the coordinated carbonyl stretch band of the FTIR (FIGURE 3). These two results strongly suggest the presence of solvent separated ion pairs (SSIP) in which a counter ion is inserted into the tetrahedral solvation shell around Li+ (Scheme 2). DFT computations (FIGURE 4) predict that a SSIP of linear carbonates (LC_SSIP) has four IR transitions in the carbonyl stretch region with three that are weakly allowed due to deformations of the solvation shell. Moreover, only three transitions are predicted to be observed for the LC_SSIP in either spectrum because of the similar frequency of two of the low frequency transitions. Since the four carbonyl stretch transitions of LC_SSIP arise from the same vibrational manifold, the presence of cross peaks in the 2DIR spectra is expected due to vibrational coupling. The strongest LC_SSIP transition is located at the same frequency as the coordinated carbonyl stretch of the free ion while the other three LC_SSIP transitions are located at either lower or higher frequency (FIGURE 4). Two LC_SSIP carbonyl stretch transitions are downshifted by ~20 cm-1 from the coordinated band and are not directly visible in the 2DIR data due to their small transition dipole. However, the cross peaks between the most intense LC_SSIP asymmetric carbonyl stretch and the two low frequency carbonyl stretch modes are visible in the DMC 2DIR spectra (FIGURE 2). In contrast, the frequency overlap between the LC_SSIP high frequency mode, the free carbonyl band, and the IR forbidden transition of the tetrahedral solvation complex does not allow us to distinguish the cross peaks signature of the LC_SSIP from that of the tetrahedrally solvated ion. Nevertheless, the cross peaks between the coordinated and free carbonyl bands are enhanced at high Li+ concentrations suggesting that the LC_SSIP transitions contribute to the cross peaks at Tw = 0 ps.

1 0

2 1 0

1 0

2 1 0

1 0

2 1 0

1 0

2 1 0

1 0 1700

1750

1800

1650

1700

1750

Transition Frequency (cm-1) FIGURE 4. DFT frequency calculations for various carbonate geometries around Li+. From top to bottom: DFT predicted intensity versus frequency for the carbonyl stretches of the

The computed DFT frequencies also predict that the presence of LC_SSIP should produce a broadening in the asymmetric carbonyl stretch band because of the 7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

mismatch between the transition frequencies with the tetrahedrally solvated Li+. The predicted broadening is experimentally observed from the modeling of FTIR carbonyl stretch peaks as function of Li+ concentration (FIGURE 3). Overall, the spectral signatures derived from DFT calculations are in very good agreement with the IR data of the linear carbonate; i.e., a cross peak between the coordinated carbonyl band and a low intensity band located at low frequency (FIGURE 2), the broadening of the IR band for the coordinated carbonyl stretch (FIGURE 3), and the cross peak enhancement in the 2DIR at high Li+ concentrations (FIGURE 2).

Page 18 of 25

broadening of the coordinated band, which is not evidenced in our data (FIGURE 4). Since the linear IR of the BC electrolyte has multiple contributing species to the coordinated carbonyl stretch band, as evidenced by the 2DIR spectra, the modeling used here may not be an accurate representation of the data. Additionally, the formation of CC_CIP should produce a splitting of the asymmetric carbonyl stretch band due to the appearance of two allowed transitions at lower frequencies. Conversely, the SSIP formed by cyclic carbonates (CC_SSIP) should have a pair of low intensity and frequency transitions (FIGURE 4). In the BC electrolyte, both CC_SSIP and CC_CIP should produce a low frequency peak in the coordinated carbonyl stretch band. However, the CC_CIP should not contribute to cross peaks between the coordinated and free carbonyl stretch peaks because the CC_CIP high frequency mode is predicted to overlap with the coordinated carbonyl stretch band of the tetrahedrally solvated ion. The spectroscopic signatures of the CC_CIP predicted by DFT are observed experimentally in the IR and 2DIR spectra of the BC electrolyte with high concentration of Li+ (FIGURE 1 and FIGURE 2). In lieu of obvious broadening of the coordinated band, the linear spectra show a second transition at ~1730 cm-1 at very high Li+ concentration, and the 2DIR spectra shows the presence of two transitions in the coordinated band. Although the two transitions are not clearly visible at early waiting times, the growth of cross peaks between the assigned CC_CIP transitions is strong evidence of the vibrational coupling between the two transitions.

Scheme 2. Structures of (a) tetrahedrally solvated Li+ ion, (b) contact ion pair, (c) solvent separated ion pair, and the space-filled optimized geometries for Li+ tetrahedrally solvated by (d) DMC and (e) EC. Structures in (a-c) are shown with EC as the model organic carbonate.

The hypothesis that DMC and BC electrolytes form different ion pairs is confirmed by measuring the P-F stretch modes of the PF6- anion via FTIR-ATR (FIGURE 5) where it is evident that the anion is sensing a more isotropic environment in the BC electrolyte where only a single band is observed. In contrast, the spectrum in the same spectral region for the DMC electrolyte displays multiple bands due to the more anisotropic interactions experienced by the anion due to the formation of LC_SSIP. Moreover, the preferred formation of CIP in the cyclic carbonate versus SSIP in the linear carbonate is consistent with the difference in cross peak intensity observed in the 2DIR spectra of the BC- and DMC-based electrolytes (FIGURE 2) where a CC_CIP will not contribute to a cross peak between the coordinated and free carbonyl stretch peaks due to the lack of a vibrational transition in the free carbonyl stretch region (FIGURE 4).

The cyclic-based electrolyte presents different behavior than its linear analogue at high Li+ concentrations. For example, the FWHM of the coordinated band decreases with increasing Li+ concentration (FIGURE 3). The changes of the IR spectra with Li+ concentration of the BC electrolyte are also attributed to the formation of ion pairs, but with different structure than those formed in the DMC electrolyte. DMC favors the formation of SSIP, but cyclic carbonates promote the creation of contact ion pairs (CIP) in which Li+ is solvated by one PF6- anion and three cyclic carbonate molecules (Scheme 2). DFT calculations predict that the occurrence of a CIP in cyclic carbonates (CC_CIP) should also be accompanied by a 8


Page 19 of 25

0.8

number of carbonates in the lithium solvation shell and simultaneously increase the number of free carbonates. Consequently, the generation of CIP in the electrolyte will reduce the ratio of areas between coordinated and free carbonyl bands. In the case of SSIP, the change in the fractional area is caused by a change in the magnitude of the transition dipole probability of the forbidden IR transition. A perfect tetrahedral solvation shell arrangement has a dark IR state with very low transition dipole probability. In contrast, the deformation of the tetrahedral complex, due to the presence of an anion in the solvation shell (SSIP), increases the transition probability of the forbidden transition and simultaneously lowers the transition probabilities of the other three SSIP allowed transitions (FIGURE 4). This results in the non-linear growth of the FTIR area ratio of the linear carbonate. Moreover, the faster growth of the area ratio of the BC electrolyte compared to its DMC analogue further supports the interpretation that the cyclic coordination shell has a structure very near to a perfect tetrahedron since only at high Li+ concentrations CIP formation becomes energetically favorable.

(a)

0.6 0.4

ATR Absoprtion

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.2 0.0 0.6

(b)

0.4 0.2 0.0 800

820

840

860

880

900

-1

Wavenumber (cm ) FIGURE 5. ATR-FTIR spectra of the PF6- anion in P-F asymmetric stretch region for (a) BC and (b) DMC samples. Solid black line represents the neat carbonate, and the green and purple lines represent Li+-carbonate solutions at X(Li+)=0.09 and 0.15, respectively.

Dynamics of carbonate molecules In an electrolyte, the organic solvent serves as a solvation medium not only for Li+ but also for the anion, though the PF6--carbonate interaction is weak relative to that of Li+-carbonate. At low salt concentration, the number of carbonate molecules per Li+ ion is large (e.g., at X(Li+)=0.01 there is 1 Li+:PF6- pair per 100 carbonate molecules), but in typical battery electrolyte concentration, the ratio of Li+ per carbonates molecules is ~1:9.56 Considering that 4 of the 9 carbonate molecules solvate Li+, the other 5 carbonate molecules do not strictly correspond to “free” molecules in solution, but to molecules that are both free and solvating the anion. It is therefore expected that the dynamics and interactions of both the free and the coordinated carbonates should play a crucial role in defining electrolyte properties such as conductivity.

From a molecular perspective, the difference in the molecular structure of the carbonate is also consistent with preferential formation of CIP and SSIP by the cyclic and linear carbonates, respectively. The flexibility of the linear carbonate alkyl chains is likely to allow the positioning of the PF6- anion close to the cation without significantly disturbing the tetrahedral structure of its solvation shell. In contrast, cyclic carbonates, such as BC, are less likely to allow PF6- to approach Li+ since the formation of an SSIP will impose a strong entropic penalty in the energy through the restriction of the rotation of the cyclic carbonates. Thus, at high salt concentrations, cyclic carbonates favor the formation of CIP to avoid the energy penalty arising from the rotation restriction, while DMC carbonates form SSIP in which the Coulombic interaction is maximized without the energetic penalty of disrupting the lithium ion-carbonate interaction. The preferential formation of CIP and SSIP in cyclic and linear carbonate-based electrolytes is in agreement with various theoretical studies showing that: CIP are energetically favored over SSIP for cyclic carbonates;71 Li+ salts tend to form SSIP in linear carbonates whereas they are likely to remain fully dissociated in cyclic carbonates;36 a higher number of PF6ions is likely to be found in the solvation shell of Li+ for linear carbonate based electrolytes than for their cyclic counterparts.34

The dynamics of the molecular environment is manifested in the 2DIR spectra by the waiting time evolution of the loss of correlation between the pump () and probe (t) frequencies. Here, the characteristic time of the frequency-frequency correlation function (FFCF) is retrieved from the waiting time evolution of either the inverse slopes of the nodal line (slope)72 or center line slope (CLS).73 FIGURE 6 displays the Tw evolution of FFCF for the free carbonyl peak in each electrolyte at various Li+ concentrations. It is apparent from the FFCF dynamics that the free carbonyl peak has a significantly different time evolution in the two electrolytes at similar Li+ concentrations. The free carbonyl band of the BC electrolyte exhibits an initial fast decay of the FFCF that rapidly converges to a constant of large amplitude, but the same band in the DMC sample presents a larger decorrelation decay time that plateaus into a constant with very small amplitude.

Finally, the formation of CIP and SSIP observed for cyclic and linear carbonates also explains the non-linear change of the fractional area of the coordinated carbonyl stretch bands with Li+ concentration in the FTIR data (FIGURE 3). The formation of CIP will decrease the 9


The Journal of Physical Chemistry The FFCF dynamics of the free carbonyl peak for both electrolytes is well modeled with a function of the form: 𝑦0 + 𝐴𝑒 −(𝑇𝑤 ⁄𝜏1 ) , where the constant, y0, represents a process that has dynamics slower than the investigated experimental time window. Results of the modeling, presented in TABLE 1, quantify the stated difference of the free carbonyl FFCF dynamics in the two electrolytes. It also shows that the slow process of the FFCF, represented by y0, has a considerably larger amplitude for BC than for DMC. Surprisingly, an increase in the Li+ concentration to X(Li+)~0.15 does not significantly affect the dynamics of the free carbonates in either electrolyte (TABLE 1). The trend observed for the FFCF dynamics of the free carbonate molecules in the BC and DMC electrolytes (i.e., (BC) >> (DMC)) is also observed in their pure solvents (FIGURE 6). However, BC molecules in the pure solvent present a decay of the FFCF which is not seen in either of the two investigated BC electrolytes.

1.0

(a)

1/CLS

0.8 0.6 0.4 0.2 0.0 0.8 X(Li+)=0.00 X(Li+)=0.09 X(Li+)~0.15

(b) 0.6

1/Slope

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 25

0.4 0.2 0.0

Differences in the dynamics of the free molecules between the linear and cyclic carbonate, of both electrolytes and pure solvents, indicate that the cyclic carbonates present a more organized structure than their linear analogues. The observed high molecular organization of the BC solvent is consistent with the IR and 2DIR spectra of the pure solvent (FIGURE S3) where a broad IR carbonyl band and cross peaks are observed due to the presence of vibrational excitonic states.74 Furthermore, the molecular ordering of the BC environment is consistent with a previous study of structurally similar organic carbonates where antiparallel dipolar ordering was observed.75

0

1

2

3

4

5

Tw (ps) FIGURE 6. (a) Inverse CLS of the free carbonyl stretch mode for BC and (b) inverse slope of the free carbonyl stretch for DMC electrolytes at various concentrations of lithium salt as a function of waiting time, Tw.

TABLE 1. Frequency decorrelation times, 1, and vertical offsets, y0, for the free carbonyl stretch mode of LiPF6 in various organic carbonates at different LiPF6 concentrations.

Slower dynamics of the free carbonate molecules in electrolytes compared to the pure solvent demonstrates that the addition of salt induces a slowdown of the motions of the BC molecular components. This salt induced slowdown is expected for a solution in which almost complete salt dissociation is occurring, since the presence of ions imposes a strong ordering through iondipole interactions (solvation shell) and Coulombic interactions between the charged species similar to that observed in ionic liquids.76 In addition, the slow dynamics seen in the free BC carbonates might have large contributions from the solvation of the PF6- anion by BC molecules.

X(Li+)

BC

DMC

1(ps)

y0

1(ps)

y0

0

6±2

0.17±0.03

1.1±0.2

0.09±0.03

0.09

0.7±0.4

0.76±0.03

1.4±0.1

0.16±0.01

0.14-0.19

No change

0.73±0.01

1.4±0.2

0.18±0.01

The FFCF dynamics for the coordinated solvent molecules was also explored in this study. FIGURE 7 displays the FFCF dynamics for the coordinated carbonyl stretch bands of the two electrolytes at different Li+ concentrations. It is clear that the two electrolytes show a significant loss of frequency correlation with waiting time, but on different time scales. The DMC coordinated peak shows an exponential decay of FFCF, while the same band in BC has an apparent linear dependence with Tw. Modeling of the FFCF dynamics with a single exponential function of the form 𝑦0 + 𝐴𝑒 −(𝑇𝑤 ⁄𝜏1 ) (TABLE 2) confirms the difference in time evolution of the two electrolytes. The FFCF characteristic times are found to be ~13 ps and ~1 ps for the BC- and DMC-based electrolytes, respectively. In addition, the coordinated carbonyl peak of DMC has a constant component corresponding to a second dynamical process with a characteristic time much longer than the 5 ps experimental time window

Free DMC molecules behave differently than BC as observed in the mere slight change of their FFCF dynamics with the addition of Li+. This small change of the FFCF dynamics validates the idea that the anion is not strongly solvated by free DMC molecules, but by DMC molecules in the solvation shell of Li+, through the formation of SSIP. The weak anion-dipole interaction observed between DMC molecules and the PF6- anion is supported by a theoretical study in which it was shown that linear carbonates do not solvate the PF6- ion as well as their cyclic analogues.36

10


Page 21 of 25

used here, which is not present in the BC sample. Interestingly, an increase in the salt concentration alters the dynamics of the frequency correlations in both the BC and DMC electrolytes, but in opposite directions. A higher concentration of salt induces faster dynamics in the FFCF of the coordinated carbonyl stretch of the BC electrolyte and a slowdown of the frequency correlation time evolution in the DMC electrolyte. Moreover, the linear carbonate electrolyte observes a reduction in the amplitude of the FFCF slow component (represented as a constant, y0) at high Li+ concentration.

solvation shell does not simply represent the motions of the solvent molecules since the carbonyl groups of the carbonates are pointing with their oxygen atoms toward Li+ (Scheme 2). In addition, the four carbonyl stretches present a strong vibrational coupling as seen from the DFT computations. Hence, the frequency correlation of the coordinated carbonates follows the evolution of the three excitonic vibrational states having a frequency of 𝜔10 + 𝛿𝜔(𝑡) − 𝛽(𝑡) which arises from the tetrahedral arrangement of the carbonates in the solvation shell (see model in the SI). As a consequence, the dynamics of the FFCF directly measures the deformation of the solvation shell through fluctuations in either the angle between carbonyl groups, 𝛽(𝑡), or the Li+-carbonyl distance, 𝛿𝜔(𝑡). Thus, the FFCF dynamics measured here demonstrates that cyclic carbonates form a more rigid solvation shell than their linear counterparts which is evidenced by the slower decay time of their FFCF.

0.8

(a)

1/CLS

0.6 0.4 0.2

From a molecular structure perspective, the difference in the FFCF dynamics demonstrates that the slight molecular difference between linear and cyclic carbonyls has far reaching consequences in the overall structure of the electrolyte. Linear carbonates, like DMC, have two untethered alkyl chains that create a slightly deformed tetrahedral solvation shell due to the arrangements of the side chains (conformers). The presence of conformers is energetically needed to minimize the entropy considering that the steric hindrance between side chains does not allow the free rotation of the carbonates, but permits a more compact solvation shell. In contrast, cyclic carbonates are planar molecules and have their dipole oriented along the carbonyl group allowing the molecule to freely rotate without being penalized energetically by steric effects. However, cyclic carbonates are restricted to geometries in which the dipole is oriented towards the Li+ to maximize the dipole-ion interaction which inhibits the deformation of the solvation shell. The presence of rotamers does not affect the FFCF dynamics their occurrence does not produce frequency fluctuations of the carbonyl stretch since rotation along the C=O bond does not change the Li+ carbonate distance or deform the tetrahedral arrangement. In addition, the presence of rotamers should be favored energetically due to their contribution to the overall entropy of the system.

0.0 0.8

(b)

X(Li+)=0.09 X(Li+)~0.15

0.6

1/Slope

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.4 0.2 0.0 0

1

2

3

4

5

TW (ps) FIGURE 7. (a) Inverse CLS of the coordinated carbonyl stretch mode for BC and (b) inverse slope of the coordinated carbonyl stretch mode for DMC electrolytes at two different LiPF6 concentrations as a function of waiting time, Tw.

TABLE 2. Frequency decorrelation times, 1, and vertical offsets, y0, for the coordinated carbonyl stretch modes of BC and DMC at different LiPF6 concentrations. x(Li+)

BC

DMC

1(ps)

y0

1(ps)

y0

0.09

12.8± 0.1(a)

0

1.4±0.1

0.25±0.01

0.14-0.19

4.3±0.5

0.09±0.03

2.3±0.2

0.17±0.02

The anomalous trend found in the FFCF of the BC coordinated peak (i.e., faster dynamics at higher concentration of Li+) is caused by the occurrence of CC_CIP which have a second transition in the same region as presented in the previous section. The fast FFCF dynamics observed for the coordinated carbonyl band at high Li+ concentration is caused by the overlap of multiple bands from the free ion and the CC_CIP, which affects the metrics used to evaluate the FFCF dynamics.77-

(a)This decorrelation time was extrapolated from a linear fit of the CLS. Note that fitting with an exponential decay gives a similar decay time, but with a larger uncertainty in the time constant.

The solvation shell defines the instantaneous frequency of the vibrational transitions. As presented in the first section, the Li+ solvation shell is composed of four carbonate molecules forming a tetrahedron. The FFCF of the coordinated carbonyl stretch band from a tetrahedral

78

CONCLUSIONS In summary, the structure around Li+ in organic carbonate-based electrolytes at lithium ion battery 11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 25

5. Xu, K., Nonaqueous Liquid Electrolytes for LithiumBased Rechargeable Batteries. Chem. Rev. (Washington, DC, U. S.) 2004, 104 (10), 4303-4418. 6. Wang, J.; Yamada, Y.; Sodeyama, K.; Chiang, C. H.; Tateyama, Y.; Yamada, A., Superconcentrated Electrolytes for a High-Voltage Lithium-Ion Battery. Nat. Commun. 2016, 7, 12032. 7. Yamada, Y.; Yamada, A., Review—Superconcentrated Electrolytes for Lithium Batteries. J. Electrochem. Soc. 2015, 162 (14), A2406-A2423. 8. Moon, H.; Mandai, T.; Tatara, R.; Ueno, K.; Yamazaki, A.; Yoshida, K.; Seki, S.; Dokko, K.; Watanabe, M., Solvent Activity in Electrolyte Solutions Controls Electrochemical Reactions in Li-Ion and Li-Sulfur Batteries. J. Phys. Chem. C 2015, 119 (8), 3957-3970. 9. Seneviratne, V.; Frech, R.; Furneaux, J. E.; Khan, M., Characterization of Crystalline and Solution Phases of Diglyme−LiSbF6. J. Phys. Chem. B 2004, 108 (24), 8124-8128. 10. Bogle, X.; Vazquez, R.; Greenbaum, S.; Cresce, A. v. W.; Xu, K., Understanding Li+–Solvent Interaction in Nonaqueous Carbonate Electrolytes with 17O NMR. J. Phys. Chem. Lett. 2013, 4 (10), 1664-1668. 11. Nie, M.; Abraham, D. P.; Seo, D. M.; Chen, Y.; Bose, A.; Lucht, B. L., Role of Solution Structure in Solid Electrolyte Interphase Formation on Graphite with LiPF6 in Propylene Carbonate. J. Phys. Chem. C 2013, 117 (48), 25381-25389. 12. von Cresce, A.; Xu, K., Preferential Solvation of Li+ Directs Formation of Interphase on Graphitic Anode. Electrochem. Solid-State Lett. 2011, 14 (10), A154-A156. 13. Qiao, H.; Fang, X.; Luan, H.; Zhou, Z.; Wu, Y.; Yao, W.; Wang, X.; Li, J.; Chen, C., Vibrational Spectroscopic and Density Functional Studies on Ion Solvation and Ion Association of Lithium Tetrafluoroborate in 4-Methoxymethyl-Ethylene Carbonate. J. Mol. Liq. 2008, 138 (1-3), 69-75. 14. Tsunekawa, H.; Narumi, A.; Sano, M.; Hiwara, A.; Fujita, M.; Yokoyama, H., Solvation and Ion Association Studies of LiBF4−Propylenecarbonate and LiBF4−Propylenecarbonate−Trimethyl Phosphate Solutions. J. Phys. Chem. B 2003, 107 (39), 10962-10966. 15. Giorgini, M. G.; Fuamatagawa, K.; Torii, H.; Musso, M.; Cerini, S., Solvation Sturcture around the Li+ Ion in Mixed Cyclic/Linear Carbonate Solutions Unvelied by the Raman Noncoincidence Effect. J. Phys. Chem. Lett. 2015, 6 (16), 32963302. 16. Hyodo, S.-a.; Okabayashi, K., Raman Intensity Study of Local Structure in Non-Aqueous Electrolye Solution-I. CationSolvent Interaction in LiClO4/Ethylene Carbonate. Electrochim. Acta 1989, 34 (11), 1551-1556. 17. Doucey, L.; Revault, M.; Lautie, A.; Chausse, A.; Messina, R., A Study of the Li/Li+ Couple in DMC and PC Solvents Part 1: Characterization of LiAsF6/DMC and LiAsF6/PC Solutions. Electrochim. Acta 1999, 44 (14), 2371-2377. 18. Kameda, Y.; Umebayashi, Y.; Takeuchi, M.; Wahab, M. A.; Fukuda, S.; Ishiguro, S.-i.; Sasaki, M.; Amo, Y.; Usuki, T., Solvation Structure of Li+ in Concentrated LiPF6-Propylene Carbonate Solutions. J. Phys. Chem. B 2007, 111 (22), 6104-6109. 19. Morita, M.; Asai, Y.; Yoshimoto, N.; Ishikawa, M., A Raman Spectroscopic Study of Organic Electrolyte Solutions Based on Binary Solvent Systems of Ethylene Carbonate with Low Viscosity Solvents Which Dissolve Different Lithium Salts. J. Chem. Soc., Faraday Trans. 1998, 94 (23), 3451-3456. 20. Cazzanelli, E.; Mustarelli, P.; Benevelli, F.; Appetecchi, G. B.; Croce, F., Raman and NMR Analysis of LiClO4 Concentrated Solutions in Ethylene Carbonate-Propylene Carbonate. Solid State Ion. 1996, 86-88, 379-384.

concentrations is found to be tetrahedral. The solvation shell of Li+ is found to be more rigid in cyclic organic carbonates than in their linear analogues. This difference in the structure is attributed to the presence of rotamers which contribute significantly to the stabilization energy via entropy for cyclic carbonate-based electrolytes and to the mobility/flexibility of the alkyl side chains which form a more compact solvation shell. The structure and dynamics of the Li+ solvation shell in linear and cyclic carbonate solvents are affected differently by salt concentration. High concentrations of the PF6- anion result in the formation of contact ion pairs in the cyclic carbonate electrolyte and in the formation of solvent separated ion pairs in the linear carbonate. The generation of different types of ion pairs is found to be related to the difference in solvation shell structure arising from the molecular structure of the carbonate. In addition, it is found that the cyclic carbonates are not completely free in solution due to their participation in the solvation of the anion which contrasts the behavior of the linear carbonates. The measured FFCF correlation times coincide with the physical model describing the solvation of Li+ and PF6- in linear and cyclic carbonate electrolytes.

Associated Content Supporting Information. Model of the carbonyl stretch modes for a tetrahedral solvation complex, details and results for the modeling of the FTIR data, results for the modeling of the FTIR fractional areas, 2DIR spectra of each electrolyte at X(Li+)=0.09 for k1 and k2 at a 45° polarization, geometries used in and frequency results of the DFT computations, and 2DIR spectra of the pure solvents. This material is available free of charge via the Internet at http://pubs.acs.org.

Author Information Corresponding Author. *Email: [email protected], Phone: 225-789-1780.

Acknowledgements D.G.K. gratefully acknowledges startup funds form the Department of Chemistry at LSU. The authors would also like to acknowledge the High Performance Computing Center at Louisiana State University for computer time.

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Solvation Structure and Dynamics of the Lithium ... - ACS Publications

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