Two-Dimensional Infrared Spectroscopy and Molecular Dynamics

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Two-Dimensional Infrared Spectroscopy and Molecular Dynamics Simulation Studies of Nonaqueous Lithium-Ion Battery Electrolytes JoonHyung Lim, Kyung-Koo Lee, Chungwen Liang, KwangHee Park, Minjoo Kim, Kyungwon Kwak, and Minhaeng Cho J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.9b02026 • Publication Date (Web): 10 May 2019 Downloaded from http://pubs.acs.org on May 11, 2019

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The Journal of Physical Chemistry

Two-dimensional Molecular

Infrared

Dynamics

Spectroscopy

Simulation

Studies

and of

Nonaqueous Lithium-Ion Battery Electrolytes Joonhyung Lim†,‡, Kyung-Koo Lee§, Chungwen Liang¶, Kwang-Hee Park†,‡, Minjoo Kim†,‡, Kyungwon Kwak†,‡, and Minhaeng Cho∗,†,‡ †Center

for Molecular Spectroscopy and Dynamics, Institute for Basic Science (IBS), Korea

University, Seoul 02841, Korea ‡Department

of Chemistry, Korea University, Seoul 02841, Korea

§Department

of Chemistry, Kunsan National University, Kunsan, Jeonbuk 573-701, Korea

¶Computational

Modeling Core, Institute for Applied Life Sciences (IALS), University of

Massachusetts Amherst, Amherst 01003, Massachusetts, USA

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ABSTRACT. Lithium ion battery (LIB) technology is undoubtedly indispensable to modern life. However, despite enormous and extended effort to improve LIB performance, our understanding of the underlying principles and mechanisms of lithium ion transport in nonaqueous LIB electrolytes remained limited until recently. There is a particular lack of knowledge of the microscopic solvation structures and fluctuation dynamics around charge carriers in real electrolytes. Typical electrolytes found in commercially available LIBs consist of lithium salts and mixed carbonate solvents, with the latter playing an essential role in promoting lithium ion transport and forming an electrically stable solid electrolyte interphase. Although a number of linear spectroscopic studies of LIB electrolytes aiming at understanding the complex nature of lithium ion solvation processes have been reported, the notion that each lithium ion is strongly solvated by carbonate molecules to form a long-lasting solvation sheath structure has remained the subject of intense debate. Here, we present the results of FTIR, fs IR pump-probe, two-dimensional IR spectroscopy, and molecular dynamics simulations reported by us and others and discuss the possible interplay of picosecond solvation dynamics and macroscopic ion transport processes within the framework of the fluctuation-dissipation relationship. Further, by measuring the time-dependent fluctuations and spectral diffusions of carbonate carbonyl stretch modes that act as excellent infrared probes for the local electrostatic environment, we show that lithium cations are not only solvated by carbonate molecules but also interact with counter anions at equilibrium depending on solvent composition. Molecular dynamics simulations support the notion that rapid chemical exchanges between carbonate solvent molecules in the first and outer solvation shells are critical for describing mobile lithium ion transport phenomena. We thus anticipate that time-resolved coherent multidimensional vibrational spectroscopy is capable of providing decisive evidence on the ultrafast solvent dynamics of various electrolytes, which is potentially helpful for designing improved and more efficient LIB electrolytes in the future.


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I. INTRODUCTION Efficient storage and usage of electrical energy have been achieved with electrochemical reactions that facilitate the conversion between chemical and electrical energy. Of the various electrochemical cells devised, the lithium ion battery (LIB) is the most widely used in modern life because of its high power to mass ratio, which is essential for portable electronics and electric transportation systems.1, 2 The electrochemical reactions during LIB operation can be separated into redox reactions on the electrodes and Li+ ion migration between the electrodes through electrolyte solution. Much research has focused on developing new electrode materials to increase energy density.3-9 However, electrolytes composed of lithium salts and carbonate solvent have received less attention despite being crucial for lithium cation transport between electrodes and in direct contact with the electrodes undergoing redox reactions. To facilitate fast Li+ ion transport, LIB electrolytes should satisfy two criteria, high lithium salt solubility and low viscosity. However, since satisfying these two conditions is often a tradeoff, finding high performance electrolytes for commercial LIBs has required a tedious optimization procedure and been achieved by trial and error. One of the most popular and widely used electrolytes in commercial LIBs consists of Li salt, often LiPF6, dissolved in organic solvent10, such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylmethyl carbonate (EMC), or a mixture of them. Organic carbonate molecules have notable advantages as LIB electrolyte solvents due to their wide electrochemical window of stable operation given the high voltages of LIBs (> 3.0 V). Furthermore, the products formed by the sacrificial electrolysis of carbonate molecules on the electrodes serve as the building blocks of the solid electrolyte interphase (SEI)11, 12, which is crucial for preventing further electrolyte solvent decomposition and for offering long-term LIB stability.13 However, the molecular level working principles of 3 ACS Paragon Plus Environment


Li+ ion transport in various electrolytes has only recently been revealed through ultrafast vibrational spectroscopic and computational studies. In particular, to elucidate the mechanism of ion transport through such LIB electrolytes, efforts have been made to connect the solvation structure of Li+ ions in carbonate solvents with macroscopic ion conductivity. The ionic conductivity of electrolytes is determined by the concentration and mobility of charge carriers. As the concentration of lithium salt dissolved in a solvent increases, the number of charge carriers and the ion conductivity increase proportionally. However, past a certain concentration of lithium salt, further addition lowers ionic conductivity because increased solution viscosity due to strong intermolecular interactions between ions in electrolyte solution decreases the mobility of Li+ ions.14 Although cyclic carbonate solvents with high dielectric constants can better dissolve lithium salt and produce more free charge carriers, their high viscosity also suppresses rapid lithium ion transport.10 Consequently, for lowering overall carbonate electrolyte viscosity, mixtures of cyclic and linear carbonate solvents have been considered the best optimized electrolytes for LIBs. For instance, an electrolyte comprised of 1 M LiPF6 in a mixture of cyclic and linear carbonate solvents shows significantly higher lithium ionic conductivity than that comprised of 1 M LiPF6 in either pure cyclic or pure linear carbonate solvent.15-17 The ionic mobility () is the velocity attained by an ion moving through a medium under unit electric field and it is determined by the charge (ze), viscosity (η), and hydrodynamic 𝑧𝑒

radius (a) as 𝜇 = 6𝜋𝜂𝑎. Here, the hydrodynamic radius is the effective radius of an ion in solution taking into account all the solvent molecules that move together with the ion. For LIB electrolytes, it is generally thought that a rigid solvation shell is formed because of strong electrostatic interactions between the Li+ ions and polar organic solvent molecules. The size of a single solvation shell is a molecular scale factor determining ionic mobility, so understanding 4 ACS Paragon Plus Environment

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the structure of the solvation shell around each Li+ ion is essential for understanding the relationship between solvent characteristics and ionic conductivity. However, spectroscopic techniques capable of providing information on solvation structure and its fluctuation time scale are scarce. Linear vibrational spectroscopy such as FTIR and Raman scattering can only provide time- and ensemble-averaged electrolyte solvent molecule and anion spectra, which cannot reflect the highly inhomogeneous and fluctuating environments of electrolyte solutions. To completely characterize solvation structures around Li+ ions, we need to determine (i) the coordination site of carbonate molecules, either carbonyl oxygen atom or one of the two etherial oxygen atoms, in the first solvation shell, (ii) number of carbonate molecules and counter anions in the first solvation shell, and (iii) the time scales of solvation and chemical exchange dynamics of surrounding carbonate molecules. In fact, the ensemble average solvation structure around Li+ ions is keenly related to the average population of contact ion pairs (CIP) between Li+ and PF6− in the first solvation shell. At a low lithium salt concentration, each Li+ ion is fully solvated by carbonate molecules. However, as lithium salt concentration increases, CIP mole fraction would increase especially in the solutions of linear carbonate having relatively small dielectric constant. In commercial LIBs, mixed solvent electrolytes with both linear and cyclic carbonates are used. Then, it becomes even more complicated to elucidate detailed solvation structures around Li+ ions because the actual chemical composition of carbonate molecules in the first solvation shell is different from the macroscopic mixing ratio of the linear and cyclic carbonate solvents. Again, it should be emphasized that correctly identifying the coordination site and the number of solvent molecules in the first solvation shell is important for calculating Li+ ionsolvent cluster size and Li+ ion desolvation energy. The former is directly related to ionic mobility and the latter is crucial for estimating the activation energy of Li+ transport between 5 ACS Paragon Plus Environment


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electrolyte and electrode. Furthermore, note that the system is very heterogeneous in all these aspects due to salt-concentration-dependent CIP formation, the conformational flexibility of alkyl groups in linear carbonate molecules, the composition of solvent and counter anions in the first solvation shell around Li+ ions, all of which causes cluster size variation in LIB mixed solvent electrolytes. Thus, to establish the relationship between microscopic solvation structure and macroscopic ionic conductivity, experimental methods need to be able to provide quantitative information on the solvation structure and dynamics of such heterogeneous systems. Recently, we have shown that time-resolved IR pump-probe and 2D IR spectroscopic techniques are suitable for addressing most of these questions. Thus, in this Feature Article, we will briefly review previous reported work and then describe our results, which provide detailed information on solvation structural dynamics and chemical exchange processes. We will present experimental FTIR and time-resolved IR spectroscopy (2D IR and IR PP) results on Li+ ion solvation structures in organic carbonate solvents, and then our DFT/MD simulation results. Perspectives and a few concluding remarks will be given in the last section VI.

II. PREVIOUS STUDIES A vast number of studies have focused on measuring macroscopic properties, such as the viscosity and glass transition temperature (Tg) of LIB electrolytes, with varying compositions of lithium salts and solvents18, 19, to establish a quantitative relationship between electrolyte composition and ionic conductivity. The general conclusion is that the microscopic solvation structure and electrostatic interactions between Li+ and carbonate molecules are important for determining the ionic conductivity of a given lithium salt electrolyte, as expected. 6 ACS Paragon Plus Environment

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To gain more insight into the detailed solvation structure surrounding Li+ ions, linear spectroscopic methods have been widely used. Because carbonate molecules have two nucleophilic sites that can interact with Li+ ions, both their carbonyl (C=O) oxygen atom and their ethereal oxygen atoms (O-C-O), the intensities and frequencies of the C=O stretch and OC-O asymmetric stretch modes extracted from FTIR spectra were useful for studying the Li+binding sites of given carbonate molecules. Previous spectroscopic investigations20-22 showed that Li+ preferentially interacts with the carbonyl oxygen atom of carbonate molecules. Bogle et al. carried out 17O NMR measurements of the chemical shifts of the ethereal oxygen and the carbonyl oxygen atoms of EC and DMC at various concentrations of LiPF6 in EC/DMC mixed solvent.23 Their experimental results supported the notion that the carbonyl oxygen atom (C=O) strongly interacts with a Li+ ion, consistent with lithium salt concentration-dependent vibration spectroscopy and DFT calculation studies. Furthermore, IR24-27 and Raman22,

26-32

experiments were conducted with various

electrolyte systems to study the coordination environment around the Li+ ions. As a result, the coordination number of each Li+ ion was posited as being 4-6 depending on the concentration of lithium salt and chemical composition of electrolyte solvent. However, the exact solvation structure is still subject to intense debate and controversy. This is mainly because of the difficulty of clearly resolving different solvation structures hidden under broadly distributed (i.e. heterogeneous) vibrational transition bands using experimental methods featuring limited time and frequency resolution. Soft electro-spray ionization mass spectrometry (ESI-MS) of LiPF6 in EC/EMC mixed solvent was used to study the solvation structure of Li+ ions.33 The ionization of LIB electrolytes using ESI produces only 1st solvation shells featuring tightly bound Li+ ions and carbonate solvent molecules, which were analyzed by mass spectrometer. The study suggests 7 ACS Paragon Plus Environment


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that cyclic carbonate molecules are the dominant species found in the 1st solvation shell. However, since the solvation shell of the Li+ ions could have been significantly perturbed by the ionization process, it was argued that the ESI-MS experimental results might not represent the real solvation structure of Li+ ions in electrolyte solutions. Independently, it is interesting that the

17O

NMR study of 1.0 M LiPF6 electrolytes in mixed solvent23 provided critical

evidence that the observed chemical shifts of the carbonyl oxygen atoms in EC and DMC with changes in the mixing ratio can be explained assuming the 1st solvation shell is mainly composed of EC not DMC. However, using attenuated total reflection (ATR) IR measurements of various LiPF6 solutions, Lucht34 found that DMC molecules can also participate in the 1st solvation shell of Li+ ions, despite their dielectric constants being quite low compared to EC. Again, due to the limited sensitivity of these techniques to solvation structure, these spectroscopic results were unable to support decisive conclusions about Li+ ion solvation structure. Furthermore, despite NMR having an exceptional ability to measure changes in the local environment affecting electron densities around nuclear spins, it has limited timeresolution capacity and so is not useful for studying ultrafast solvation dynamics and the chemical interconversion processes between states separated by small energy barriers. To study the various properties of LIBs, over the past two decades extensive computer simulations have been performed complementing experimental observations. Especially, much attention was paid to the details of the underlying mechanisms in electrolytes35 that govern the efficiency and performance of LIBs, such as ion-electrode interactions36-38, solid-electrolyte interphase formation39-41, and ion solvation structure and dynamics42-44. Two major modeling approaches have commonly been employed to study LIB systems: electronic structure calculation and molecular dynamics simulation.


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To elucidate the solvation structure of Li+ ions in electrolytes, DFT calculations were performed and showed that the most energetically favorable configuration is a tetrahedral solvation structure of carbonate molecules around Li+ ions.45-48 However, such quantum chemistry calculations were carried out for gas-phase clusters meaning that neither long-range intermolecular interactions with solvent molecules nor entropic effects were taken into consideration properly.49 Although certain implicit solvent models50,

51

were used to

approximately take into account the interactions between solute and solvent molecules, they could not quantitatively describe specific interactions such as ion-dipole terms. Therefore, the energetically favorable configuration according to quantum chemistry calculation may not be the most populated one experimentally observed and studied. Furthermore, oftentimes energetically less favorable configurations play an important role in ion transportation. Thus, any conclusion drawn from only quantum chemistry calculations, which reflect a limited number of Li+-carbonate cluster species, needs to be made with caution. Indeed, recent DFT calculations have revealed that the energy differences between various solvation structures around Li+ ions in LIB electrolytes could be well below 5 kcal/mol even when employing the continuum solvation model.46 This clearly indicates that energetically less favorable structures may be non-negligibly populated. To carry out accurate MD simulations, classical force fields are commonly chosen to model a system compatible with experimental conditions, in terms of size, concentration, and timescale. However, in the description of classical force fields, electric polarization, which is crucial to determining solvation structures, is generally treated in a mean-field way. Ignoring instantaneous polarization effects would result in a biased solvation structure distribution as well as ion mobility. A recent simulation study52 of highly concentrated electrolytes pointed out that reproducing experimentally measured ion mobility requires enhancing ion dynamics 9 ACS Paragon Plus Environment


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by modifying the partial charges of lithium cations and counter anions. However, generally such modifications lack rigorous theoretical support. Applying polarizable models or ab initio approaches to molecular dynamics simulations will ultimately allow the problems mentioned above to be overcome. However, these approaches are currently too expensive and computationally demanding to be applied to systems comparable to the experimental conditions of LIB electrolytes. Therefore, developing new models that efficiently account for instantaneous polarization will greatly enhance the accuracy of MD simulations of LIB systems. It is by now accepted that electrolytes are highly heterogeneous solutions with a broad distribution of solvation structures. Spectroscopic methods with sufficiently high time resolution have thus been used to separately observe individual solvation structures, especially using molecular spectroscopic probes highly sensitive to the details of configurations of the solvation shells around Li+ ions. Although time-resolved IR spectroscopy, particularly pumpprobe, is one of the most useful techniques with fs time resolution, spectral congestion impedes the separate observation of each solvation structure in the frequency domain. Two-dimensional infrared spectroscopy (2DIR) can spread spectral information across two-dimensional frequency space and provide information on the time-correlation of different frequency components. In addition, ultrafast solvent dynamics known as spectral diffusion can be traced by extracting the frequency-frequency correlation function from nodal53 or center line slope54, 55

analyses of the waiting time-dependent 2DIR spectra. Furthermore, the time-dependent rise

and decay of cross peak intensities and changes in diagonal peak intensities and shapes provide crucial and direct information about the equilibrium dynamics of solvation structure changes in the 1st solvation shell around Li+ ions. In the present Feature Article, we will show how 2DIR and IR-PP spectroscopy combined with MD simulation can be used to reveal solvent chemical 10 ACS Paragon Plus Environment

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exchange dynamics and to investigate the heterogeneity of solvation structures in LIB electrolyte solutions.

III. LINEAR SPECTROSCOPY Vibrational spectroscopy is the primary research tool for studying the solvation structures of Li+ ions. In particular, certain vibrational modes of carbonate molecules and anions interacting with Li+ cations are sensitive probes to the local environment around Li+ ions. Previous IR experiments27, 34, 56 adopted an attenuated total reflection (ATR) method to avoid the saturation of vibrational absorption bands (C=O and O-C-O stretching) in carbonate electrolytes. However, because IR pump-probe (IR-PP) and 2DIR measurements usually require the detection of transmitted IR radiation, we needed to have an ultrathin sample cell (approximately 1 µm thickness), which was prepared using radio-frequency (RF) magnetron sputtering technique.57 With this home-built IR cell, the optical densities of C=O stretch and O-C-O asymmetric stretch modes could be set at about 0.3, which is suitable for time-resolved IR spectroscopic measurements. The FTIR spectra of the C=O and O-C-O stretching modes of DEC in LiPF6 electrolyte solutions are shown in Figures 1a and 1b, respectively. As the concentration of LiPF6 in DEC increases, the shoulder peaks at 1715.4 cm-1 in the C=O stretch IR band and 1305.6 cm-1 in the O-C-O asymmetric stretch IR band increase with simultaneous decreases in the peak intensities at 1747.0 cm-1 (C=O) and 1258.9 cm-1 (O-C-O). The new peaks appearing in LiPF6/DEC solutions were assigned to those stretch modes of DEC molecules interacting with Li+ ions. Our DFT calculations showed that such simultaneous frequency shifts of C=O and O-C-O stretch modes upon electrostatic interaction of DEC with



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Li+ ions can only be induced when the carbonyl oxygen atom of DEC interacts with Li+ ions.23, 29, 58

However, at high concentrations of LiPF6 it is still possible for Li+ ions to interact with ethereal oxygen atoms. Furthermore, in polymer electrolytes like poly ethylene oxide (PEO), it was found through FTIR spectroscopy that Li+ ions strongly interact with ethereal oxygen atoms.59 DFT calculations show that, if Li+ forms additional electrostatic bidentate interactions with the two ethereal oxygen atoms of a single carbonate molecule whose carbonyl oxygen atom is already interacting with Li+, the overall frequency shifts of the C=O and O-C-O bands become very small. From this, we expect that the absorption peaks at 1715.4 cm-1 (C=O···Li+) and 1305.6 cm-1 (O-C-O of DEC in C=O···Li+ state) become weaker and weaker as lithium salt concentration becomes higher than the saturation concentration at which there are no more carbonyl groups available for electrostatic interaction with Li+ ions. To verify this prediction, it is assumed that a single Li+ ion is fully solvated by four carbonyl groups adopting a tetrahedral geometry. Then, at the limit of complete dissociation of dissolved LiPF6, the increase of C=O peak intensity upon increasing Li+ concentration should reach its maximum at a 0.2 mole fraction of lithium salt. As the mole fraction of lithium salt increases beyond this estimated value, any further increase in Li+ ion concentration should result in a decrease of the low-frequency carbonyl stretch (C=O···Li+) band intensity and a simultaneous increase of the high-frequency free C=O stretch IR band intensity because the population of DEC molecules with both carbonyl oxygen and ethereal oxygen atoms interacting with two different Li+ ions increases. In Figure 1c, we plot the percent ratio of the integrated C=O stretch IR band of the DEC···Li+ complex to that of free DEC, which is referred to as area fraction in the figure, with respect to lithium salt concentration. Inconsistent with the above hypothesis based on tetrahedral geometrical solvation shell formation, the peak area ratio exhibits almost linear and 12 ACS Paragon Plus Environment

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monotonic increase beyond a mole fraction of 0.2. The linear line in Figure 1c indicates that the solvation structure of DEC around Li+ ions does not undergo an abrupt change with increasing lithium salt concentration. As described in the following section, CIP formation at high LiPF6 concentrations reduces the average number of DEC molecules in the first solvation shell around Li+ ions, which may increase the saturation mole fraction of the carbonyl oxygen site beyond 0.2. Thus, within the experimental lithium salt concentration range, it is safe to ignore the possibility of the formation of a strong electrostatic interaction between Li+ ions and the ethereal oxygen atoms of DEC. To estimate the average number of DEC molecules in the first solvation shell surrounding the Li+ ion, we examined in detail the carbonyl stretch IR spectra with respect to lithium salt concentration. In Figure 1d, the ratio of the free C=O band area to the lithiumcomplex C=O band area is plotted with respect to the inverse of Li+ ion concentration. Assuming that the absorbance is linearly proportional to the square of the transition dipole moment and the number of molecules and the transition dipole moment of the C=O stretch mode do not depend on lithium salt concentration, one can easily obtain the average coordination number nLi of the Li+ ions via linear fitting analysis of the area ratio values to the linear equation for 1/CLi given in Figure 1d. From this least square fitting analysis, we found the average number of DEC molecules in the first solvation shell around Li+ ions to be about 3.3. Noting that the average coordination number of each Li+ ion is approximately four due to its formation of tetrahedral solvation structure with carbonate molecules and/or counter anion, it is believed that lithium cations forming direct contact ion pairs with PF6− anions coexist with those fully solvated by DEC molecules only. Indeed, this is consistent with the previous experimental results proving the presence of CIP between Li+ and PF6− in DEC solutions due to the low polarity (dielectric constant) of the DEC molecule.25,

56

It is expected that the 13

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population of CIP increases with Li+ ion concentration, suggesting that one of the coordination sites in the first solvation shell around the Li+ ion is occupied by a PF6− ion. This estimated coordination number differs from those previously reported. For example, an NMR study23 revealed that the Li+ coordination number ranges from 6 to 10, depending on the concentration of Li+, carbonate solvent, and counter anion. Our estimated coordination number is rather in good agreement with that reported in one of the ATR based IR study,34 even though such ATRIR spectroscopy is only capable of providing information on solvation structures around Li+ ions that are close to the surface of the ATR crystal. In this regard, it is believed that our FTIR spectroscopy using an exceptionally thin sample cell is better suited to the study of the bulk solution properties of LIB electrolytes. Instead of carbonate C=O and O-C-O stretch modes, FTIR data on the P-F asymmetric stretch modes of PF6− are also useful for investigating CIP formation between Li+ and PF6− ions, because their frequencies are quite sensitive to local environments around PF6−. We measured the P-F asymmetric stretch IR spectra for lithium salt concentrations from 0.25 to 2.0 M. The P-F stretching vibrations appear in the frequency range from 800 to 900 cm-1. Thus, any sample cells composed of a CaF2 window cannot be used because of their strong absorption of IR photons (< 1200 cm-1) by CaF2. Therefore, we built an IR sample cell out of NaCl since NaCl crystal can provide a transparent region down to 700 cm-1. The P-F stretch IR spectra of LiPF6/DEC solutions are shown in Figure 2. The absorption spectrum exhibits two prominent peaks at 845 cm-1 and 870 cm-1. Furthermore, there is a peak at 890 cm-1 whose magnitude increases with increasing lithium salt concentration. Previous studies using ATR-IR and DFT calculations revealed that various ion species including CIP can contribute to the P-F stretch IR band of PF6− in carbonate LIB electrolyte solutions.60, 61 Although it becomes clear that the P-F stretching vibrations of PF6− are sensitive to its solvation state, the spectral congestion of 14 ACS Paragon Plus Environment

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the P-F stretch IR bands prohibits simple interpretation of the FTIR results. Nonetheless, the new peak at 890 cm-1 is clear evidence supporting the notion that a new PF6− solvation species is generated in the electrolytes at high lithium salt concentrations and that the species is likely to be Li+··· PF6− CIP. Thus, at high concentrations of lithium salt, the population of CIP species should not be ignored. This means that, if the number of solvating species, either the DEC carbonyl group or PF6−, is about 4, making the geometry of the first solvation shell close to a tetrahedral structure, the average number of DEC molecules found in the first solvation shell may be smaller than 4 because of the increased probability of CIP formation. The data accumulated clarifies the existence of three distinct DEC species: (i) DEC molecules directly interacting with Li+ solvated by DEC molecules only, (ii) those interacting with Li+··· PF6−DECn, and (iii) those that are solvated by DEC molecules only. Species (iii) has been referred to as free DEC, but the first two species (i) and (ii) are not distinguishable by FTIR spectra so are denoted together as Li+···DEC complex. However, DFT calculations have shown that the C=O stretch frequency of (i) is slightly smaller than that of (ii).

From these

FTIR studies and DFT calculations, the two bands at 1745 cm-1 and 1715 cm-1 can be assigned to the C=O stretch mode of free DEC molecules and that of Li+···DEC complexes, respectively. To further investigate the vibrational properties of these solvation structures and their chemical exchange kinetics, we carried out time-resolved IR spectroscopy.

IV. INFRARED PUMP-PROBE SPECTROSCOPY Polarization selective IR pump-probe spectroscopy is an excellent time-domain technique for measuring the vibrational lifetimes and rotational relaxation rates of molecules in condensed phases. Vibrational lifetimes are sensitive to not only molecular structure and 15 ACS Paragon Plus Environment


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conformation but also the intermolecular interactions with neighboring solvent molecules. If two or more species with distinctively different solvation structures have different vibrational lifetimes, they can be distinguished in time-resolved IR PP spectra. Therefore, time-resolved IR PP spectra can provide information about heterogeneously distributed molecules in condensed phases. Furthermore, from the Stokes-Einstein-Debye equation for the rotational diffusion constant, the rotational relaxation time constant is inversely proportional to the effective volume of the rotator. If it is not just a single molecule but a cluster of molecules due to strong intermolecular-interaction-induced formation of a solvation complex, the measured rotational relaxation time extracted from the analysis of the anisotropic IR PP signal can provide critical information about the approximate size of each solvent cluster around Li+. In particular, the number of solvent molecules in the first solvation shell of Li+ can be estimated from the effective hydrodynamic volume of solute (Li+)-solvent clusters assuming that the shape of the solvation shell is approximately spherical.62 Prior to the recent timeresolved IR studies of LIB electrolytes, FTIR and Raman scattering spectroscopy was used to quantify the number of carbonate solvent molecules in the first solvation shell, assuming that the composition of solvent molecules and counter anions in the first solvation shell remains the same throughout a range of salt concentrations. However, as mentioned in Section III, the solvation structure itself varies with lithium salt concentration. Thus, IR PP, which can separate different species by their differing vibrational lifetimes, is useful for investigating changes in the solvation structure and composition of constituent species with respect to lithium salt concentration. Figures 3a and 3b show the probe frequency-resolved pump-probe spectra of the C=O stretch modes in pure DEC and 1.0 M LiPF6 DEC solution, respectively. A positive signal, which indicates that the experimentally measured probe beam intensity in the presence of 16 ACS Paragon Plus Environment

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preceding pump pulse is stronger than that in the absence of pump pulse, originates from pump beam-induced ground state bleaching (GSB) and probe beam-induced stimulated emission (SE) from vibrationally excited states. On the other hand, a negative IR PP signal means that the experimentally measured probe beam intensity is weaker when the system interacted with pump pulse before than when did not, and this effect can be produced by the excited state absorption (ESA) of the probe beam that creates a population in a second vibrationally excited state. As can be seen in Figure 3a, the positive GSB and SE signals are contaminated by local heating contributions over long waiting times. Therefore, to measure the vibrational lifetime of the first excited state accurately, numerical fitting analyses were performed considering the time-dependent change of the negative ESA signal. Figures 3c and 3d are the isotropic IR PP signal of the C=O stretch mode (1726.5 cm-1) of pure DEC liquid and that (1696.2 cm-1) of 1.0 M LiPF6 DEC solution, respectively. A single exponential function fits the decay signals well and the corresponding vibrational lifetimes are found to be 2.1 ps for DEC in pure DEC liquid and 1.1 ps for DEC in Li+···DEC complexes. The vibrational energy of the excited C=O stretch mode can be transferred to other intramolecular modes including O-C-O symmetric and asymmetric stretching vibrations. The C=O and O-C-O stretching modes share the same carbon atom so vibrational couplings between them through potential anharmonicity are likely to be large. Therefore, the notable difference in the vibrational lifetimes of C=O stretch modes in free DEC and DEC in Li+···DEC can be understood by noting that, upon the formation of Li+···DEC complexes, the vibrational frequencies of stretch modes are shifted. The C=O stretch mode is red-shifted by 31.6 cm-1 from 1747.0 cm-1 (free DEC) to 1715.4 cm-1 (Li+···DEC) and the O-C-O stretch mode is blueshifted by 46.7 cm-1 from 1258.9 cm-1 (free DEC) to 1305.6 cm-1 (Li+···DEC). That is to say, the frequency difference between the two modes significantly decreases from 488.1 cm-1 to 17 ACS Paragon Plus Environment


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409.8 cm-1 as an Li+ ion forms an electrostatic interaction with the carbonyl group. This may increase the rate of intramolecular vibrational energy transfer from the C=O stretch mode to the O-C-O stretch mode. As shown in our DFT calculation results (Figure 4), the calculated frequencies of the C=O and O-C-O stretching vibrations are highly dependent on the distance between Li+ and carbonyl group as well as the angular geometry of the Li+···DEC complex. Thus, detailed knowledge of the solvation structure is likely necessary to understand the mechanism of the vibrational energy relaxation processes in detail. An important clue about solvent dynamics in the vicinity of Li+ ions can be extracted from anisotropic IR PP data. The average rotational relaxation time constant of free DEC molecules is found to be 1.5 ps, whereas that of DEC in Li+···DEC complexes is significantly longer, i.e., 8.5 ps. From the Stokes-Einstein-Debye equation, this experimental observation suggests that the effective volume of an Li+···DEC solvation complex is about 5.7 times larger than that of DEC without Li+ ion interaction. Zheng and coworkers tried to estimate the size of the solvation structure by analyzing their experimentally measured orientational relaxation times.62 However, it remains a difficult task because of the presence of various ion pair states. Although the IR PP results, i.e., vibrational and rotational relaxation rates, provide invaluable information about the solvation structure around Li+ ions, it has been shown that twodimensional IR spectroscopy is indispensable for further elucidation of the ultrafast dynamics within the first solvation shell around the Li+ ion.

V. TWO-DIMENSIONAL INFRARED SPECTROSCOPY 2D IR spectroscopy is now a mature technique that enables the measurement of the time-correlation function of the frequency of an IR oscillator interacting with probe beam after 18 ACS Paragon Plus Environment

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a waiting time Tw later with that of an IR oscillator excited by a pump beam at time zero. The diagonal and cross peak intensities in a series of time-resolved 2D IR spectra directly provide information such as on state-to-state energy transfers and conformational transitions between multiple conformers with different resonant frequencies. In particular, because the resonant frequency of a given IR oscillator is sensitive to changes in molecular structure and the surrounding environment, tracking frequency shifts and cross peak intensities in the time domain allows the study of chemical exchange dynamics in condensed phases. Figure 5a shows a set of representative normalized 2D IR spectra of 1.0 M LiPF6 in DEC at four different waiting times. At a short waiting time (Tw=0.3 ps), the positive diagonal peak in the 2D IR spectrum is elongated diagonally, indicating that both free DEC molecules and DEC molecules in Li+···DEC complex are structurally heterogeneous. The IR, Raman, and DFT calculation studies63 showed that the linear carbonates can have cis-cis and cis-trans conformers in LIB electrolyte solutions. Perhaps, such conformational distribution is related to the spectral inhomogeneity found in the 2D IR spectrum. Another origin of the spectral inhomogeneity causing the diagonally elongated spectral shapes of diagonal peaks may be the variation in the numbers of coordinated carbonate molecules and counter anions in the first solvation shell. As expected, as the waiting time increases the diagonal peaks become less and less diagonally elongated and approach round shapes due to spectral diffusion and random transitions among heterogeneous structures. One of the most notable features in the 2D IR spectra of this LIB electrolyte solution is that the cross-peak intensities increase with waiting time, Tw. This can be interpreted by considering two different mechanisms. The cross peaks could result from vibrational energy transfers between the C=O stretch normal modes delocalized over the coupled carbonate molecules in the first solvation shell. They could also result from the chemical exchanges 19 ACS Paragon Plus Environment


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between the DEC molecules outside the first solvation shell and the DEC molecules in the first solvation shell forming the Li+···DEC complex. If the vibrational energy transfer mechanism is the dominant process responsible for cross peak generation, one might expect the C=O stretch vibrational lifetimes of the DEC molecules in the Li+···DEC complex to depend on the concentration of LiPF6 salt because the participation of PF6− as a coordinating molecule around each Li+ ion breaks the rigid carbonate molecule solvation structure. Therefore, a series of IR PP measurements were performed varying lithium salt concentration and the ESA contributions to the IR PP signals quantitatively analyzed. It turned out that the vibrational lifetimes of free DEC and DEC in the Li···DEC complex are 2.1 ps and 1.1 ps, respectively, and that they do not change with respect to lithium salt concentration. Thus, it is possible to rule out the vibrational energy transfer mechanism as an explanation of the picosecond dynamics found in the 2D IR spectroscopy of 1.0 M LiPF6 in DEC. On the other hand, the chemical exchange mechanism assumes the molecular exchange of DEC molecules in and out of the first solvation shell surrounding the Li+ ion. The volume fitting analyses with Gaussian functions were performed (Figure 5b) to quantitatively analyze the cross peak intensity changes suggested that the time constants associated with the breaking of Li+···DEC and the making of Li+···DEC are 2.2 ps and 17.5 ps, respectively. Here, it is worth noting the recent work by Kuroda and coworkers.25, 64 They carried out time-resolved IR spectroscopic studies of LIB electrolytes and suggested that vibrational coupling between the carbonyl groups of the first solvation shell is solely responsible for the growth in the experimentally measured 2D IR cross peaks. Their conclusion was drawn from the assumption that the first solvation shell around the Li+ ion is very rigid with four carbonate carbonyl groups forming a nearly perfect tetrahedral geometry. Therefore, C=O stretch normal modes delocalized over the four strongly coupled carbonyl oscillators are spontaneously 20 ACS Paragon Plus Environment

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formed with non-degenerate frequencies and they contribute to the two diagonal peaks in the corresponding FTIR and 2D IR spectra. Thus, the rising cross peaks originate from the vibrational energy transfer from the high-frequency carbonyl stretch normal modes to the lowfrequency normal modes. However, the assumption that the solvation shell is rigid and does not undergo any chemical exchanges during the experimental time window still requires experimental and computational confirmation. If solvent exchange dynamics occur on a picosecond time scale, the thermal fluctuation of the first shell solvent molecules would make such delocalized carbonyl stretch modes rapidly localize onto each solvent molecule. This is analogous to the localization of excitonic states in coupled chromophore systems, such as lightharvesting protein complexes, where delocalized excitonic states are initially excited but chromophore-solvent interactions and fluctuations localize them to an individual pigment. Although the electrolyte system of LiPF6 dissolved in DEC solvent is an excellent model solution for 2D IR studies because the carbonyl stretch modes of free DEC and Li+···DEC species are well separated in FTIR and 2D IR spectra, such pure DEC solvent is not actually used in commercial LIBs. A more realistic LIB electrolyte is a mixture of linear and cyclic carbonates where the cyclic carbonate molecules with larger dipole moments are likely to preferentially participate in the solvation of Li+ ions. Figure 6 shows the carbonyl stretch IR spectra of 1.0 M LiPF6 in DEC:PC (=1:1 in volume) mixed solvent (black line), 1.0 M LiPF6 in DEC (red line), and 1.0 M LiPF6 in PC (blue line). The spectrum of 1.0 M LiPF6 in DEC:PC mixed solvent exhibits four discernible peaks at 1725, 1760, 1780, and 1800 cm1 and their intensities were found to depend on lithium salt concentration. By comparing these peak positions with the two carbonyl stretch modes of DEC and PC, it is possible to assign these four peaks to the C=O stretch modes of DEC in Li+···DEC complex, free DEC, PC in Li+···PC, and free PC, respectively. In fact, the observation that there are four distinctive peaks in the 21 ACS Paragon Plus Environment


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FTIR spectrum of 1.0 M LiPF6 in DEC:PC mixed solvent is strong evidence that even in this mixed solvent system DEC molecules can participate in the formation of the first solvation shell around Li+. Recent DFT calculations showed that the carbonyl stretch mode undergoes such a large red-shift only when the carbonyl oxygen atom of DEC is within 3 Å of Li+. The old notion was that, in mixed PC/DEC electrolyte solutions, PC molecules, due to their strong dipole moment and high dielectric constant, form a rigid and strong solvation sheath around each Li+ ion and that linear carbonate molecules such as DEC just serve as a medium for Li+···PCn solvation sheath transport. However, this idea is contradicted by the appearance of the red-shifted carbonyl stretch band of DEC at 1.0 M LiPF6 concentration. For the 2D IR study, instead of a DEC:PC mixture, we used DMC:PC (1:1 in volume) mixed solvent electrolyte because the carbonyl stretch vibrational lifetime of DMC is longer than that of DEC enabling us to monitor diagonal and cross peak intensity changes for longer waiting times. Figure 7a shows the FTIR spectra of 1.0 M LiPF6 in DMC:PC (1:1 in volume) mixed solvent, 1.5 M and 2.0 M LiPF6 in DMC:PC (1.5:1 in volume). The C=O stretching bands of free PC and free DMC appear at 1800 cm-1 and 1760 cm-1, respectively, whereas those of Li···PC and Li···DMC appear at 1774 cm-1 and 1730 cm-1, respectively. To further elucidate the solvent dynamics of such a mixed solvent electrolyte, we carried out 2D IR measurements on LiPF6 in DMC:PC (1:1) mixed solvent. Figure 7b shows four 2D IR spectra of 1.0 M LiPF6 in DMC:PC (1:1) mixed solvent at different waiting times. In addition, we also obtained the 2D IR spectra of 1.5 and 2.0 M LiPF6 solutions. They exhibit increasing cross peaks (see the 2D IR features in the red box, Figure 7b) between free DMC and Li+···DMC complex, indicating the chemical exchange of DMC molecules in and out of the first solvation shell. Interestingly, the cross peaks between free PC and Li+···PC complex could not be identified within our experimental time (Tw) range. This can be explained by 22 ACS Paragon Plus Environment

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strength of the electrostatic interactions between Li+ and PC molecules in the first solvation shell. In summary, we found experimental evidence that the lithium solvation shell consists of not only cyclic carbonate (PC) molecules but also linear carbonate (DMC and DEC) molecules. The Coulomb interaction making the Li+···PC complex stable is found to be sufficiently strong to last for ~10 ps or longer, but the strength of interaction between a lithium ion and DEC or DMC is comparatively weak so the solvation structure fluctuates due to chemical exchange processes on several to tens of picosecond time scales.

VI. MOLECULAR DYNAMICS SIMULTION STUDY 2D IR spectroscopy is undoubtedly one of the most incisive spectroscopic tools for probing detailed molecular structures and ultrafast dynamics of complex systems like LIB electrolytes. However, interpreting experimentally measured 2D IR spectra is still difficult due to spectral congestion.65, 66 Therefore, the interpretation of 2D IR experimental data strongly relies on computational spectroscopy with a state-of-the-art spectrum modeling approach. Several studies on a wide variety of complex systems have already demonstrated that the simulated spectra are in excellent agreement with those from 2D IR spectroscopy measurement.67-72 Models of the 2D IR spectra of complex systems generally apply a mixed quantum/classical scheme. In this approximation, the vibrational chromophores that involve optical excitation are treated as quantum oscillators, and the rest of the system is assumed to behave as a fluctuating classical environment. Then, in order to construct the time-dependent vibrational Hamiltonian of the system, a vibrational exciton model is used to describe how the quantum states of the vibrational chromophores evolve in a classical bath. By solving the time23 ACS Paragon Plus Environment


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dependent Schrodinger equation numerically, the optical response functions in the time domain that describe the corresponding interaction diagrams can be derived. The frequency domain spectrum is then obtainable after Fourier transformation. This protocol is described in detail in the literature.70,

71, 73-75

Following it, the structural and dynamical information of MD

simulations can be translated into spectral information, allowing the direct interpretation of and detailed explanations for the specific spectral line shapes generally measured by experiments. As mentioned in previous sections, probing the carbonyl stretching band of carbonate molecules produces 2D IR spectra of carbonate electrolyte systems showing two diagonal peaks (low and high frequencies) and two sets of off-diagonal cross peaks (Figure 5a). A positive correlation between the intensity of the cross peaks and the waiting time was observed. Intuitively, the low frequency diagonal peak can be assigned to the carbonyl groups that directly interact with lithium ions in the first solvation shell, and the high frequency diagonal peak can be assigned to those not involved with lithium ion binding.25, 64, 76 However, as briefly mentioned earlier, whether the origin and time evolution of the cross peaks are caused by vibrational energy transfer64 between delocalized carbonyl stretch normal modes or by the chemical exchange76 of carbonate molecules in and out of the first solvation shell is still under debate. A recent simulation study77 based on 2D IR modeling aimed to provide a clear understanding of which mechanism plays the dominant role in determining the cross peak signals. It indicated that the chemical exchange process involving the formation and dissociation of Li+···DEC complexes is responsible for the growth of the 2D IR cross peaks with increased waiting time. On the other hand, at very short waiting times, weak cross peak signals appear due to the vibrational coupling between the carbonyl groups of the carbonate molecules in the first solvation shell. Therefore, both vibrational coupling and chemical 24 ACS Paragon Plus Environment

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exchange contribute to different features of the 2D IR spectra. Classical MD simulations clearly confirmed that in the case of 1.0 LiPF6 in DEC solvent the increased cross peaks at long waiting times resulted from chemical exchange processes.

VII. PERSPECTIVES, SUMMARY, AND A FEW CONCLUDING REMARKS In this feature article, we have demonstrated how the complex solvation structure and ultrafast solvation dynamics of lithium ions in carbonate electrolytes can be revealed by timeresolved IR spectroscopy, DFT electronic structure calculation, molecular dynamic simulation, and spectrum modeling approaches. The new findings from these techniques have greatly improved our microscopic view of LIB electrolytes and challenge the previously believed hypothesis. The solvation sheath of a lithium ion consisting of carbonate molecules in the first shell is neither rigid nor stable at the picosecond time scale. The coordination number of a lithium ion fluctuates in response to the instantaneous electrostatic environment, which is significantly varied by carbonate polarity/conformation structure, ion concentration, and the nature of the counter anion. A solvent exchange process is observed between the carbonates within the first shell solvation sheath and those beyond it. Furthermore, in binary carbonate electrolytes with a mixture of cyclic and linear carbonates, linear carbonates are found in the solvation sheath, contrary to the notion that the lithium ion is solvated by cyclic carbonate molecules only due to its dipole moment being large compared to that of linear carbonate. These key findings will shed light on understanding the relation between solvation structure/dynamics and ion mobility, with the ultimate goal of improving LIB efficiency and performance.



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As discussed in the previous sections IV and V, though time-resolved IR spectroscopy can be used to probe ultrafast structural dynamics, which cannot be accessed by traditional experimental techniques, the interpretation of spectral features relies highly on spectrum modeling approaches. Probing the solvation structural dynamics of (pure or binary) carbonate electrolyte systems to extract dynamical information directly from experimentally measured time- and frequency-resolved IR spectra is a challenge if the C=O stretching bands associated with the free C=O’s and Li+-bound C=O’s of different carbonate species spectrally overlap. Therefore, spectrum modeling approaches should be used to decompose those entangled signals and to identify the specific timescales of the system. However, the quality of spectrum modeling heavily relies on both the treatment of electrostatic interactions in MD simulations, and the estimation of the frequency shift and vibrational coupling using ab initio mapping. Future work should focus on the development of efficient treatments for incorporating instantaneous polarization (e.g. polarizable force fields) and improving the quality of ab initio mapping using more sophisticated electronic structural calculation methods. Carbonate electrolytes are commonly used in current commercial LIBs. However, carbonates tend to decompose under higher voltages or temperatures and suffer from a lack of high oxidative stability78. In addition, under humid environments, the hydrolysis of the commonly used lithium salt, LiPF6, not only decreases battery performance but also produces poisonous hydrogen fluoride gas can cause explosions79. Therefore, exploring new LIB materials for improving the safety issue has been a pressing problem of considerable interest. To solve it, materials such as ionic liquids, polymer electrolytes, solid-state electrolytes, and electrolyte additives have been developed. Therefore, for those new materials, obtaining a perspective of lithium-ion solvation and its interplay with the ion transport mechanism, i.e., the fluctuation-dissipation relationship, at the molecular level is highly desirable. Therefore, as 26 ACS Paragon Plus Environment

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demonstrated in this feature article, the combination of time-resolved infrared spectroscopy, MD simulation, and spectrum modeling should be considered a promising approach to studying these new systems, which could shed light on future LIB design and development.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (MC) ORCID Minhaeng Cho: 0000-0003-1618-1056 Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by IBS-R023-D1.



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Figure captions

Figure 1. IR spectra of (a) C=O stretch and (b) O-C-O asymmetric stretch of DEC in LiPF6/DEC solutions. (c) Area fraction of Li+···DEC, which is obtained by analyzing the C=O and O-C-O stretch bands, is plotted with respect to mole fraction of LiPF6. Here, the area fraction corresponds to the percent ratio of the integrated C=O stretch IR band of the DEC···Li+ complex to that of free DEC. The two linear lines result from least square fitting analyses. (d) The coordination number of Li+ in LiPF6 in DEC solution can be estimated from the ratio of integrated IR absorption band of Li+···DEC to that of free DEC.

Figure 2. Normalized P-F stretch IR spectra for LiPF6/DEC solutions in the salt concentration range from 0.25 to 2.00 M.

Figure 3. Carbonyl stretch IR PP spectra of DEC molecules in (a) pure DEC and (b) 1.0 M LiPF6/DEC solution. (c) From (a), the isotropic IR PP signal of pure DEC at a probe wavenumber of 1726.5 cm-1 is plotted with respect to waiting time. (d) From (b), the isotropic IR PP signal of 1.0 M LiPF6/ DEC solution at a probe wavenumber of 1696.2 cm-1 is plotted with respect to waiting time. Adapted with permission from ref 76. Copyright 2017 Nature Publishing Group.

Figure 4. (a) Li+···DEC complex structure for quantum chemistry calculations. (b) The frequency of C=O stretch mode is plotted with respect to the distance (R) between Li+ and carbonyl oxygen atom. As the inter-atomic distance (R) between Li+ and carbonyl oxygen atom of DEC increases, the C=O stretch frequency increases (blueshift), whereas the O-C-O asymmetric stretch frequency decreases (redshift). These patterns are consistent with the FTIR data shown in Fig. 1. Adapted with permission from ref 76. Copyright 2017 Nature Publishing Group.

Figure 5. (a) Four representative C=O stretch 2D IR spectra of DEC molecules in 1.0 M LiPF6/DEC solution. The waiting times are 0.3, 2.0, 6.0, and 10.0 ps. The 2D IR spectra are normalized to the maximum peak intensity. (b) The time-dependent diagonal and cross peak volumes obtained from the experimentally measured 2D infrared spectra are plotted with respect to waiting time. The fitted curves (solid lines) are also shown here. The time-dependent cross peak intensity is separately plotted in the inset, to show the rise-and-decay pattern of the cross peak. Adapted with permission from ref 76. Copyright 2017 Nature Publishing Group. 33 ACS Paragon Plus Environment


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Figure 6. Carbonyl stretch IR spectra of 1.0 M LiPF6 solutions in DEC, PC, and DEC:PC (1:1 in volume) mixed solvent. Adapted with permission from ref 76. Copyright 2017 Nature Publishing Group.

Figure 7. (a) Carbonyl stretch IR spectra of 1.0, 1.5 and 2.0 M LiPF6 solutions in DMC:PC binary solvents. (b) Time-resolved 2D IR chemical exchange spectra for the solution of 1.0 M LiPF6 in DMC:PC (=1:1 in volume). In the red box, the diagonal and cross peaks of carbonyl stretch modes of DMC molecules in Li+···DMC and free DMC are shown, which indicates that DMC molecules even in such a mixed solvent electrolyte participate in the solvation of Li+. Adapted with permission from ref 76. Copyright 2017 Nature Publishing Group.


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Figure 1.



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Figure 2.


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Figure 3.



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Figure 4.


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Figure 5.



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Figure 6.


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Figure 7.



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TOC


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Biographies

Joonhyung Lim received B.S from Korea University and is currently Ph.D. graduate student of Korea University under the direction of Professor Minhaeng Cho. His research interests are the solvation structures in electrolyte of lithium ion battery with polarization controlled pump– probe spectroscopy and 2DIR spectroscopy.

Kyung-Koo Lee was born in Kunsan (Korea). He received his Ph.D. in 2010 at Korea University under the supervision of Professor Minhaeng Cho. After postdoctoral research at Toronto University in Canada, he worked as a researcher at LG Chem Battery Research Institute. He is currently an associate Professor in Chemistry at Kunsan National University. His main research interests are the thermodynamics and kinetics of solution electrolyte.

Chungwen Liang is the director of Computational Modeling Core at Institute for Applied Life Sciences (IALS), University of Massachusetts Amherst, Massachusetts, USA. He works on modeling of chemical/biological systems and computational spectroscopy. He obtained his Ph.D. in 2012 from University of Groningen, the Netherlands. After a few years postdoc experience in University of Basel and EPFL in Switzerland, he joined Institute for Applied Life Sciences (IALS) at University of Massachusetts Amherst in 2018. His current research interests focus on understanding the complex structural dynamics when chemical/biological processes take place.

Kwang-Hee Park received B.S. from Korea University and M.S. and Ph.D. from Korea University under the research guidance of Professor Minhaeng Cho. He is currently a research fellow at IBS Center for Molecular Spectroscopy and Dynamics (CMSD). His research interests are time-resolved vibrational spectroscopy, IR probe development, solvation dynamics of ions in electrolytes.

Minju Kim received her M.S. degree from Korea University in 2019 under the research supervision of Professor Kyungwon Kwak. She has carried out vibrational spectroscopy and quantum chemistry calculation studies of lithium ion battery system to understand ultrafast solvent dynamics around lithium cations in various nonaqueous electrolytes.

Kyungwon Kwak received B.S. and M.S. degrees in Chemistry from Korea University and his Ph.D. from Stanford University, where his research advisor was Professor Michael D. Fayer, in 2008. After postdoctoral training at U.C. Berkeley in Stephen R. Leone’s group from 2008 to 2010, he became as assistant professor in Chuang-Ang University in 2011. He moved to Korea University in 2016 and has been on the faculty of Korea University. His research interests are ultrafast nonlinear vibrational spectroscopy. 43 ACS Paragon Plus Environment


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Minhaeng Cho was born in Seoul. He received B.S. and M.S. degrees in chemistry from Seoul National University in 1987 and 1989 and a Ph.D. from University of Chicago in 1993 under the direction of Graham R. Fleming. After two years of postdoctoral training at MIT in Robert J. Silbey’s group from 1994 to 1996, he has been on the faculty of Korea University since March 1996, where he was the director of the Center for Multidimensional Spectroscopy from 2000 to 2009. His research interests are theoretical and computational chemistry and ultrafast nonlinear optical spectroscopy. He was elected as a member of the Korean Academy of Science and Technology (KAST) in 2002. He is a recipient of the Nobel Laureate Signatures Award (American Chemical Society) in 1995, Young Scientist Award (Ministry of Science and Technology, Korea) in 1999, Kyung-Am Prize in 2010, KAST Award in 2011, and Korea National Academy of Science Award in 2012.


Two-Dimensional Infrared Spectroscopy and Molecular Dynamics

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