Letter Cite This: J. Phys. Chem. Lett. 2017, 8, 5779-5784
pubs.acs.org/JPCL
Revealing the Solvation Structure and Dynamics of Carbonate Electrolytes in Lithium-Ion Batteries by Two-Dimensional Infrared Spectrum Modeling Chungwen Liang,† 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, Republic of Korea
‡
S Supporting Information *
ABSTRACT: Carbonate electrolytes in lithium-ion batteries play a crucial role in conducting lithium ions between two electrodes. Mixed solvent electrolytes consisting of linear and cyclic carbonates are commonly used in commercial lithium-ion batteries. To understand how the linear and cyclic carbonates introduce different solvation structures and dynamics, we performed molecular dynamics simulations of two representative electrolyte systems containing either linear or cyclic carbonate solvents. We then modeled their two-dimensional infrared (2DIR) spectra of the carbonyl stretching mode of these carbonate molecules. We found that the chemical exchange process involving formation and dissociation of lithium-ion/carbonate complexes is responsible for the growth of 2DIR cross peaks with increasing waiting time. In addition, we also found that cyclic carbonates introduce faster dynamics of dissociation and formation of lithium-ion/ carbonate complexes than linear carbonates. These findings provide new insights into understanding the lithium-ion mobility and its interplay with solvation structure and ultrafast dynamics in carbonate electrolytes used in lithium-ion batteries.
L
NMR study of 1.0 M LiPF6 in pure EC and DMC solvents revealed that the average coordination numbers of EC and DMC molecules around a lithium ion are 5.63 and 8.19, respectively.4 It also suggested that in a mixture of EC/DMC electrolyte DMC molecules not only act as cosolvent but also partially participate in the first solvation shell of lithium ions, even though the solvation is predominated by cyclic carbonate molecules.4 On the other hand, computational studies based on electronic structure calculation using density functional theory (DFT) suggested that a tetrahedral solvation structure of EC molecules around a lithium ion is energetically the most favorable configuration.5,6 Molecular dynamics (MD) simulations of dilute lithium salt solutions, where the lithium-ion concentration is much lower than those in commercial LIBs, were conducted and showed that in addition to cyclic carbonates DMC molecules also form a tetrahedral structure around a lithium ion.7 Another MD study of 1.0 M LiPF6 in EC solvent also reported that lithium ions are tetrahedrally coordinated by EC molecules.8 Here, it should be noted that the lithium-ion solvation structures probed by NMR and Raman spectroscopies are highly averaged ones over a microsecond time scale and that computational studies are often limited by the system size (e.g., clusters in vacuum for DFT calculations). Overall, it is believed that the solvation
ithium-ion batteries (LIBs) are widely used as power supplies for numerous portable electronic devices. The positive and negative electrodes and an electrolyte are the three primary functional components in a LIB. Among them, electrolytes play an important role in facilitating the movement of lithium ions passing between the positive and negative electrodes during electric charge and discharge. To achieve high ionic conductivity, high chemical stability, and a wide range of operational temperatures, electrolytes in commercial LIBs consist of lithium salts in mixed carbonate solvents.1 It is generally assumed that the combination of linear and cyclic carbonates enhances the ionic conductivity due to the fact that the two types of carbonates play distinctively different but crucial roles in facilitating lithium ion transport between electrodes.2 Cyclic carbonates (e.g., propylene carbonate (PC) and ethylene carbonate (EC)) dissolve lithium ions efficiently and prevent ion pairing due to their high polarity and high dielectric constants. Linear carbonates (e.g., diethyl carbonate (DEC) and dimethyl carbonate (DMC)) act as a medium (socalled cosolvent) for transporting ion−solvent complexes, often referred to as lithium ion solvation sheaths, due to their lower viscosity and less polarity compared to cyclic carbonates. To validate this hypothesis, numerous studies focused on probing the solvation structures of lithium ions in different carbonate electrolytes. A previous Raman spectroscopy experiment of LiClO4 in a mixed PC/DEC electrolyte showed that a lithium ion is preferentially solvated by three PC molecules and one DEC molecule, where the direct interaction between Li+ and ClO−4 is not favored in the first solvation shell.3 A previous © XXXX American Chemical Society
Received: October 4, 2017 Accepted: November 13, 2017 Published: November 13, 2017 5779
DOI: 10.1021/acs.jpclett.7b02623 J. Phys. Chem. Lett. 2017, 8, 5779−5784
Letter
The Journal of Physical Chemistry Letters
structural and dynamical information on MD simulations can be translated into the spectral information, which bridges the gap between vibrational spectroscopy experiments and simulations. Several studies have already demonstrated that the simulated spectra are in excellent agreement with those from 2DIR spectroscopy measurements of soluble peptide dynamics,15,16 membrane protein structural changes,17,18 and water hydrogen bonding dynamics.19,20 In this Letter, we aim at both interpreting the previous 2DIR experiments and providing new insights into solvation structure and dynamics of lithium ions in carbonate electrolytes. We first performed MD simulations of two representative carbonate electrolyte systems, LiPF6/DEC and LiPF6/PC, and then modeled their 2DIR spectra of the carbonyl stretching modes of DEC and PC molecules. Here, classical MD simulations were performed using the GROMACS 2016 package21 with the CHARMM general force field22 to obtain the instantaneous molecular configurations (simulation details can be found in the Supporting Information (SI)). These configurations provide rich structural and dynamical information that can then be used to model the carbonyl stretching IR spectrum of carbonate molecules. First, the vibrational Hamiltonian of the system can be described as a floating vibrational exciton model, where the carbonyl groups are treated as quantum oscillators in a fluctuating classical environment
structure of lithium ions surrounded by cyclic or linear carbonate molecules has not been fully characterized yet and is currently under intense debate. In addition, understanding how microscopic solvent dynamics plays a role in lithium-ion transportation is as equally crucial as understanding the solvation structure itself. Therefore, a technique with high time resolution is prerequisite for probing the instantaneous solvation structure and ultrafast dynamics in carbonate electrolytes. Two-dimensional infrared (2DIR) spectroscopy is one of the time-resolved experimental techniques that has been used to probe molecular processes of complex systems on time scales down to subpicoseconds.9 By carefully selecting time delays between a series of laser pulses, the system dynamics can then be extracted by monitoring the spectral changes as a function of the time delays. A recent 2DIR spectroscopic study of carbonate CO stretching modes focused on probing the ultrafast dynamics of linear carbonate molecules around lithium ions.10 It was shown that the chemical exchange of carbonate molecules in the first solvation shell of lithium ions occurs on the picosecond time scale. The time constants of the formation and dissociation of the ion−solvent complex were extracted from two-state analyses of the growth of 2DIR cross peaks with respect to the time delays (waiting times).10 Independently, another research group used 2DIR spectroscopy to investigate the differences in the solvation structure and dynamics of lithium ions in various cyclic and linear carbonates.11,12 They suggested that both cyclic and linear carbonate molecules form slightly distorted tetrahedral solvation structures around lithium ions. Therefore, the vibrational energy transfer between strongly coupled modes of carbonyl groups (in this tetrahedral solvation shell) occurring on the picosecond time scale is responsible for the growth of the experimentally measured 2DIR cross peaks.12 Therefore, there exist two distinctively different interpretations about the growth of 2DIR cross peaks, i.e., chemical exchange of solvent molecules in the first solvation shell versus vibrational energy transfer between coupled carbonyl stretching modes of solvating carbonate molecules in the first solvation shell. Schematic illustrations of these two mechanisms are shown in Figure 1.
N
H (t ) =
⎡
∑ ⎢⎣ωi(t )B†i Bi − i=1
⎤ Δ † † Bi Bi Bi Bi ⎥ + ⎦ 2
N
∑ Jij (t )B†i Bj i,j
N
−
∑ μi⃗ (t )·E⃗(t )[B†i + Bi] i=1
(1)
where Bi† and Bi are Bosonic creation and annihilation operators, respectively, of the ith oscillator. ωi(t) is the vibrational frequency of the ith oscillator, and Jij(t) is the vibrational coupling between the ith and the jth oscillators. μ⃗i(t) is the transition dipole responsible for the coupling to the applied laser field E⃗ (t). Δ represents the anharmonicity of the potential energy surface of the carbonyl stretching, which accounts for the fact that the energy gap between the single and double excited vibrational states is smaller than that between the ground state and the single excited state. The vibrational frequency ωi(t) and transition dipole μ⃗i(t) were predicted using an electrostatic mapping procedure that is constructed by performing multiple electronic structure calculations using density functional theory (the detailed mapping procedure is described in the SI).23−27 The vibrational coupling Jij(t) were calculated through the transition dipole coupling (TDC) scheme.28 The vibrational frequency ωi(t) and transition dipole μ⃗i(t) are strongly correlated to the instantaneous configurations extracted from MD snapshots, which account for the local fluctuating electrostatic environment. After constructing the time-dependent vibrational Hamiltonian H(t), IR spectra were calculated by using the numerical integration of the Schrödinger equation (NISE) approach. The detailed description can be found elsewhere.18,19,29−31 This quantum−classical method is based on numerically solving the time-dependent Schrödinger equation for the vibrational Hamiltonian H(t) and using the solution to calculate optical response functions in the time domain. The frequency domain IR spectra are obtained from the Fourier transform of the time domain response functions.
Figure 1. Schematic illustrations of two distinct mechanisms that are responsible for the growth of 2DIR cross peaks: (a) chemical exchange and (b) vibrational energy transfer. The yellow spheres represent lithium ions. The carbonate molecules and PF−6 anion are shown as sticks.
While 2DIR spectroscopy is an extremely useful and promising tool for probing detailed molecular structures and ultrafast dynamics of complex systems, the 2DIR spectra are generally hard to interpret due to the spectral congestion.13,14 Therefore, interpreting 2DIR experimental data strongly relies on computational spectroscopy with a state-of-the-art spectrum modeling approach. By directly modeling the 2DIR spectrum with molecular configurations obtained by MD simulations, the 5780
DOI: 10.1021/acs.jpclett.7b02623 J. Phys. Chem. Lett. 2017, 8, 5779−5784
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line shape of diagonal peaks is much broader than that of LiPF6/DEC, and the cross peaks are also more pronounced. This indicates that the local solvent environment of carbonyl groups is more inhomogeneous in LiPF6/PC than that in LiPF6/DEC electrolytes, which is consistent with the fact that PC is more polar than DEC. It is well-known that the appearance of 2DIR cross peaks mainly attributes to two different mechanisms: vibrational population transfer and chemical exchange. Vibrational population transfer occurs when vibrational chromophores are close enough in space and carry similar transition frequencies. The increase of 2DIR cross peaks with finite waiting times due to the vibrational population transfer between amide I modes in polypeptides was demonstrated by both experiments32 and simulations.33 On the other hand, the chemical exchange process takes place when vibrational chromophores experience different local chemical environments, which results in a shift of the transition frequency during the waiting time. Several studies illustrated that the hydrogen bond switching mechanism in organic solvents34 and liquid water35 gives rise to the growth of 2DIR cross peaks with an increase of the waiting time. By monitoring the growth of cross peaks, the rate constants of chemical exchange processes can be determined.36 For the present study, which mechanism plays a more important role in the growth of 2DIR cross peaks is the most important key question to be answered. This can be easily addressed by deliberately suppressing either of the two mechanisms in the simulations to see whether the growth of cross peaks remains. Figure 3a shows that the 2DIR cross peaks of LiPF6/DEC electrolyte are clearly visible at Tw = 10 ps (in contrast to that no cross peaks can be seen at Tw = 0 ps), even in the limit that the vibrational couplings Jij(t) are set to zero in the 2DIR spectrum calculations. This strongly indicates that the chemical exchange process is responsible for the growth of 2DIR cross peaks, which is consistent with the conclusion drawn in ref 10. To further examine the effect of vibrational population transfer on the 2DIR cross peaks, we applied strong position restraints on heavy atoms in MD simulations (see the SI), which essentially prohibit chemical exchange processes. In this case, the 2DIR cross peaks do not change their intensities with respect to waiting time (Figure 3b). Nonetheless, it is interesting to note that the 2DIR cross peaks are not negligible at Tw = 0, and their intensities remain the same even at Tw = 10 ps. This indicates that strong intermolecular coupling does contribute to the occurrence of the 2DIR cross peaks at a short waiting time. However, vibrational population transfer does not make the intensity of the 2DIR cross peaks increase at a longer waiting time. To identify whether the strong intermolecular coupling arises from the interaction between binding CO’s and free CO’s, or only between binding CO’s, we additionally calculated 2DIR spectra (Figure 3c) with the contribution of binding CO’s (by neglecting the contribution from free CO’s). The 2DIR cross peaks remain unchanged (compared to Figure 3b), which indicates that the strong intermolecular vibrational coupling between binding CO’s gives rise to the 2DIR cross peaks and weak high-frequency peaks along the diagonal. Interestingly, these high-frequency peaks due to the intermolecular coupling spectrally overlap with the contribution from free CO’s. Once this coupling was neglected, neither cross peaks nor high-frequency peaks exist (the right panel of Figure 3c). Therefore, the cross peaks at zero waiting time indeed arise from vibrational couplings
Comparison between simulated and experimental FTIR and 2DIR spectra is described in the SI. In general, the simulated FTIR and 2DIR spectra reproduce well all of the features (peak positions, line shapes, and growth of cross peaks with increasing waiting time Tw) of experimental spectra. However, for pure DEC liquid, the central frequency of the carbonyl stretching vibration predicted from simulation is slightly smaller than that probed by experiment. The frequency shift due to binding of lithium ions shown on simulated spectra is larger than that on experimental spectra. These discrepancies are discussed in the SI. The simulated 2DIR spectra with the parallel polarization of LiPF6/DEC and LiPF6/PC electrolytes at different waiting times are shown in the left column of Figure 2. The low-
Figure 2. Simulated parallel (left column) and perpendicular (right column) polarization 2DIR spectra with two waiting times Tw (0 and 4 ps) of (a) LiPF6/DEC and (b) LiPF6/PC electrolytes.
frequency peak associates with the carbonyl stretching modes of carbonate molecules forming strong electrostatic interactions with lithium ions, which we shall refer to as binding CO’s in the first solvation shell. The high-frequency peak originates from the contribution of carbonyl groups that do not directly interact with lithium ions, which will be referred to as free C O’s. It can be noted that in the 2DIR spectra of LiPF6/PC the 5781
DOI: 10.1021/acs.jpclett.7b02623 J. Phys. Chem. Lett. 2017, 8, 5779−5784
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polarization 2DIR spectra at short waiting times, we calculated the correlation between the vibrational coupling Jij(t) and the relative angle between transition dipoles of any pairs of binding CO’s (Figure 4a). In general, the coupling between binding
Figure 3. Perpendicular polarization 2DIR spectra of carbonyl stretching vibration in LiPF6/DEC electrolyte, (a) by neglecting the vibrational couplings between carbonyl groups (left: Tw = 0 ps; right: Tw = 10 ps), (b) calculated from position-restrained MD simulations by suppressing chemical exchange dynamics (left: Tw = 0 ps; right: Tw = 10 ps), and (c) containing only the contribution of binding CO’s in position-restrained MD simulations (left: Tw = 0 ps; right: Tw = 0 ps by neglecting the vibrational couplings).
Figure 4. (a) Correlation plots between the vibrational coupling and the angle of binding of CO’s in LiPF6/DEC (left) and in LiPF6/PC (right) electrolytes. (b) Population of the binding number of PF−6 , DEC, and PC. The most populated solvation structures are shown in the insets. The yellow spheres represent the lithium ions. The carbonates and PF−6 are shown as sticks.
CO’s ranges from 15 to 35 cm−1 for DEC and from 30 to 60 cm−1 for PC. It shows that the coupling strength between PC molecules is stronger than that of DEC molecules. For DEC, the configurations at around θ = 97.2° are much more populated than those at around θ = 138°, which is important evidence explaining the observation that the perpendicular polarization reveals more pronounced cross peaks than the parallel one. For PC molecules, the binding CO’s have relative angles of 109.5 and 60°. To further determine the solvation structure of a lithium ion with respect to surrounding PF−6 , DEC, and PC molecules, their coordination numbers were calculated from replica exchange MD simulations (see the SI) and are plotted in Figure 4b. A cutoff of 2.25 Å was chosen to determine whether neighboring molecules directly interact with a lithium ion. In LiPF6/DEC solution, the chance to find a PF−6 anion in the first solvation shell of a lithium ion is around 70%, which indicates that the possibility for contact ion pair formation is quite high. The most populated solvation structure is a lithium ion surrounded by three DEC molecules and one PF−6 anion. On the other hand, in LiPF6/PC electrolyte, lithium ions are solvated by PC molecules only so that no ion pair formation is found. The most populated solvation structure is then a lithium ion surrounded by five PC molecules. To quantitatively analyze the time-dependent 2DIR spectra with perpendicular polarization, we calculated the intensity of 2DIR cross peaks as a function of waiting time Tw (Figure 5). Cross peak I in the upper-left corner of the 2DIR spectrum (see Figure 2) corresponds to the dissociation of lithium-ion/ carbonate complexes because the carbonyl vibrational frequency is blue-shifted during the waiting time Tw, resulting from the configurational change of binding CO’s to free C
between carbonyl stretching modes of carbonate molecules in the first solvation shell of lithium ions. However, the growth of 2DIR cross peaks originates from the chemical exchange process of carbonate molecules. Because intermolecular couplings between carbonyls of carbonate molecules bound to lithium ions give rise to the signal of cross peaks at short waiting times, where the chemical exchange process is too slow to happen, it would be interesting to study whether solvation structures of binding carbonyls can be revealed by analyzing the intensity of cross peaks in 2DIR spectra by applying different polarization schemes. For instance, to enhance the cross peaks between vibrational chromophores that are close to perpendicular to each other, it is advantageous to use the perpendicular polarization scheme, where the two pump pulses are polarized identically and perpendicularly to the probe and detection pulses.37 The perpendicular polarization 2DIR spectra of LiPF6/DEC and LiPF6/PC electrolytes with different waiting times are shown in the right column of Figure 2. It can be noticed that for both LiPF6/DEC and LiPF6/PC cross peaks are more pronounced in the spectra with perpendicular than parallel polarization at Tw = 0 ps. However, at longer waiting time (Tw = 4 ps), the difference between parallel and perpendicular 2DIR spectra is negligible. This is additional important evidence that the growth of 2DIR cross peaks at longer waiting times is induced by the chemical exchange process, which reduces the polarization dependence of the 2DIR signals and smears out the difference in intermolecular couplings probed by different polarizations. In order to understand the relationship between solvation structure and pronounced cross peaks in the perpendicular 5782
DOI: 10.1021/acs.jpclett.7b02623 J. Phys. Chem. Lett. 2017, 8, 5779−5784
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Letter
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b02623. Detailed description of simulations (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
Figure 5. 2DIR cross peak intensity as a function of waiting time Tw of (a) LiPF6/DEC and (b) LiPF6/PC. The red and blue data points represent the intensity of cross peak I (upper-left corner) and cross peak II (lower-right corner) in a 2DIR spectrum, respectively.
ORCID
Chungwen Liang: 0000-0001-9721-6411 Minhaeng Cho: 0000-0003-1618-1056 Notes
The authors declare no competing financial interest.
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O’s. Similarly, cross peak II in the lower-right corner of the 2DIR spectrum corresponds to the formation of lithium− carbonate complexes. The plots in Figure 5 were fitted to an exponential function with an offset I(Tw) = A × (1 − exp(−Tw/ τ)) + B, where the offset represents the vibrational couplinginduced cross peak intensity. Fitting parameters are summarized in the SI (Table S3). The time constants τd and τf are associated with the dissociation and formation processes. For DEC molecules, the dissociation and formation time scales are very similar (τd= 9.38 ps and τf = 10.5 ps). For PC molecules, the dissociation time constant is about half of the formation time constant (τd= 1.77 ps and τf = 3.51 ps), and both time scales are significantly shorter than those of DEC molecules. This suggests that both formation and dissociation processes of lithium-ion/PC complexes are faster than those of lithium-ion/ DEC complexes. In summary, we have performed MD simulations and modeled the carbonyl stretching 2DIR spectra of LiPF6/DEC and LiPF6/PC electrolytes. We identified that the growth of cross peaks in the 2DIR spectrum is due to the chemical exchange process between binding CO’s and free CO’s. However, the intermolecular coupling between binding CO’s in the first solvation shell gives rise to the 2DIR cross peaks at short waiting times. By modeling the 2DIR spectra with perpendicular polarization, 2DIR cross peaks at short waiting times are more pronounced. For structural information, we found that the most populated solvation structure of a lithium ion in LiPF6/DEC electrolyte is surrounded by three DEC molecules and one PF−6 anion. For LiPF6/PC electrolyte, the most populated structure is a lithium ion coordinated with five PC molecules. Because previous studies based on various experimental techniques or different simulation approaches suggested different solvation structures, it would be crucial to validate them by performing high-level ab initio MD simulations38,39 that account for a more accurate electrostatic interaction and polarization effect between ions and solvent molecules. For dynamical information, we could estimate the time constants associated with the dissociation and formation processes of lithium-ion/carbonate complexes by analyzing the time-dependent 2DIR spectra. We found that both processes occur faster in LiPF6/PC electrolyte than those in LiPF6/DEC. To validate this hypothesis, further time-resolved experiments are needed to investigate the different dynamics in both pure and mixed carbonate systems. By doing so, revealing the solvation structure and dynamics of lithium ions in carbonate electrolytes will shed light on designing a new generation of high-performance LIBs.
ACKNOWLEDGMENTS This work was supported by IBS-R023-D1. REFERENCES
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