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Dec 1, 2012 - Electrochemistry Branch, Power and Energy Division, Sensor and Electron Devices Directorate, U.S. Army Research Laboratory, 2800. Powder...
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Correlating Li+ Solvation Sheath Structure with Interphasial Chemistry on Graphite Arthur von Wald Cresce, Oleg Borodin, and Kang Xu* Electrochemistry Branch, Power and Energy Division, Sensor and Electron Devices Directorate, U.S. Army Research Laboratory, 2800 Powder Mill Road, Adelphi, Maryland 20783, United States S Supporting Information *

ABSTRACT: In electrolytes with unique electrochemical signature, the structure of Li + solvation sheath was quantitatively analyzed in correlation with its electrochemical behavior on graphitic anodes. For the first time, a direct link between Li+ solvation sheath structure and formation chemistry of the solid electrolyte interphase (SEI) is established. Quantum chemistry calculations and molecular dynamics simulations were performed to explain the observed reversed preference of propylene carbonate (PC) over ethylene carbonate (EC) by Li+.

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INTRODUCTION Although the critical role of interphase in Li ion intercalation chemistry has been well recognized since the inception of the technology, it remains the least understood component in the device due to its ad hoc nature of formation, nanometric presence, and sensitive chemical nature that is elusive to most characterizing tools.1−3 In particular, the knowledge about its formation mechanism on atomistic level remains sketchy at best. Among the limited efforts, Besenhard and Winter et al. have proposed a 3D process that seems to be increasingly supported by most of the recent experimental observations.4 This model involves an intermediate compound of cointercalation by solvent molecules into the graphitic structure before permanent interphase forms at lower electrochemical potential and the resultant SEI (solid electrolyte interphase, named after its electrolyte nature) that should consist of the reduction products from these cointercalated solvent molecules. Apparently, interphase thus formed would partially penetrate into the interior of graphite from the edge sites, rendering it some 3D characteristic. The most convincing evidence for this model came from the in situ electrochemical-XRD experiments, where expanded interlayer distance between graphene sheets during the initial lithiation process were observed due to such cointercalations at potentials 200−300 mV higher than the known reduction potential of the solvent.5 Considering that these cointercalating solvent molecules, before serving as precursors of SEI, should be within the primary solvation sheath of Li+, Xu et al. further suggested that the eventual interphasial chemistry on graphitic anodes is actually dictated by the solvation sheath composition of Li+ in typical nonaqueous electrolytes.6,7 As a refined version of the Besenhard−Winter mechanism, this solvation-driven model has been based on circumstantial evidences from electroThis article not subject to U.S. Copyright. Published 2012 by the American Chemical Society

chemical impedance studies, spectroscopic analysis on SEI, as well as quantitative analysis of the Li+ solvation sheath using a novel mass spectrum technique; 6−8 however, a direct verification of the solvation model has not been available. In this work, we attempted to address this issue by attempting to establish such a direct link between Li+ solvation sheath structure and interphasial chemistry of the corresponding electrolytes. Using propylene carbonate (PC) as an SEI marker due to its notorious inability to form SEI, we investigated the quantitative change of solvation members within Li+ sheath, while PC in the bulk electrolyte is gradually replaced by ethylene carbonate (EC), a solvent reputed for its SEI formation efficacy. This dependence was then quantitatively correlated with the electrochemical behavior of the corresponding electrolytes on a graphitic anode with the intent to find their interdependence. To better understand the observed preferential solvation of Li+, computational approaches were used to explore the relative stability of and the first reduction in ECnPCm/Li+ complexes (n + m = 3 or 4).

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EXPERIMENTAL SECTION All electrolyte solutions were prepared in glove-boxes or in a dryroom with moisture level below 5 ppm. While lithium hexafluorophosphate (LiPF6) was purchased from Morita and used as received, all carbonate solvents purchased from Aldrich or Ferro were further dried over 3A molecular sieves and redistilled. After such treatment, the moisture level is anticipated to be below 10 ppm as indicated by Karl Fischer titration. Typical electrolyte solutions were prepared by dissolving a calculated amount of LiPF6 (1.0 M) in premixed Received: April 14, 2012 Revised: November 29, 2012 Published: December 1, 2012 26111

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Figure 1. Schematic drawing of quantitative analysis of Li+ solvation sheath structure using mass spectrum with a soft ionization technique electrospray ionization (ESI-MS). Reproduced with permission from ref 9. Copyright 2011 Electrochemical Society.

carbonate as compared to its acyclic counterparts, as it was believed that a naked Li+ could be best stabilized by the more polar EC molecule as it is removed from salt lattice in an imaginary Bohn−Haber experiment.12 The resultant preference relationship as described in Figure 3 was tentatively correlated to both SEI composition analysis performed on cycled graphitic anode with NMR7 and Fourier-transformed infrared (FTIR)13 spectroscopies, as well as activation energy barriers measured on graphitic or layered titanate anode surfaces with electrochemical impedance spectroscopy (EIS),6−8 providing strong but, nonetheless, circumstantial support for the Li+-solvationdriven model describing the first formation of interphase. In order to establish an unambiguous link between the Li+ solvation structure and interphasial chemistry, in this work, we selected PC as an SEI marker, leveraging its notorious inability in forming protective SEI and the accompanied characteristic 0.8 V exfoliation of graphitic structure as a unique electrochemical signature.14 By gradually increasing EC concentration in its binary mixture with PC from 0 to 90% (weight), the change in Li+ solvation sheath structure is investigated using ESI-MS. Figure 2 showed MS graphs for selected electrolyte compositions. It becomes immediately obvious that the preference of Li+ shifts from EC to PC. For example, the EC-only species, such as Li(EC) and Li(EC)2, were not detected until EC concentration in bulk reaches 70%; however, PC presence was persistently found even when EC concentration reaches 90%, where Li+ solvation sheath with mixed participation by both solvent molecules, Li+(EC)(PC) and Li+(EC)2(PC), remains the most abundant species. One particular aspect worth noting is that ESI-MS technique did not detect any Li+(solv)4 species, even though they were believed to be stable solvating structures based on earlier spectroscopic studies and quantum calculations.15−18 This discrepancy very likely stems from the fact that spectroscopic means such as FT-IR, Raman, or NMR are all time-averaged techniques, which are unable to differentiate the solvent molecules in the primary sheath that have varying association strength. In this sense, the solvation sheath picture revealed by ESI-MS in this work should be snapshots of the Li+ solvation sheath that only contains these molecules tightly associated to Li+. Needless to say, it is those tightly associated molecules that are most significant to interphasial chemistry and processes, as

solvents that consist of EC and PC at weight ratios between 0:100 to 90:10, which were converted to molar fractions upon data analyses. Electrospray ionization mass spectrum was taken with a JEOL AccuTOF mass spectrometer. Typically, under flow of dry nitrogen, neat electrolyte solutions were injected with a syringe into the nebulizing chamber. A DC bias of 2300 V was applied at the nozzle to generate an electrospray jet. Another DC field of 30 V was applied at the aperture to accelerate the cationic species into the spectrum tube (Figure 1). Time-offlight (TOF) technique was employed to detect the cationic species. Graphitic anode materials were obtained gratis from ConocoPhilips. Electrodes were coated onto copper foil using typical protocols with carbon additive and binder. Graphite/Li anode half cells were assembled in a dryroom with standard Hohsen CR2032 coin cell hardware. Galvanostatic tests were carried out using typically C/10 rate based on the nominal capacity. With the exception of the initial lithiation cycle where the cells were discharged from their own individual open circuit voltages, all subsequent cyclings were conducted between 1.5 and 0.002 V vs Li.

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RESULTS AND DISCUSSION Li+ Preference of PC. In previous publications, we have shown that electrospray ionization mass spectrometry (ESIMS) is an effective and reliable tool for quantitative analysis of Li + solvation sheath structure.9,10 By measuring exact composition of individual Li+-containing species and their abundances (Figure 1), it is possible to precisely map the statistical distribution of various carbonate molecules within the Li+ solvation sheath. In all typical binary electrolyte systems analyzed, i.e., mixture of EC with a linear carbonate such as DMC or EMC, an overwhelming EC preference by Li+ has been identified, which was represented by a rather positive deviation of EC population as found in the Li+ solvation sheath when plotted against the bulk electrolyte composition (Figure 3). The same preference was also found by Lucht and coworkers in 13C and 1H nuclear magnetic resonance (NMR) studies of similar carbonate solutions.11 This preference was attributed to the much higher permittivity of the cyclic 26112

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Figure 3. Preferential solvation of Li+ by PC: EC populations as found within the Li+ solvation sheath for the EC/PC binary system. Also plotted for comparison are corresponding EC populations in typical binary electrolyte systems (EC/DMC and EC/EMC).

Correlation to Electrochemistry. Highly graphitic carbon anodes were galvanostatically lithiated and delithiated in these same EC/PC electrolytes, and their voltage profiles against capacity were summarized in the panels of Figure 4a. It is wellknown that the graphitic structure can be exfoliated due to PC reduction, marked by a characteristic plateau at ∼0.8 V,14,20 which exists more or less for nearly all the solutions investigated here with the only two exceptions at EC population of 80% and 90%, respectively. This should be expected from the fact that PC dominates the Li+ solvation sheath in most situations, and this solvation sheath is the electrolyte entity that a graphitic interstitial accommodates. When PC population in the bulk electrolyte is incrementally displaced by EC, the above irreversible process diminishes but does so in a nonlinear manner. While a glimpse at Figure 4a immediately tells that there seem to be two sudden changes around 40% and 80% EC bulk population (by weight, or 46% and 84% by molar), respectively, a closer look at the voltage depressions near lithiation degree x = 0.5 reveals these more clearly (Figure 4b). The electrochemical performance of graphitic anodes in these EC/PC systems provides us with two independent quantities to evaluate the correlation of the Li+ solvation sheath to interphasial properties. The first quantity is the voltage depression at an arbitrarily chosen lithiation degree x = 0.5, where about half of the available graphite sites are populated with Li+, most likely in Stage II configuration.21 In the presence of a protective SEI, the above lithiation degree corresponds to an electrochemical potential of 0.05−0.10 V; however, if no stable SEI is present to suppress the irreversible PC decomposition and exfoliation, the cell potential would remain around 0.7−0.8 V without Li+ intercalation. In the intermediate ranges where EC and PC compete for interphasial dominance, the potential would lie between these two extremes, as shown in Figure 4b. For the convenience of comparison, we would plot this value on a reversed y-axis against EC content in the bulk electrolyte. The second such quantity is the Coulombic efficiency (CE) for the graphitic anode in those electrolytes during the first lithiation/delithiation cycle, as defined by the

Figure 2. ESI-MS graphs for selected solutions of 1.0 M LiPF6 in EC/ PC mixed solvents.

suggested by the solvation-driven model.6−8 In other words, the solvation sheath structure as revealed by ESI-MS should be closer to the ones that edge-sites of a graphitic anode would see during the charging process of Li-ion batteries. Statistical distribution of EC population in the solvation sheath was analyzed based on peak abundances obtained by ESI-MS, which provided further quantitative confirmation for the PC preference. As shown in Figure 3, when EC population as found in the Li+ solvation sheath is plotted against EC% in bulk electrolyte composition, a rather negative deviation from the so-called “ideal non-preferential solvation behavior,” represented by the diagonal straight line, was revealed, which almost mirror-imaged that of the EC preferential behavior as found in EC/DMC or EC/EMC systems.9,10 This complete reversal in the relative solvation competitivity predicts a Li+ solvation sheath structure dominated by PC in most of the compositions of EC/PC system. It should be pointed out that, not only the rationale underneath remains unclear but the above preference of PC over EC by a Li+ is by no means selfapparent either. Rather oppositely, there has been a computational study pointing the other way, reflecting how controversial the topic is.19 Nevertheless, the experimental results shown in Figure 3 unequivocally clarify this issue. 26113

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Figure 4. (a) Effect of successive replacing PC with EC on interphasial chemistry. (b) Details of voltage profiles associated with the irreversible capacities caused by PC reduction and graphite exfoliation as shown in panel a. Note the nonlinear transition from exfoliating modes at EC% = 0−70 to intercalating modes at EC% = 80−90.

of Li+). In reality, CE for the most typical graphitic carbon materials ranges between 0.85−0.90.22 These two independent quantities, both describing how effectively the formed SEI supports lithiation/delithiation chemistry, are plotted against the EC population as found in

capacity ratios between delithiation and lithiation (Cdeli/Cli). This value would be nearly zero when protective SEI is absent (as there is no Li+ intercalation) or ideally 1.0 when a perfect SEI is formed (which supports reversible insertion and removal 26114

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the Li+ solvation sheath as shown in Figure 5, and an excellent agreement is found in their dependences on the latter with a

Table 1. Comparison of Physical Parameters Believed to Dictate Solvating Power of Solvents solvent

permittivity (25 °C)

donicity

EC PC DMC EMC DEC

90 65 3.1 2.9 2.8

16.4 15.1 15.1 16

electron availability (donicity) or polarizability (permittivity) of individual solvent molecules, we believe that a more accurate understanding about the Li+ solvation sheath could be obtained by considering the sheath as a complete cluster. Quantum Chemistry and Molecular Mechanics Calculations. Total binding energies ΔE of Li+(ECnPCm) complexes with n + m = 1,3,4 were calculated using Minnesota M05-2X and range separated LC-ωPBE density functionals, Møller− Plesset perturbation theory (MP2), G4MP2 composite method, and molecular mechanics (MM) using many-body polarizable APPLE&P force field25,26 as discussed in detail in the Supporting Information. Total binding energy ΔE(Li+(ECnPCm)) for the complex of Li+(ECnPCm) was defined as

Figure 5. Direct correlation of Li+ solvation sheath structure (as represented by EC population found in Li+ solvation sheath) and interphasial chemistry (as represented by (1) Coulombic efficiency in the 1st lithiation/delithiation cycle and (2) voltage at x = 0.5 in the 1st lithiation.

ΔE(Li+(ECnPCm)) = E(Li+(ECnPCm)) − E(Li+) − nE(EC) − mE(PC) (1)

where n and m are the numbers of EC and PC molecules, respectively, and E is the absolute energy. Optimized geometries of Li+(ECnPCm) were first obtained in MM calculations using APPLE&P force field, followed by geometry optimization using DFT and G4MP2. Optimized structures had no imaginary frequencies. Optimized geometries are given in the Supporting Information. Basis set superposition error (BSSE) correction was applied to all binding energies using the counterpoise method. Frozen core calculations were performed. The most accurate estimate of the EC/Li+ and PC/Li+ binding energy from G4MP2 calculations was found in good agreement with MP2/aug-cc-pvTz energies in accord with (acetonitrile)n/LiSalt calculations.27,28 Less expensive MP2/ aug-cc-pvDz calculations slightly underestimated solvent/Li+ binding energies, while DFT calculations (M05-2X and LCωPBE) overestimated it. All calculations, however, yielded PC/ Li+ being more stable than EC/Li+ by 1.3−1.4 kcal/mol. As G4MP2 calculations are too computationally expensive for calculating binding energies of Li+(ECnPCm) complexes with n + m = 3, only MP2 and DFT methods were utilized for these complexes (see Table 2). Replacement of EC by PC in Li+(ECnPCm), n + m = 3 resulted in monotonic stabilization of complexes by 0.6−0.9 kcal/mol. A similar trend was observed for Li+(ECnPCm) complexes with n + m = 4 as shown in Figure 6a, indicating that, for all studies, Li+(ECnPCm) complexes in the gas-phase replacing EC with PC always stabilize the complex. Table 2 and Figure 6a also indicate that MM calculations using APPLE&P force field accurately reproduced not only relative stability of all studied Li+(ECnPCm) complexes n + m = 1,3,4 but also total binding energies calculated at MP2 level and G4MP2. MM calculations also allowed us to estimate contributions from Coulomb interactions due to fixed charges and polarization to the binding energy difference of 2.2 kcal/ mol between EC4/Li+ and PC4/Li+. We conclude that the higher stability of PC4/Li+ compared with EC4/Li+ mainly

slope of near unity. We believe that underneath the unity slope dependences of these two independent quantities lies a single common foundation, i.e., the opportunity of graphitic edge sites seeing a PC molecule equals the probability of a PC molecule appearing in a solvated Li+, which is the central piece that brings these solvent molecules into reductive environment of graphite interior through the open edge sites. In fact, the above relationship would have been predicted by the solvation-driven model for SEI formation based on spectroscopic evidences.6−8 Thus, Figure 5 elegantly establishes the very first direct link between Li+ solvation sheath structure and its impact on interphasial chemistry in a quantitative manner. One can also immediately tell from Figure 5 that, for a decent SEI to be formed (decent as defined by the nonwritten standard of Li-ion battery industry, i.e., CE > 0.8), the population of EC in the Li+ solvation sheath needs to be at least ∼70% in order for the sustained decomposition of PC to be effectively suppressed. Of course, this number is only meaningful in the EC/PC binary system. In binary or higher order systems of EC and acyclic solvents (DMC, EMC, etc.), the presence of EC might not need to be at such high level because the decomposition products of these acyclic carbonates, differing from the extreme example of PC, also participates in forming a protective SEI, although the stability of SEIs originated from acyclic carbonate could not outcompete that from EC.18,22 In our previous work, we have resorted to solvent permittivity to explain the observed EC preference over linear carbonates (DMC and EMC), rendering it with more significance in stabilizing a naked Li+ than does donicity.9,10 However, the observed preference of PC over EC in this work cannot be accounted for as EC is superior to PC in both parameters (Table 1).23,24 Instead of overemphasizing the significance of single physical properties related to either 26115

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Table 2. BSSE-Corrected Binding Energy for Li+(ECnPCm) Complexes from QC Calculations and MM Optimization Using APPLE&P Force Field; the Most Accurately Calculated Are Shown in Bold

a

energy calculation

M05-2X/6-31+G**

LC-ωPBE/631+G**

MP2/Dza

MP2/Tza

geometry optimization

M05-2X/6-31+G**

LC-ωPBE/631+G**

LC-ωPBE/631+G**

LC-ωPBE/631+G**

G4MP2

MM(FF)

−47.4 −48.9 −106.6 −107.4 −108.2 −108.9

−47.5 −48.9

−45.4 −46.6 −103.6 −104.3 −105.0 −105.7

EC/Li+ PC/Li+ EC3/Li+ EC2PC/Li+ EC,PC2/Li+ PC3/Li+

−50.3 −51.7 −113.3 −114.0 −114.7 −115.4

EC/Li+ PC/Li+ EC3/Li+ EC2PC/Li+ EC,PC2/Li+ PC3/Li+

0.0 −1.4 0.0 −0.7 −1.4 −2.1

Total Binding Energy (kcal/mol) −49.5 −46.4 −50.9 −47.8 −110.4 −104.8 −111.1 −105.7 −111.9 −106.5 −112.5 −107.1 Binding Energy Relative to ECn/Li+ Complex (kcal/mol) 0.0 0.0 −1.4 −1.4 0.0 0.0 −0.8 −0.8 −1.5 −1.6 −2.1 −2.2

0.0 −1.4 0.0 −0.9 −1.6 −2.3

−1.3

0.0 −1.1 0.0 −0.7 −1.4 −2.1

Dz denotes aug-cc-pvDz and Tz denotes aug-cc-pvTz basis sets.

with increasing solvent coordination from 1 to 3 and 4, we expect that, in the condensed phase, the Li+ coordination will be preferred by PC. This supposition will be confirmed by performing MD simulations of mixed EC:PC/LiPF6 electrolyte in the condensed phase. It is important to compare DFT results with ESI-MS data. This is accomplished by estimating the ratio of cluster populations using Boltzmann factors assuming degeneracy of one: ⎛ ΔEij ⎞ Pi = exp⎜ − ⎟ Pj ⎝ kT ⎠

(2)

where Pi and Pj are populations of clusters i and j, ΔEij is the energy difference between clusters i and j, k is the Boltzmann constant, and T is temperature. At room temperature, we estimate using DFT energies that populations of Li+EC3, Li+(EC2,PC), and Li+(EC,PC2) will be 0.015, 0.06, and 0.25 of the population of Li+PC3. Thus, Li+PC3 and Li+(EC,PC2) clusters are expected to have significant populations among three solvent-coordinated Li+ in accord with ESI-MS data for EC/PC = 5:5. Similarly, ΔE for Li+EC and Li+PC is 1.3 kcal/ mol from G4MP2 calculations that yields PC/Li+ population being 9 times the Li+EC population. Our quantum chemistry calculations and analysis of ESI-MS data contradict the previous conclusion by Li et al.29 drawn from B3LYP/6-31G** calculations on Li+(ECnPCm) solvation clusters (n + m = 2) and Bhatt et al.19 calculations in which the interactions between Li+ and EC molecules are shown to be stronger than those with PC. The reduction potential was calculated for the EC−Li+ and PC−Li+ complexes in gas-phase and surrounded by the polarized continuum model (PCM) with ε = 20 and 78 using G4, G4MP2, and LC-ωPBE/6-31+G** levels with calculation details given in the Supporting Information. In agreement with previous reports,30 reduction adiabatic potentials of EC−Li+ and PC−Li+ were similar, ∼0.5 eV for ε = 78 with the difference between EC−Li+ and PC−Li+ being less than 0.1 eV. Using a more recent and often more accurate SDM solvation model31 instead of PCM model resulted in the increased reduction potential by 0.2 eV to 0.7−0.8 eV and brought calculated results in good agreement with the measured values.

Figure 6. (a) Binding energies of Li(EC)n(PC)m with n + m = 4 as calculated from QC at two levels using LC-ωPBE/6-31+G** geometry as well as from MM using APPLE&P force field.21,22 QC binding energies were corrected for BSSE using the counterpoise method. (b) Representative optimized structures of the singly reduced ECnPCm/Li+ complexes from LC-ωPBE/6-31+G** calculations with PCM (ε = 78). For details, see Supporting Information

comes from the 1.3 kcal/mol contribution from Coulomb interactions and 1 kcal/mol from polarization, while repulsion− dispersion contributions were similar. Because stabilization of Li+(ECnPCm) clusters by replacing EC with PC was preserved 26116

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A single reduction process of a number of Li+(ECnPCm) clusters, n + m = 4, was studied at the LC-ωPBE/6-31+G** level. In all studied clusters, one PC molecule was reduced. The reduction potential was in the range of 0.42−0.66 eV. Figure 6b shows the representative optimized geometries of the singly reduced clusters with PC being reduced. These results indicate a slight preference for PC reduction over EC in the studied clusters. Molecular Dynamics Calculations. MD simulations were also carried out on EC:PC(1:1)/LiPF6 electrolyte at solvent/Li = 10 at 298 and 363 K to understand details of the Li+ solvation shell in bulk electrolytes. MD simulation utilized APPLE&P force field25−28 that adequately predicted structural and transport properties of EC/LiPF6 electrolyte25 and ANLiSalts.27,28 It also accurately described binding energy of the Li+(ECnPCm) clusters for n + m = 1,3,4 as shown in the Supporting Information and Figure 6a. MM calculations are described above. Simulation run lengths were longer than 16 ns that allowed us to obtain a well-equilibrated lithium solvation shell. Each simulation cell contained 48 LiPF6 and 480 solvent molecules. The number of carbonyl groups from PC and EC in the first solvation shell of 2.8 Å around Li+ indicates a slight preference by 0.2 molecules for PC compared to EC. Such preference is consistent with what is observed in the QC and MM calculations summarized above. Note that good agreement between MM calculations using APPLE&P force field with QC results (see Figure 6a) provides additional validation of the ability of APPLE&P force field to reproduce Li+ competitive solvation in ECnPCm/Li+.

ACKNOWLEDGMENTS This work was supported by Advanced Battery Research for Transportation Program of Department of Energy (DOEABRT) via an Interagency Agreement between the U.S. Department of Energy and the U.S. Army Research Laboratory DE-IA01-11EE003413. We also want to thank Dr. Yue Li of University of Maryland, College Park, who provided ESI-MS services, and Dr. Zhenhua Mao of ConocoPhilips, who provided complementary graphitic anode materials.

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CONCLUSIONS The ESI-MS experiments performed on a series of EC:PC containing electrolytes showing a preference for a Li+ to be coordinated by PC in expense to EC in the gas-phase. In accord with these results, QC calculations of ECnPCm/Li+ clusters for n + m = 1,3,4 indicated that EC displacement reactions by PC are energetically favorable for all cluster sizes with the energy difference around 2.2 kcal/mol between PC4/Li+ and EC4/Li+. The relative stability of the ECnPCm/Li+ clusters was accurately described by molecular mechanics calculations using APPLE&P force field that was used in MD simulations to explore composition of the Li+ cation solvation shell in EC:PC(1:1)/ LiPF6 electrolyte. On average, the Li+ solvation shell contained 0.2 more PC compared to EC. This preference for PC solvation by Li+ together with the observed preference for PC reduction to occur in ECnPCm/Li+ clusters surrounded by implicit solvent using PCM (ε = 70) allows us to attribute the preference to PC reduction in mixed EC:PC/LiPF6 electrolytes compared to EC. ASSOCIATED CONTENT

* Supporting Information S

Detailed description of the quantum chemistry studies of reduction potential of Li+ECnPCm complexes and binding energies; molecular dynamics simulation methodology. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] or [email protected]. Notes

The authors declare no competing financial interest. 26117

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