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What Makes Fluoroethylene Carbonate Different? Ilya A. Shkrob, James F. Wishart, and Daniel P. Abraham J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b03591 • Publication Date (Web): 16 Jun 2015 Downloaded from http://pubs.acs.org on June 22, 2015

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The Journal of Physical Chemistry

What Makes Fluoroethylene Carbonate Different? Ilya A. Shkrob*1, James F. Wishart,2 and Daniel P. Abraham*1 1

Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 S.

Cass Ave, Argonne, IL 60439 2

Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973-

5000, USA

Received: April 15, 2015 Revised: June 1, 2015

Corresponding authors: *I.A.S.: tel, (630) 252-9516; e-mail, [email protected]. *D.P.A.: tel, (630)252-4332; e-mail, [email protected].

ACS Paragon Plus Environment

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ABSTRACT Rechargeable lithium-ion batteries containing silicon-based negative electrodes have the potential to revolutionize electrical energy storage, but the cyclic and acyclic organic carbonate solvents (such as ethylene and propylene carbonates) that are commonly used in graphite Li-ion batteries, yield unsatisfactory performance when used with such Li alloying electrodes. It has been found by trial-and-error that additions of the closely related carbonate additive, fluoroethylene carbonate (FEC) to conventional electrolytes, yields a robust solid electrolyte interphase (SEI) on the LixSiy alloy surface. Several mechanisms for this protective action have been considered in the literature and modeled theoretically; however, at present these mechanisms remain hypothetical. In this study, we use radiolysis, laser photoionization, electron paramagnetic resonance, and transient absorption spectroscopy to establish the redox chemistry of FEC. While the oxidation chemistry is similar to that of other organic carbonates, the reduction chemistry of the fluorinated molecule is strikingly different. Specifically, one-electron reduction of bulk FEC causes the fission of two (instead of one) C-O bonds, resulting in concerted defluorination and decarboxylation; in contrast, the reduction of the ethylene and propylene carbonate results in ring-opening and the formation of a radical anion. For FEC, the reduction yields the vinoxyl radical that can abstract an H atom from another FEC molecule, initiating both the chain reaction causing FEC decomposition and radical polymerization involving the reaction products. The resulting polymer can further defluorinate yielding the interior radicals that migrate and recombine to produce a highly cross-linked network. This feature implies that the outer SEI resulting from FEC reduction may exhibit elastomeric properties, which would account for its cohesion during expansion and contraction of silicon particles in the course of Li alloying/dealloying cycling.


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1. INTRODUCTION Silicon-based rechargeable Li-ion batteries (LIBs) have significant advantages over graphite-based LIBs, as silicon has the highest known specific capacity for room temperature electrochemical lithiation, which is roughly an order of magnitude greater than that of graphite used in current commercial devices.

1-3

Large volume changes ( >

3X) during the alloying/dealloying cycles remain a challenge even after the introduction of porous nanowire and nanoparticle silicon electrodes 2, that can accommodate such an expansion. There is still the need to provide a robust solid electrolyte interphase (SEI) 4-6 that protects the liquid electrolyte from runaway reduction on the LixSiy alloy electrode. 7 This protective action is due to “rectifying” properties of the SEI that passes Li+ cations but blocks the solvent molecules. In the graphite based LIBs, the electrolyte is typically a mixture of ethylene carbonate (EC) with acyclic dialkyl carbonate diluents, such as dimethylcarbonate and/or ethylmethylcarbonate containing 1 M LiPF6 4, 8-10 Even in these extensively studied electrolytes, minor variations in composition can have dramatic consequences for cell performance. For instance, an important milestone in the development of LIBs was the discovery that EC reduction on graphite yields a robust SEI, whereas the previously used propylene carbonate (PC) forms an SEI that allows solvent molecules to co-intercalate between the graphene sheets causing their eventual exfoliation, which leads to electrode failure. 11 This trial-and-error approach has been replayed in the development of silicon LIBs. It has been found that the common EC based electrolytes are inadequate for silicon LIBs, while the partial replacement of EC by a closely related compound, fluoroethylene carbonate (FEC, see structure 1 in Scheme 1), has surprisingly beneficial effects.

12-16

particular, the addition of FEC co-solvent facilitates formation of an SEI coating

17

In

that

withstands many cycles of expansion and contraction during the electrode’s lithiation/delithiation cycles. This effect was rationalized through the preferential breakdown of FEC over other organic carbonates in the same solvent.

13, 17, 18

Somehow

this selective reduction leads to a better quality SEI. Similar, but less pronounced effects were observed for vinylene carbonate, VC.


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1

2a

3b

4d

2b

4a

5

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3a

4b

4c

6

7

Scheme 1. Structural Formulas for Reaction Intermediates Derived by Oxidation and Reduction of FEC (Structure 1).

FEC is not a new electrolyte additive: it was already considered for LIBs as early as the 19

1990s.

Studies of electrolytes containing FEC co-solvent suggest improved

performance of graphite electrodes,

20, 21

including the suppression of Li0 dendrites.

20

However, while for graphite LIBs the addition of FEC is an improvement, for silicon LIBs it seems to make all the difference. The issue that we address in this study is “what makes FEC special”? Is the redox chemistry of FEC analogous to that of EC and PC, or is it different? To answer these questions, matrix isolation continuous wave Electron Paramagnetic Resonance (cw EPR) spectroscopy and pulse radiolysis – transient absorbance were used to directly observe radicals that are generated during the photolytic and radiolytic oxidation and reduction of FEC solutions. The present examination follows the general approach of our previous two-part study on the mechanistic aspects of SEI formation during redox reactions of EC and PC.

22, 23

Throughout the discussion, the chemistry of FEC will be compared and

contrasted to that of EC and PC in order to answer the title question.


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To contrast the two behaviors, we briefly examine the results of these previous studies,

22, 23

which in turn, incorporate many of the previously reached mechanistic and

theoretical insights (we suggest the excellent reviews of Xu

4, 8

subject). The oxidation of EC and PC yields the H loss radical.

for introduction to the 23, 24

In radiolysis, this

species is formed via rapid deprotonation of the parent radical cation generated via ionization of the solvent molecules.

23, 25

An isolated H loss radical (7 in Scheme 1) is

unstable, eliminating CO2, eventually yielding the vinoxyl radical,

●

CH2CHO. In

contrast, the reduction reaction can proceed in several different ways, depending on the energetics. For EC, the prevalent pathway is ring-opening with the fission of a single C-O bond and the formation of the ●CH2CH2OCO2Li radical that can undergo the 1,2-shift (that appears to be concerted with the reduction) to CH3●CHOCO2Li radical (Scheme 2). Alternatively, it abstracts H from the parent compound, yielding yet another H loss radical, in this way initiating a chain reaction. For PC (but not for EC), the radical anion (PC-●) is stable below 120 K in a glassy frozen matrix. 26-29 Ring-opening of the reduced PC also occurs, but it is always concerted with the 1,2-shift.

23

All of these radical

intermediates recombine and disproportionate, yielding multiple secondary products and initiating radical and/or anionic polymerization.

22

The resulting chemistry is complex,

and the end result of electrolyte reduction is the formation of an inner mineral layer near the electrode (built of inorganic species, such as Li2CO3 crystallites) and an outer polymeric network that hinders diffusion of solvent molecules towards the electrode. 30 The prevalent explanation for the beneficial effect of FEC is the selective defluorination of this molecule on the LixSiy electrode, but no consensus has emerged as to how this process occurs and the mechanism by which it protects the Li alloying electrode that other carbonate solvents cannot achieve. The conclusion that FEC promotes the deposition of solid LiF as well as fluoride modification of the silicon oxide overlayer covering the Si particles researchers.

12-14, 18, 32

31

has been reached independently by several

Aurbach and co-workers

15

and Zhao et al.

33

have speculated that

the alkoxide products of electrolyte breakdown react with FEC, yielding LiF and VC. The latter facilitates radical polymerization (see section 3.4), contributing to the formation of the outer SEI. Subsequent research puts this rationale in question, as the direct addition of VC does not bestow the same beneficial effect, 17, 34 so the only crucial


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difference seems to be in the formation of LiF. This salt, however, is also formed by the breakdown of fluorinated ions (such as hexafluorophosphates) present in the electrolyte; given that, it is not clear why FEC makes a qualitative difference. Nakai at al., more recently Xu et al.,

35

13

and

suggest instead that the fluoride is released via one-electron

ring opening reaction of FEC yielding radical 4a in Scheme 1, which is followed by another reduction yielding the carbonate dianion and fluoroethylene. The latter is involved in radical oligomerization yielding a fluorinated polymer. Finally, this fluoropolymer is reduced releasing the fluoride that is deposited as LiF. Unlike the mechanism suggested by Aurbach and co-workers, 15 the fluoride release is envisioned as a slow, many-stage process. Chen et al.

18

suggested another variant of this mechanism,

where the resulting radicals 4a and/or 4b recombine forming the corresponding dicarbonates, and the fluoride is subsequently released from these products via a substitution reaction (in the same manner as envisaged by Aurbach et al.)

15

In contrast,

Lucht and co-workers 14 suggested direct defluorination through a one-electron reduction yielding LiF and F loss radical 7 in Scheme 1 (which is identical to the H loss radical for EC). By far the most elaborate scenario of FEC reduction was suggested in the computational study of Balbuena, Leung and co-workers

36-39

who used density

functional theory (DFT) and ab initio molecular dynamics (AIMD) to examine multiple reaction pathways. On a Li13Si4 slab, FEC reduced in a two-electron process that yielded fluoride and CO; Li4SiO4.

36

37, 38

the same reaction was observed on a thin (< 1 nm) overlayer of

This result suggests that like EC and PC, FEC completely mineralizes on the

electrode surface (in the inner SEI); incomplete reduction can only occur further away, in the outer SEI. In AIMD calculations, one-electron reduction in the bulk preferentially yields the LiOCHFCH2O●CO radical, while DFT and ab initio methods for the gas phase clusters suggested that FEC yields a metastable FEC-● anion that subsequently dissociates. The dissociation to the fluoride, CO2, and the vinoxyl radical was the most exergonic reaction (Scheme 2), yet different computational methods gave vastly different estimates for the reaction barriers. The defluorination with the formation of radical 7 (envisioned by Lucht and co-workers)

14

is next in the exothermicity. The heat of this

reaction, however, is close to the C-O ring-opening which yields radical 4b. The


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formation of the acyl radical (that was favored by AIMD calculations) exhibits the lowest reaction barrier, but this reaction has low exothermicity. As the resulting acyl radicals can readily eliminate CO2, this intermediate can further decompose to the LiOCHF●CH2 radical that subsequently dissociates to LiF and the vinoxyl radical. Since these reactions are likely to be quite rapid, Leung et al. 36 conjectured that dissociation resulting in LiF + CO2 + vinoxyl should be the prevalent reaction (Scheme 2); the subsequent reactions of the acyl radical were too slow to observe on the time scale of AIMD calculations, which is in the tens of picoseconds.

+ Li+, e-

Li+

+ Li+, e- - LiF + CO2

Scheme 2. Hypothesized Mechanistic Difference between the One-Electron Reduction of Organic (top) and Fluoro Organic (bottom) Carbonates in the Bulk Electrolyte.

Given these differences of opinion, we decided to settle the matter experimentally. Our results strongly support the conclusions reached in the computational studies of Leung et al. 36 over the more heuristic models suggested by others. This study is organized as follows. In section 3.1 we examine the methods that were used to fingerprint radicals in our EPR experiments. In section 3.2, fast electrons and laser light are used to initiate redox reactions in FEC solutions and EPR spectroscopy is used to establish the radical products using the structural insights from section 3.1. In section 3.3, these low-temperature measurements are complemented with roomtemperature pulse radiolysis – transient absorption measurements. In section 3.4, chemical ramifications of these results for LIBs are discussed. To save space, many of the


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supporting schemes, tables, figures, and the lists of abbreviations and reactions have been placed in the Supporting Information. When referenced in the text, these materials have the designator "S", as in Figure 1S.

2. EXPERIMENTAL AND COMPUTATIONAL METHODS 2.1. Materials. Fluoroethylene carbonate (FEC) was obtained from Solvay (for EPR studies at Argonne) or Aldrich (for pulse radiolysis studies at Brookhaven). Anthracened10 and naphthalene-d8 were obtained from Cambridge Isotope Laboratories. Battery grade

LiPF6

was

obtained

from

Novolyte

Technologies.

N,N,N',N'-

tetramethylphenylenediamine (TMPD), was twice sublimed in vacuum immediately before use. All other reagents were obtained from Aldrich and used as supplied. Deuterated solutes and co-solvents were used where possible to minimize spectral congestion in the EPR spectra of irradiated samples. 2.2. Low temperature radiolysis and photolysis. FEC solutions were placed in Suprasil glass tubes and deaerated through repeated freeze-thaw cycles in vacuum, and the tubes were flame sealed. The samples were warmed to 60-70 oC and then frozen solid by quenching in liquid nitrogen. At low temperature, FEC yields a polycrystalline solid, while the FEC solutions containing > 1 M LiPF6, LiBr and LiNO3 yield high-quality vitreous samples. Another way to vitrify FEC is through the addition of polar co-solvents, such as acetone and dimethylsulfoxide (DMSO). The best quality samples were obtained for 5:1 and 1:1 mol/mol mixtures of these two co-solvents, respectively. These samples were irradiated to 3 kGy at 77 K using 3 MeV electrons from Argonne's Van de Graaff accelerator. Alternatively, 1-10 mM of aromatic photosensitizer was added, and the frozen solution was photolyzed using 35 mJ, 6 ns, 355 nm pulses from an Nd:YAG laser (Quantel Brilliant). 2.3. Pulse radiolysis – transient absorption spectroscopy. Pulse radiolysis - transient absorption studies were carried out at Brookhaven’s Laser-Electron Accelerator Facility (LEAF).

40

This accelerator can supply 7 ps pulses of 8.7 MeV electrons. The

measurements were performed on Ar-purged liquid samples in septum-sealed, Suprasil


8

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self-masking semi-micro spectrophotometer cuvettes with 1 cm optical path; the analyzing light was collinear with the electron beam. All measurements were carried out at 20 oC. The analyzing light passed through 8 nm optical bandpass filters and was detected using FND-100Q silicon photodiode, a CLC-100 amplifier, a Teledyne LeCroy HRO 66Zi 12-bit oscilloscope, and locally-written LabVIEW (National Instruments, Inc.) and Igor Pro (Wavemetrics, Inc.) code for acquisition and analysis, respectively. The absorbed dose was calculated on the basis of the absorbance of the thiocyanate dimer radical anion, (SCN)2-● (=7,950 M-1 cm-1 at 480 nm) produced in N2O-saturated aqueous 10 mM KSCN solution assuming the radiolytic yield (G-value) of 5.92 anions per 100 eV absorbed (which is equivalent to 0.613 mol/J). This dose was adjusted by the electron density in the material. The absorption of the glass container has been numerically removed from the transients. With the transient absorption spectroscopy, only the product G of the G-value (in molecules or ions per 100 eV of absorbed radiation) and the molar extinction coefficient (in M-1 cm-1) can be determined. 2.4. Matrix isolation cw EPR spectroscopy. The measurements were conducted using a 9.44 GHz Bruker ESP300E spectrometer, with the sample placed in a flow helium cryostat (Oxford Instruments CF935) so that the temperature could be changed from 4 to 200 K. The magnetic field and the hyperfine coupling constants (hfcc’s) are given in the units of Gauss (1 G = 10-4 T). If not stated otherwise, the first-derivative EPR spectra were obtained at 50 K using 2 G modulation at 100 kHz and 0.02 mW microwave power. The radiation-induced EPR signals at g~2 from the Suprasil sample tubes (that overlapped with the resonance lines of the organic radicals) are not shown in the EPR spectra presented in Figures 1 to 4 and the SI.

To assist radical identification, programmed warming of irradiated samples was

used. As the temperature increases, the matrix softens, and the radicals begin to migrate, react (yielding secondary radicals, e.g. by H abstraction) and recombine. Before this occurs, thermally activated transformations (including radical fragmentation) can occur. Some of these radicals exist in the rigid matrix as an ensemble of frozen rotamers/conformers, which complicates the analyses of congested EPR spectra. At higher temperature, the conformation dynamics and rotations of groups are facilitated,


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and the resonance lines frequently become better resolved. Furthermore, as there are several radicals contributing to the EPR spectrum, it is sometimes possible to find the conditions when some radicals decay while others persist, in this way separating their contributions. The calculations of the hfcc’s for 1H and

19

F nuclei in the radicals and radical

structures and energetics (Tables 1 in the text and 1S to 3S in SI) were carried out using a density functional theory (DFT) method with the B3LYP functional 31+G(d,p) basis set from Gaussian 09.

43

41, 42

and 6-

In the following, aiso denotes the isotropic part

of the hfc tensor A with principle axes (a,b,c), and B is the anisotropic part of this tensor. Powder EPR spectra were simulated using first-order perturbation theory. In the simulation of the experimental EPR spectra the orientations of hfc tensors as computed by DFT were retained, and the principal values of these tensors were optimized. For some radicals (see section 3.1) the rotation barriers were low, and so configuration averaging was required. To this end, the dihedral angles of rotation were changed in a systematic way while all other degrees of freedom were optimized; this ensemble was then averaged using the calculated energies for Boltzmann weighting at a set temperature.

3. RESULTS AND DISCUSSION 3.1. Strategy for radical identification. In radiolysis, the solvent is simultaneously reduced and oxidized, so both kinds of radical species are present in the reaction mixture. Thus, the ability to fingerprint the radicals is imperative, as the resulting EPR spectra are congested. Since the nuclear magnetic moment is significantly larger for

19

F than for the proton, the hfcc’s in the

fluorinated radicals are typically much higher than in the organic radicals (Table 3S in SI), which makes it easier to identify such radicals. In analogy to the nonfluorinated carbonates, 22, 23, 25 one can expect that the radical cation undergoes prompt deprotonation yielding the corresponding H loss radicals 2a or 2b. 44, 45 Our DFT calculations (Tables 1S and 2S in SI) indicate that radical 2a (which is a C 2p radical) is 0.17 eV more stable than radical 2b (which is a -radical). Thus, even if radical 2b is formed in a solid matrix, it can eventually undergo the 1,2-shift and


10

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rearrange to radical 2a, provided that the reaction barrier is sufficiently low. Both of these radicals are thermodynamically unstable, eventually eliminating carbon dioxide and yielding radicals 3a or 3b. As the latter radical is 0.6 eV more energetic, the formation of radical 3a should be overwhelmingly favored. In Figure 1S, we present simulated matrix EPR spectra of these oxidation channel radicals assuming no motional averaging. Since in radical 3a the C-F bond is perpendicular to the C 2p orbital bearing the unpaired electron, the corresponding hfcc is only 9.2 G (Table 3S in SI) vs. 21 G for the methylene protons, and the resulting spectrum is a triplet (Figure 1S in SI). Due to the large isotropic hfcc (96 G) for 19F in radical 2a, the corresponding EPR spectrum is roughly a doublet. The EPR spectrum for 2b is more complex due to the anisotropy of hfcc in the fluorine, but the outer lines corresponding to A||(19F) ≈ 250 G are easy to recognize. For FEC reduction, several radicals need to be considered. If defluorination proceeds via the formation of radical 7, as suggested by Lucht and co-workers,

14

this

radical can be readily identified, as it has been observed in radiolytic oxidation of EC. 23 So is the vinoxyl radical, postulated by Leung and co-workers.

36

If, on the other hand,

the ring opening involves C-O bond dissociation (as is the case for EC and PC), the formation of radicals 4a-d needs to be considered (Table 1). In this case, radicals 4a and 4b would be the primary products and radicals 4c and 4d would be products of their rearrangement via the 1,2-shift (as these interior radicals are more stable, see Table 1). In Figure 2S in SI we show simulated EPR spectra for these species; as the hfcc’s for 1H and 19

F nuclei in the rotating groups strongly depend on the conformation of these radicals

(see calculations presented in Figure 3S in SI), such EPR spectra may not be representative of the entire ensemble. Indeed, DFT calculations shown in Figures 3S and 4S in SI suggest that the rotation barriers for the methylene group in radical 4b and the methyl group in radical 4d are < 40 meV, so conformational averaging is required. This averaging is carried out in the EPR simulations shown in Figures 5S and 6S in SI. Since the rotational barriers in radicals 4a and 4c are much higher, these two radicals remain “locked” in their lowest energy conformations unless the rotation is activated at high temperature (thus, it is imperative to study the temperature dependence of the EPR spectra). As seen from these plots, the resulting EPR spectra can be quite similar to the


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one shown for radical 2a in Figure 2S, and here lies the challenge of distinguishing these two families of radicals. The approach taken in section 3.2 is based on (i) using selective electron scavengers to suppress the electron attachment to FEC (preventing formation of the corresponding radicals, see Appendix 1S in SI) and (ii) expected transformations caused by thermal annealing of the matrix that frees the rotation of the terminal groups.

3.2. Matrix isolation EPR experiments. The lower trace in Figure 1 exhibits the typical EPR spectrum obtained in radiolysis of frozen polycrystalline FEC. Four groups of the resonance lines can be distinguished. In the center, there is the triplet (radical I), which corresponds to a species with the anisotropic g-tensor with the principal values of 2.0126, 2.0031, and 1.9936 (see simulated EPR spectrum in Figure 7S in SI).

with 1.5M LiBr X10

X5 X5

radical I radical II radical III

3200

3300

3400

3500G

Figure 1. The first-derivative X-band EPR spectrum of 3 MeV electron irradiated frozen neat FEC (lower traces) and 1.5M LiBr solution (upper traces) obtained at 50 K, 0.02 mW, 2 G modulation at 100 kHz. The signal from the E’ center in the irradiated sample tube is removed from the plot. The open circles, filled and open squares indicate resonance lines from radicals I, II, and III.

There are no discernible hfcc’s on the 1H or 19F nuclei, which excludes all radicals other than the carbonate radical anion 5; 46 i.e. the reaction is


12

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FEC + e-● → CO3-● + CH2CHF

(1)

Superimposed on the resonance lines of radical I there is a doublet of multiplets from radical II, which exhibits a large hfcc on 19F. The double integration of this EPR signal suggests approximate parity in the yields of radicals I (which can only be the product of reduction) and II, suggesting that the latter is the product of oxidation. In the wings of the outer lines from radical II (circled in Figure 1) there are weak resonance lines separated by 262 G that correspond to the outer components of a multiplet with an hfcc of 13 G. Such a feature can result either from the outermost resonance lines of radical 2b (Figure 2S in SI) or conformationally arrested radicals 4a and 4d (Figures 5S and 6S in SI, respectively). Due to the weakness of this signal and the strong spectral overlap with radical II, we were unable to identify the progenitor. As the yield of this species is negligibly low, it is excluded from further consideration.

FEC 200 K, X10

EPR signal, 1st derivative

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


175 K 150 K 125 K 100 K 75 K 50 K

3300

3400

3500G

Figure 2. First-derivative X-band EPR spectrum of frozen FEC irradiated by 3 MeV electrons at 70 K obtained during the programmed warming from 50 to 200 K. The temperatures are indicated in the plot. The dashed vertical lines indicate the resonance lines from radical I.

It is expected that FEC reduction takes different routes, depending on whether the fluoride released via dissociative electron attachment (DEA) can be bound by the matrix,


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as otherwise the energy penalty for such dissociation can be prohibitive. This consideration led us to examine FEC solutions containing lithium salts. Another advantage of studying such solutions is that the resulting matrices are vitreous rather than polycrystalline, resulting in improved averaging of magnetic anisotropies. In our previous studies of EC and PC, no difference was found between the neat solvents and lithium salt solutions, but FEC proved to be different. As seen from the upper trace shown in Figure 1, radiolysis of 1.5 M LiBr solution still yields radical II, but instead of radical I another species (that we designated radical III) is observed, and it is also a triplet. Once again, there is parity in the yields of radicals II and III, suggesting that radical III is the product of reduction. In addition to this triplet there is a narrow, asymmetric singlet line (radical IV) that will be examined later. When the EPR spectrum is swept over a wide field range (Figure 8S in SI) the multiple resonance lines from Br2-● radical (generated via bromide oxidation) can be observed in addition to these radicals; the yield of this species is low and its contribution to the features shown in Figure 1 is negligible. As the irradiated neat FEC sample is slowly warmed (Figure 2), the triplet of radical I disappears around 150 K, while radical II persists up to the temperature at which the matrix softens and the radicals begin to migrate and recombine (~ 200 K). The resolution in the M(19F)=±1/2 components of the doublet markedly improves above 125 K (Figure 9S in SI), and unequal spacing between the resonance lines in the quartet suggests that radical II cannot be radicals 4a-c (as freeing of the rotation would result in a much different spectral transformation, as was discussed in section 3.1 and demonstrated in Figures 5S and 6S in SI). Around 175 K, another radical is observed (Figure 10S in SI) with a highly anisotropic g-tensor (2.006, 2.006, 1.98) and 19G pattern in the high-field component that strongly overlaps with the M(19F)=-1/2 line of radical II. Due to this strong overlap we were unable to identify this secondary radical, but our simulations indicate that neither radicals 3a or 3b (with a highly anisotropic g-tensor) are the progenitors. When thermal annealing is carried out for the 1.5 M LiBr solution, both radicals II and III persist to 155 K, which is the glass transition point for this matrix (see Figure 11S in SI).


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Out of all radicals considered in section 3.1, radical 2a provides the closest match to radical II (see the simulation in Figure 12S in SI; the table in the legend compares the simulated and computed hfc tensors). We remind the reader that this is the most stable form of the H loss radical (Table 1S), and the parity argument suggests that radical II is indeed the product of oxidation. The electron scavenging experiments further suggesting that radical II is, specifically, the product of oxidation are examined in more detail in Appendix and Figures 13S to 16S in SI. We, therefore identify radical II with radical 2a. This, in turn, implies that radicals I and III are the products of the reduction. Figure 3 shows the EPR spectra obtained from irradiated FEC solution containing 1 M LiPF6 (trace i) and the solution that was saturated with LiPF6 and subsequently quenched at 77 K (trace ii). Both of these frozen solutions were clear glasses, and the former solution yielded the resonance lines of radicals II and III which were similar to those observed in 1.5 M LiBr solution in Figure 1. As this 1 M LiPF6 glass is gradually annealed, radical II decays, and the residual EPR signal observed at 175 K corresponds exactly to the triplet from radical III (Figure 4). This EPR spectrum can only originate from an almost freely rotating radical with two equivalent protons. Only ●CH2CHO and ●

CH2CFO radicals can yield such a spectrum, as the in-plane XCO nucleus has negligible

coupling (section 3.1). As the latter radical exhibits considerable anisotropy of hyperfine tensor for 19F (Figure 1S in SI), by process of exclusion we conclude that radical III is the vinoxyl radical. As this radical is the product of one-electron reduction (see above), we conclude that, in the presence of Li+ cations, this reduction proceeds in the manner deduced by Leung et al. 36 (Scheme 2) FEC + Li+ + e-● → LiF + CO2 + ●CH2CHO

(2)

Radical III was also observed in two-photon laser photoionization of various aromatic solutes in FEC solutions (see Appendix 2S and Figures 17S to 21S in SI), where it is the prevalent product even in the absence of lithium salts.


15


IV EPR signal, 1st derivative

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

Page 16 of 38

X20

FEC with LiPF6 3300

(i) (ii) 3400

3500G

Figure 3. The first-derivative EPR spectra of 3 MeV electron irradiated frozen vitreous FEC containing 1 M LiPF6 (trace i) and saturated LiPF6 solution (trace ii). The insets demonstrate magnified sections of trace i indicating the presence of radical II. The filled circles indicate the outer resonance lines of radical III. The narrow line is from radical IV.

In the irradiated FEC saturated with LiPF6 (Figure 3) radical III is not seen; instead, a narrow singlet resonance line of radical IV is observed. This feature persists to 165 K, at which temperature the glass softens (Figure 22S in SI). Similar features were observed in glassy PC,

23, 26-28

where it was attributed to the PC-● radical anion (for which the proton

couplings are negligible, but there is non-negligible 13C coupling). 23, 27 For vitreous PC, this radical anion, once it forms in the matrix, does not undergo ring opening upon thermal or isothermal annealing; instead, it gradually decays via recombination. We suggested

23

that the ring opening occurs only in an excited precursor via the DEA

involving incompletely thermalized electrons. Radical IV has the same behavior as this PC-● radical anion: once it is formed, it remains stable at 50 K. This species can be simulated as a radical with g=(2.0070, 2.0054, 2.0000), which is coupled to a single spin1/2 nucleus with Axx,yy ≈0 and Azz≈6 G. We identify radical IV as the FEC-● radical anion in which all of the hfcc’s with the exception of 19F nucleus are negligible. This attribution apparently contradicts our DFT calculation for a gas-phase anion (Table 3S in SI) suggesting aiso≈54 G for fluorine-19. We remind, however, that FEC has no electron affinity, so the very existence of a bound anion in the gas phase is an artifact of the tight


16

Page 17 of 38

binding basis set. The spin distribution in a polar liquid that stabilizes this radical anion can be quite different from that computed in such a manner, so we do not consider this disagreement to be a problem.

EPR signal, 1st derivative

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


FEC with 1M LiPF6

(i) (ii)

50 K 100 K 150 K

3300

3400G

Figure 4. The evolution of the first-derivative EPR spectra of 3 MeV electron irradiated frozen vitreous FEC containing 1 M LiPF6 solution observed as the sample was gradually warmed from 50 to 175 K. The temperatures are indicated in the plot. Trace i was obtained at 175 K, and (normalized) trace ii was obtained after cooling down to 50 K. The rectangles highlight the resonance lines of radical II, which decays above 150 K. The filled circles and the vertical lines indicate the resonance lines of radical III.

Another consideration that bears on the attribution made above is the color of the irradiated FEC solids. Both the carbonate radical and PC-● exhibit the characteristic blue green color

28

originating in the * transitions in the planar

47, 48

O2●C-O- moiety. This

color is also observed in the irradiated frozen FEC. In contrast, the irradiated saturated LiPF6 solution, where radical IV is the prevalent spin center, has a yellow tint (see section 3.4 below). This coloration is another reason to conclude that radical IV is indeed the FEC-● anion.

3.3. Room temperature pulse radiolysis. To relate the spectroscopic observations in section 3.2 to processes occurring in roomtemperature liquid FEC, pulse radiolysis – transient absorption spectroscopy was used, as


17


it provides time resolution down to ~1 ns. This time resolution is necessary, as the radical species rapidly decay through recombination. With this technique, a short pulse of radiation is used to generate the reaction intermediates, and continuous wave

FEC 3

G

4x10

neat FEC 2 ns 20 ns 200 ns 1500 ns

-•

CO3

2

-•

x10

(a)

0 400

FEC 4x10

-•

3

G

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

Page 18 of 38

600

800nm

0.66 M Li PF6 in FEC 2 ns 20 ns 200 ns 1500 ns FEC, 2ns

2

(b) 0 400

500 600 wavelength, nm

700

800nm

Figure 5. Pulse radiolysis - transient absorption spectra obtained for (a) Ar-purged room temperature FEC and (b) solution containing 0.66 M LiPF6 at different delay times that are indicated in both panels. The arrows indicate the decay of the FEC radical anion whose absorption band is centered at 430 nm. At 1.5 s, when this absorption fully decays, one can observe a smaller signal centered at 600 nm that is attributed to the carbonate radical anion (the filled diamonds indicate this absorption magnified by ten times in order to facilitate the comparison). In panel b, the gray bold line indicates the absorbance of FEC-● in neat solvent.

probe light is used to detect their absorbances on the submicrosecond time scale.

40

By

combining many of such transients obtained at different probe wavelengths it becomes possible to follow the time evolution of the entire spectrum between 350 and 1600 nm. Figure 5a exhibits the transient absorption spectra obtained in pulse radiolysis of FEC at delay times of 2, 20, 200, and 1500 ns. Immediately after the electron pulse, there


18

Page 19 of 38

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is a band that is centered at 430 nm that decays within 500 ns, yielding a species with spectrum stretching between 450 to 750 nm peaking at 600 nm. This weak signal strongly resembles the absorption spectrum of the CO3-● radical anion

48

and the presence of this

species (radical I in section 3.2) was also suggested by our matrix isolation studies (as it is the only radical (ion) without hfcc’s on 1H and 19F that is plausible in this system). As radical II cannot absorb in the visible, the initial absorbance originates from FEC-● radical anion that we identified as radical IV in section 3.2. No transient absorbance was found to the red of 800 nm, which is where the band of the solvated electrons was found for the diethylcarbonate.

49

It seems unlikely, therefore, that FEC supports metastable

solvated electrons; the progenitor of the 430 nm band is likely to be solvent supported monomer radical anion that is FEC-●. The decay of the 430 nm absorbance in neat FEC is nearly second-order out to 100 ns (Figure 23S) suggesting that the progenitor species decays by recombination or disproportionation, which is consistent with our attribution of this species to the solvent radical anion. The addition of 0.66M LiPF6 had little effect on the FEC-● absorption band at 430 nm, but it appears to suppress the yield of the CO3-● radical anion (Figure 5b), which is fully consistent with the low-temperature EPR observations discussed previously.


19


6x10

3

3

(a)

An* FEC

-•

G

4

72 mM An in FEC 2 ns 20 ns 200 ns 1500 ns

-•

2

An

0 400

1.5x10

500 600 700 wavelength, nm

800nm

-2

(b) 1.0

OD

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

Page 20 of 38

0.5

420 nm 430 nm 710 nm

0.0 0

500

1000 delay time

1500ns

Figure 6. Pulse radiolysis - transient absorption (a) spectra and (b) kinetic traces obtained for Arpurged room temperature FEC containing 72 mM anthracene-h10 (An). The delay times for the absorption spectra are indicated in panel a, and the selected wavelengths  for the kinetic traces are indicated in panel b (where OD is the corresponding optical density).

The addition of 72 mM anthracene-h10 (An) resulted in complete scavenging of the FEC-● anion (observed at 430 nm) and the formation of An-● over the first 300 ns (observed at 710 nm, see Figures 6a and 6b). The matching kinetics of this decay and formation (Figure 6b) suggest the occurrence of electron transfer from the FEC-● anion to the aromatic solute. In addition to the absorbance band of An-● at 710 nm, the narrow triplet-triplet absorption band at 420 nm from the anthracene T1 state is also observed (labeled 3An*, Figure 6a). Apparently, there is both the prompt formation of this excited triplet state (via the direct excitation of the aromatic solute to a singlet excited state followed by the intersystem crossing to the T1 state) and the delayed formation of this triplet state as the anthracene ions generated via the charge transfer reactions with the solute (Figure 6b) in their turn decay in recombination reactions. From the known molar extinction coefficients for the reaction products, we estimated that the radiolytic yields


20

Page 21 of 38

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for An-●, 3An*, and CO3-● were all ~ 0.1 molecules/100 eV, whereas the ionization yield is expected to be 3-5 ions/100 eV; this suggests (in agreement with our EPR studies) that only a small fraction of the radiolytically generated FEC-● anions transfer charge to the anthracene even at this high concentration; the yield of the carbonate radical anion in neat FEC is also quite low, suggesting that it is a side product, i.e. most of the FEC-● anions decay by recombination. In radiolysis, both positive and negative ions are generated as pairs in 1-5 nm clusters of several such pairs that are called radiolytic spurs (which are separated by 100-200 nm along the track of the fast electron). All reactions of these primary ions in spurs compete with their recombination; this competition does not occur in electrochemical reduction of the solvent. On the strength of these results, we surmise that most of the FEC-● anions rapidly decay by recombination/disproportionation. Whether some of these species decay to the CO3-● radical anion or the latter species is generated promptly within the duration of the electron pulse (through DEA reactions of the energetic electrons) cannot be concluded from these data. The concord between the pulse radiolysis – transient absorption and matrix isolation EPR spectroscopy studies suggest that the same reactions occur both in the frozen solid solutions and in room temperature liquid solutions, justifying the transferability of our results to the practically relevant temperature conditions in the electrochemical cells.

3.4. Chemical ramifications. At this point, the direct answer to the title question has been obtained: under the relevant conditions, one-electron reduction of FEC causes the elimination of LiF and CO2 with the formation of the vinoxyl radical (Scheme 2), whereas for EC, PC and acyclic carbonates it causes the ring-opening with the formation of the radical anions (Scheme 2). In this section, we scrutinize the ramification of this mechanistic difference. We first observe that reaction 2 is consistent with the theoretical calculations of Leung et al.

36

We are agnostic as to the precise route that this concerted reaction takes

(whether the C-F or C-O bond dissociates first), but this mechanistic detail is inconsequential from a practical standpoint. The C-F dissociation with the formation of radical 7 that was postulated by Lucht and co-workers

50


is excluded by our EPR

21


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Page 22 of 38

observations. Ditto for the ring opening reactions suggested by Chen et al. 18 and Nakai et al.

13

The scenario suggested by Aurbach and co-workers

15

is neither supported nor

rejected by our results, but this scenario tacitly assumes that other carbonates in the electrolyte are reduced before FEC providing the bases attacking this molecule, which is not supported experimentally. 13, 17, 18 We suggest that the SEI formation in FEC solutions is initiated by reaction 2. The beneficial effects of fluorinated anions (including the constituent anions in ionic liquid electrolytes) in LIBs

31, 51, 52

are well established, and the release of fluoride

via the reduction of these anions is the common thread. For example, our studies of bis(fluorosulfonyl)imide (N(SO2F)2-) reduction suggested that fluoride release, and the subsequent reduction of the residual ●SO2NSO2F radical to LiSO2F and NSO2-, would account for the formation of an all-mineral SEI in the ionic liquid electrolyte consisting of this anion, which can arrest eruptive Li0 dendrite growth.

53 54

This mechanistic

pathway is not the case for FEC: although there is ample evidence for LiF deposition and fluorination of the silicon oxide network on the surface, there is also the polymeric outer SEI (section 1). The unique property of FEC may be in facilitating both mineral deposition/modification in the inner SEI (where 2-electron reduction occurs and FEC is completely mineralized) and changing the character of the polymeric outer SEI (where FEC is incompletely reduced), as we proceed to discuss. The interesting feature of reaction 2 is that it yields the vinoxyl radical that in solution readily abstracts H from another FEC or carbonate molecule, yielding radical 2a. These radicals can recombine and disproportionate (Tables 2S and Scheme 1S in SI) yielding numerous secondary products, including fluorovinylene carbonate (FVC), structure 6 shown in Schemes 1 and 3, which is the product of radical disproportionation involving radical 2a. For most of these reactions, the heats calculated for FEC and EC are comparable (Table 2S in SI). However, the elimination of CO2 from radical 2a with the formation of radical 3a is significantly more exergonic for FEC than for EC (Table 2S in SI). This reaction yields the ●CH2CFO radical that can also abstract H from FEC, initiating chain reaction (X=H, F)


22

Page 23 of 38

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1 3a

- CO2 + 2a -1

2a

6 +R

+6 - CO2

Scheme 3. Initiation of Radical Polymerization By Radical 3a Generated in One-Electron Reduction of FEC (X=H or F).

●

CH2CXO + FEC → MeCXO + 2a● ●

(3)

●

2a → CO2 + CH2CFO

(4)

which can sustain living polymerization, as shown in Scheme 3 (where R● stands for any of the radicals involved). Reactions 2 to 4 provide the “fodder” for the radical polymerization that results in the incorporation of –CHFCH2- units into the polymer backbone, as these radicals add to the unsaturated products of radical disproportionation, such as FVC, as shown in Scheme 3.

22

The formation of such a polymer is strongly

suggested by the recent X-ray photoelectron spectroscopy study by Edstrom and coworkers.

35

At each step of the reaction, the growing radical attaches to FVC monomer,

and the resulting species can grow further, decarboxylate, recombine or disproportionate (see Scheme 2S in SI for a specific example).

22

We observe that the addition of VC

similarly introduces a monomer for the radical polymerization, which accounts for its frequent use as an SEI forming additive. For EC and like carbonates, the one-electron reduction causes C-O bond dissociation that frees the carbonate group, leading to the eventual formation of the terminal organic carbonates ROCO2- and the products of their decarboxylation, RO-. Both


23


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Page 24 of 38

of these bases can react with the cyclic carbonates, initiating anionic polymerization (see 22

and references therein). According to the product analyses, most of the oligomerization

in such electrolytes appears to be anionic, with internalization of the carbonate in the growing chains as ROC(O)OR’ groups (see below). The branching and cross linking of these polymer chains occurs only when the polymer internalizes certain products of radical reactions.

22, 23

In the reduction of FEC (reaction 2), terminal carbonates are not

formed, and so radical polymerization should be the favored route. Li+

+ Li+ + 1e-

+ Li+ + 1e- LiF - CO2 1,2-shift - CO2

cross-linking

Scheme 4. One-Electron Reduction of the Polymer Consisting of VC or FEC Units.

Here lies another peculiarity of FEC, which was implicit in the scenario considered by Nakai et al.

13

. Namely, reduction of the resulting polymer leads to

defluorination yielding the interior radical (Scheme 4). In a linear polycarbonate polymer, the reduction breaks the C-O bond dissociating this chain Li+ + 1e+ Li+


24

Page 25 of 38

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When two such terminal radicals recombine, another linear chain is formed, so the cross linking is inefficient and the cohesion of the network is low. In our previous study

22, 23

we surmised that for EC the cross linking occurs through side reaction products, such as VC (see Scheme 3), that become incorporated into the polymer chains, and for PC even this possibility is unlikely. For FEC and VC, however, cross linking through the recombination of interior polymer radicals (Scheme 4) should be efficient. Indeed, in both cases the reduction yields secondary radicals that can readily recombine. It is also likely that some of these interior radicals can undergo 1,2-shift followed by decarboxylation (as shown in Scheme 4), which further facilitates cross linking by allowing radicals in the matrix to migrate along the chains and subsequently recombine. Thus, not only the radicals generated in the decomposition of FEC can facilitate polymerization, but the polymer itself (as it becomes further reduced, see Scheme 4) has the inherent ability to morph into a 3D cross-linked network as it becomes further reduced.

4. CONCLUSION To summarize the results of sections 3.1 to 3.3, one-electron oxidation of FEC yields the H loss radical 2a, while the outcome of one-electron reduction depends on the energetics and the presence of fluoride binding Li+ cations in the matrix. If Li+ cations are not present and the electron energy is low, the reduction proceeds via reaction 1, otherwise it proceeds via reaction 2. In addition to these two reactions, the radical anion of FEC-● is formed; this species is sufficiently stable to decay in reactions (e.g. recombination) other than the C-O bond ring opening. The major mechanistic difference between the nonfluorinated and fluorinated carbonates hypothesized in Scheme 2 has been demonstrated experimentally.Our examination of the chemical ramifications of this crucial difference in section 3.4 suggest that the polymeric matrix formed by reduction of FEC in the outer SEI would be substantially more cross linked than in the polymeric matrix derived from EC and like carbonates. This high degree of cross linking, in turn, implies that this polymeric matrix can have elastomeric properties. We remind that in a polymer, elasticity and high failure strain always requires a high degree of cross-linking.


25


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Page 26 of 38

Estimates for bulk and shear moduli for different components of SEI obtained using atomic force spectroscopy and molecular dynamics calculations suggest that the polymeric matrix has the lowest elastic modulus of 1-2 GPa (compared to 10-20 GPa for lithium organocarbonates);

55

. This can be compared with 40-70 GPa for the inner

mineral layer of SEI on lithiated silicon.

56

The peculiarity of the outer SEI formed by

FEC may originate less from the degree of polymerization and/or the structure of the polymer chains than the efficient cross-linking of these polymer chains. The ability to efficiently form 3D cross-linked polymer networks (Scheme 4) is the inherent property of FEC reduction in the outer SEI that can be traced back to reactions shown in Scheme 2, and it inevitably makes this polymer a better elastomer. We conjecture that the “secret” of the SEI that is formed by FEC (and, to a lesser degree, by VC) is that it has elastomeric properties.

Scheme 5. Cross-Linked Polymeric Matrix Generated via Electrolyte Reduction on Li-Si Electrode as a Protective Elastomer Coating.

While much attention has been given to the deposition of LiF in the inner SEI and fluorination of the silicate layer (see above), by itself such a process may not always prevent runaway electrolyte reduction under the expansion and contraction of the Li alloying electrodes;

52

the excessive deposition of LiF may even cause SEI failure.

33

Since large volume changes in LixSiy nanoparticles are unavoidable in practical electrodes,

2

the outer SEI needs to withstand the stress induced by these changes

(Scheme 5); rupture of the outer SEI exposes the reductive LixSiy surface to the electrolyte, causing continuous Li+ consumption and, thereby, capacity fade. As illustrated in Scheme 5, the ideal SEI needs to be an elastomer that can be stretched


26

Page 27 of 38

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


without yielding during stress while remaining impermeable to the organic solvent. In comparison, the outer SEI that is formed on intercalation graphite electrodes only needs to exhibit impermeability to the solvent but not necessarily this elasticity. While defluorination reaction 2 can be important for conditioning of the inner SEI (e.g. through LiF deposition), this fluoride release may, actually, be a means to an end rather than the end in itself. The great advantage of FEC additive can be in the fortuitous use of this defluorination to facilitate radical polymerization and cross linking of polymer chains yielding an elastic coating, which is capable of withstanding stresses caused by cycling of the LixSiy electrodes.

ACKNOWLEDGMENT IAS thanks K. Quigley, R. Lowers, and S. Chemerisov for technical support. DPA thanks Solvay for providing the FEC used in this study, and K. Pupek, S. Trask, M. Klett and J. Gilbert for technical support. JFW thanks S. D. P. Dhiman for assistance with the pulse radiolysis experiments. This article is based upon work supported by the U.S. Department of Energy Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences under Award Numbers DE-AC02-06CH11357 (Argonne) and DE-SC0012704 (Brookhaven), which also supported use of the LEAF Facility of the Brookhaven Accelerator Center for Energy Research. The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.

ASSOCIATED CONTENT Supporting Information: A PDF file containing a list of abbreviations, two appendices, the tables presenting DFT results, and the additional schemes and figures. This material is available free of charge via the Internet at http://pubs.acs.org.


27


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Page 28 of 38

Table 1. Energetics of the gas phase Li0 initiated reduction reactions of FEC and EC according to B3LYP/6-31G(d,p) calculations. reactions

structure a

LiX + EC(-H)● LiCO3-● + CH2CHX LiX + ●CH2CXO + CO2 ● CHXCH2CO3Li CXH2●CHCO3Li ● CH2CHXCO3Li CH3●CXCO3Li

2a 5 3a 4b 4a 4c 4d

X=F FEC, eV b 0.85 0.86 1.48 1.58 1.78 1.79 1.97

X=H EC, eV b 1.06 1.78 2.07

a) as shown in Scheme 1; b) the column gives the negative enthalpies for a given reaction channel (Li0 + FEC) in the gas phase.


28

Page 29 of 38

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FIGURE CAPTIONS.

Figure 1. The first-derivative X-band EPR spectrum of 3 MeV electron irradiated frozen neat FEC (lower traces) and 1.5M LiBr solution (upper traces) obtained at 50 K, 0.02 mW, 2 G modulation at 100 kHz. The signal from the E’ center in the irradiated sample tube is removed from the plot. The open circles, filled and open squares indicate resonance lines from radicals I, II, and III.

Figure 2. First-derivative X-band EPR spectrum of frozen FEC irradiated by 3 MeV electrons at 70 K obtained during the programmed warming from 50 to 200 K. The temperatures are indicated in the plot. The dashed vertical lines indicate the resonance lines from radical I.

Figure 3. The first-derivative EPR spectra of 3 MeV electron irradiated frozen vitreous FEC containing 1M LiPF6 (trace i) and saturated

LiPF6 solution (trace ii). The insets

demonstrate magnified sections of trace (i) indicating the presence of radical II. The filled circles indicate the outer resonance lines of radical III. The narrow line is from radical IV.

Figure 4. The evolution of the first-derivative EPR spectra of 3 MeV electron irradiated frozen vitreous FEC containing 1M LiPF6 solution observed as the sample was gradually warmed from 50 to 175 K. The temperatures are indicated in the plot. Trace i was obtained at 175 K, and (normalized) trace ii was obtained after cooling down to 50 K. The rectangles highlight the resonance lines of radical II, which decays above 150 K. The filled circles and the vertical lines indicate the resonance lines of radical III.


29


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Page 30 of 38

Figure 5. Pulse radiolysis - transient absorption spectra obtained for (a) Ar-purged room temperature FEC and (b) solution containing 0.66 M Li PF6 at different delay times that are indicated in both panels. The arrows indicate the decay of the FEC radical anion whose absorption band is centered at 430 nm. At 1500 ns, when this absorption fully decays, one can observe a smaller signal centered at 600 nm that is attributed to the carbonate radical anion (the filled diamonds indicate this absorption magnified by ten times in order to facilitate the comparison). In panel b, the gray bold line indicates the absorbance of FEC-● in neat solvent.

Figure 6. Pulse radiolysis - transient absorption (a) spectra and (b) kinetic traces obtained for Arpurged room temperature FEC containing 72 mM anthracene-h10 (An). The delay times for the absorption spectra are indicated in panel a, and the selected wavelengths  for the kinetic traces are indicated in panel b (where OD is the corresponding optical density).


30

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O O F O

O

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What Makes Fluoroethylene Carbonate Different? - The Journal of

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