Reduction of Carbonate Electrolytes and the Formation of Solid

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Reduction of Carbonate Electrolytes and the Formation of Solid-Electrolyte Interface (SEI) in Lithium Ion Batteries. 1. Spectroscopic Observations of Radical Intermediates Generated in One-Electron Reduction of Carbonates. Ilya A. Shkrob, Ye Zhu, Timothy W. Marin, and Daniel P. Abraham J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp406274e • Publication Date (Web): 27 Aug 2013 Downloaded from http://pubs.acs.org on August 27, 2013

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The Journal of Physical Chemistry Shkrob et al.

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Reduction of Carbonate Electrolytes and the Formation of SolidElectrolyte Interface (SEI) in Lithium Ion Batteries. 1. Spectroscopic Observations of Radical Intermediates Generated in One-Electron Reduction of Carbonates. Ilya A. Shkrob*1, Ye Zhu, 1 Timothy W. Marin, 1,2 and Daniel Abraham 1 1

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

Cass Ave, Argonne, IL 60439 2

Chemistry Department, Benedictine University, 5700 College Road, Lisle, IL 60532

Received: July 1, 2013 Revised: August 12, 2013

ABSTRACT While there are numerous experimental and computational studies of electrochemical reduction leading to the formation of solid-electrolyte interface (SEI) in lithium ion batteries, so far there have been no direct spectroscopic observations of radical intermediates involved in the SEI formation. In Part 1 of this series, radiolysis and laser photoionization of carbonate electrolytes are used to observe and identify these reaction intermediates using electron paramagnetic resonance spectroscopy. Our study indicates that the suggested scenarios for electrolyte reduction require elaboration. In particular, we establish the occurrence of efficient H abstraction and 1,2-migration involving radicals generated through the reductive ring-opening. Instead of the primary radicals postulated in the current models, secondary and tertiary radicals are generated, biasing the subsequent chemistry to radical disproportionation. The consequences of this bias for radical and anionic polymerization are examined, and it is suggested that branching and the formation of a polymer network is favored. We argue that this chemistry accounts for some of heretofore unexplained properties of SEI, including the dramatic difference in solvent permeability for SEIs derived from ethylene carbonate and propylene carbonate.

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*) To whom correspondence should be addressed: Tel. (630) 252-9516, e-mail [email protected].


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1. INTRODUCTION Despite rapid advances in the understanding of the chemistry underlying the operation of lithium batteries,

1-10

the structure of solid-electrolyte interface (SEI) on

lithiated graphite anodes in contact with carbonate electrolyte (Scheme 1) continues to be insufficiently well understood.

11-17

There is a regrettable lack of direct spectroscopic

insight into the complicated chemistry of electrolyte reduction,

10, 18

which is only

partially mitigated by state-of-the-art quantum chemistry modeling, given the many approximations needed to model such complex systems. 19-30 Experimentally, it has been established that SEI is an (electron) insulating layer (5-10 nm)

25, 31

of organic/inorganic composite that enables Li+ conduction while

remaining impermeable to electrolyte molecules, keeping them from co-intercalation into the graphitic anode and further electrolytic breakdown of the solvent. The organic component of SEI consists of various products of electrolyte reduction; the electrolyte is typically a mixture of ethylene carbonate (EC) and a diluent, such as dimethyl carbonate (DMC) or ethylmethyl carbonate (EMC), as are shown in Scheme 1. Beyond this general description, significant divergence of opinion as to the nature of SEI exists in the literature. Some researchers suggest that the organic component of SEI consists mainly of dicarbonate molecules (Scheme 2 and the discussion below), 12, 14, 32

while the others believe it to be composed of much larger oligomers

polymers.

36, 37

23, 33-35

and/or

Even the initial stages of electrolyte reduction remain conjectural.

16, 17

One of the many unsolved puzzles about SEI is its striking structural sensitivity: while solutions containing EC (Scheme 1) form SEI that prevents further transfer of EC molecules to the anode surface, the same solutions containing PC (Scheme 1) yield SEI that remains permeable to PC molecules, resulting in graphite exfoliation and anode destruction.

38

It was the replacement of PC by EC in the early 1990s that (along with

other important advances in battery design) permitted wide use of lithium ion batteries in consumer electronics. How can so small a change result in such drastically different behavior? 39 In this paper, we use matrix isolation Electron Paramagnetic Resonance (EPR) spectroscopy to directly observe radicals that are generated during the reduction of carbonate molecules and establish the initial steps towards SEI formation. Before


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proceeding further, we briefly review previously suggested reaction mechanisms (Schemes 2 and 3). In the gas phase, PC and EC have no electron affinity, forming dipole-bound anions.

40

In liquid solutions and solid matrices, a solvent-supported molecular anion of

the >C●O- type is formed by one-electron reduction. This is suggested by quantum chemistry calculations (e.g., 29) and supported by EPR observations of PC-● in irradiated solid PC. 41 In the EPR spectra (see section 3.2), there is a narrow resonance line that was initially attributed to trapped electrons.

42, 43

However, subsequent EPR studies revealed

satellites from the coupled carbon-13 nucleus in the carbonate group with a hyperfine coupling constant (hfcc) characteristic of the radical anion. methods

19-21, 29, 44

41

Other computational

also suggest the formation of an intermittent radical anion of cyclical

carbonate (cC-●): cC + e-● → cC-●

(1)

In the optical spectra of irradiated PC, there is an absorption band

41-43

that was attributed

to a carbonate radical, CO3-●, suggesting the occurrence of a dissociative electron attachment (DEA) reaction cC + e-● → RCHCH2 + CO3-●

(2)

that may or may not be mediated by the radical anion. Retrospectively, it is clear that this 500-700 nm absorption band was not from CO3-● (whose spectrum was not known in the early 1970s) but from the PC-● radical itself. Nevertheless, the olefin eliminated in reaction 2 is one of the known volatile products observed in the electrolytic breakdown of carbonates. 45 Reaction 2 is not the only possible outcome of one-electron reduction. The prevalent opinion

17, 20, 23, 29, 44, 46

is that cC-● is inherently unstable to ring-opening,

undergoing C-O bond dissociation cC-● → QCH(R)●CH2 or QCH2●CHR

(3)

where we introduced the abbreviation Q for the terminal -OCO2- (Li+) group. Such DEA reactions have abundant precedence in radiolytic reduction of esters in low-temperature aqueous glasses 47 e-● + R'CO2R → RCO2- + R●


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Some chemists envision RCHCH2 elimination (cf. reaction 2) as the sequential reduction of an intermediate radical generated in reaction 3 14, 19, 21, 48 QCH(R)●CH2 + e-● → RCHCH2 + CO32tentatively assuming that the intermediate radical is sufficiently long lived to accept another electron. It is hard to see how such a reaction can actually occur given the high reactivity of such terminal radicals (section 4); a more logical scenario would be the occurrence of reaction 2 followed by the reduction of the carbonate radical, CO3-● + e-● → CO32that has much higher stability. Another suggestion 23, 49 is that DEA also proceeds by EC-● → O●COCH2CH2O-

(4a)

that is followed by the elimination of CO (Scheme 2) O●COCH2CH2O- → CO + (CH2O)2-●

(4b)

This reaction was suggested to account for carbon monoxide that was found among volatile products. Reaction 4b would be unprecedented in radical chemistry, as it is the reverse reaction that commonly occurs.

50

Alkoxycarbonyl radicals typically decompose

by CO2 elimination, 51 that is O●COCH2CH2O- → CO2 + ●CH2CH2OIt has been suggested, e.g. by Xu

16, 32, 52

(4c)

, that C-centered radicals generated in ring-

opening reaction 3 subsequently recombine via reaction (Scheme 2) 2 QC(R)H●CH2 → QCH(R)CH2Q + RCHCH2

(5a)

that yields an olefin and dicarbonate. The same product can be generated in recombination of this C-centered radical with the carbonate radical (Q●) generated in reaction 2 QCH(R)●CH2 + Q● → QCH(R)CH2Q

(5b)

although such a possibility has not been discussed. Reaction 5a would compete with the cross recombination that yields long-chain dicarbonate molecules, e.g. 2 QCH(R)●CH2 → (CH2CH(R)Q)2

(6)

The shorter dicarbonates generated in tentative reactions 5a and 5b have been observed using 1H and 13C NMR and mass spectrometry,

10

while these longer-chain dicarbonates

have not been observed. This challenges Scheme 2, as according to the calculations of Bedrov et al. 29 and Wang et al. 20 reaction 6 is ~0.5 eV more exergonic than reaction 5a


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and it has a ~0.65 eV lower reaction barrier. Such prohibitive energetics cast doubt that short-chain dicarbonates are indeed generated through reaction 5a. This difficulty would not exist for reaction 5b; however, in order for this reaction to greatly outpace reaction 6, the reduction must proceed almost exclusively via reaction 2, so a large excess of the carbonate radical generated in reaction 2 remains over the C-centered radicals generated in reaction 3 (Scheme 2). We will return to this issue in section 4 (where it is suggested that neither reaction 5a or 5b actually occur). The most popular view to date is that the organic component of the SEI are these short-chain dicarbonates.

10, 16

This would be surprising on several counts. First, such

dicarbonates have relatively high solubility in the organic electrolyte (which, incidentally, explains why these molecules can be so readily extracted for analyses).

28, 53

Second, no

interaction between these molecules can readily explain selective permeability of SEI for solvent molecules. Third, there is no obvious difference between the dicarbonates derived from EC and PC that accounts for the dramatic differences in the solvent permeability of the corresponding SEIs. That the dicarbonates are generated in the electrolyte reduction is indisputable, while their role in the SEI formation is tenuous. An alternative view regards the dicarbonates as initiators of anionic polymerization. 33-35, 54 It is known 55 that cyclical carbonates react with organic bases, 56 including the alkoxides and alkylcarbonates, in ring opening reactions (Scheme 3) RO- + EC → ROCH2CH2Q

(7a)

RQ + EC → ROC(O)OCH2CH2Q

(7b)

The two species, RO- and RQ, are coupled through equilibrium reaction RQ ← → RO- + CO2

(8)

In this paradigm, RQ anions (including the dicarbonates and free carbonate anion) and/or RO- anions (or their radical anion precursors) generated in reactions 3, 4, 5 and 8 subsequently react with the parent molecules via reaction 7a and 7b generating larger R'Q anions that once again enter reaction 7b or undergo reaction 8 followed by reaction 7a (Scheme 3). For EC, the growing oligomer/polymer consists of ethylene oxide (EO) and internalized ethylene carbonate units; the ratio of these two units depends on the relative rates of reactions 7a, 7b, and 8. Co-polymers consisting of the EO and EC units have been observed in situ using high-resolution electrospray ionization mass spectrometry, in


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the electrochemical reduction of EC/DMC electrolytes, 33, 34 and even larger polymers of unknown composition have been found in the SEI.

37, 57

Importantly, free carbonate

anions (see above) can participate in reaction 7b, and it has been suggested 20, 58 that the dicarbonates are formed from this nucleophilic attack rather than reaction 5a. Reactions 7 and 8 do seem to occur in the electrochemical breakdown of the electrolyte, but (as is the case with the dicarbonates) there is (i) no explanation of how such soluble, weakly interacting oligomer chains account for the unique SEI properties, and (ii) no account for the dramatic differences between EC and PC. While many of the important pieces of the jigsaw puzzle that comprise SEI formation have been discovered, these pieces do not yet quite connect. We argue that one of the "missing pieces" of this puzzle is the occurrence of secondary radical reactions that strongly change the subsequent chemistry. In Part 1 of this series, we examine one-electron reduction of linear (section 3.1) and cyclical (sections 3.2 and 3.3) carbonates and detail the likely consequences of the revised chemistry (section 4). In Part 2,

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extensive polymerization in radiolysis of ethylene

carbonate is observed, validating our deductions. These studies provide circumstantial evidence for the occurrence of cross linking and network formation in the SEI matrix and establish the conceptual framework in which heretofore unexplained observations pertaining to the SEI formation are rationalized. To save space, many of the 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. Unless stated otherwise, the reagents were obtained from Aldrich and used as supplied without further purification. N,N,N',N'-tetramethylphenylenediamine (TMPD), was twice sublimed in vacuum before use. Anthracene-d10 (d10An) and perylene-d14 (d14Pe) were obtained from Cambridge Isotope Laboratories. Battery grade LiPF6 and carbonates (Scheme 1) were obtained from Novolyte Technologies. Since the direct detection of short lived radicals in SEI was too challenging, three other methods were used to generate electrons in the bulk of the solvent: (i) 2.5 MeV


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electron radiolysis, (ii) biphotonic laser photoionization of aromatic molecules (Scheme 1S), and (iii) photodetachment from anions (Scheme 2S). For EPR spectroscopy, the solutions were deaerated through repeated freeze-thaw cycles in vacuum, and flame sealed samples placed in Suprasil tubes were frozen solid by quick immersion in liquid nitrogen. In radiolysis experiments, these samples were irradiated over 1 min to 3 kGy at 77 K using 2.5 MeV electrons from Argonne's Van de Graaf accelerator. Alternatively, 1-10 mM of aromatic photosensitizer was added (prepared under normal conditions) and the frozen solution was photolyzed using 35 mJ, 6 ns fwhm, 355 nm pulses from a Nd:YAG laser (Quantel Brilliant) to initiate biphotonic ionization of the solute, as shown in Scheme 1S. For electron photodetachment from ferrocyanide anion (Scheme 2S), filtered output of a 60 W Xe arc lamp was used. In the latter experiments, EC was stirred with potassium ferrocyanide overnight and the solution was filtered through a 0.2 µm pore Teflon filter. Alternatively, aqueous solutions of EC containing potassium ferrocyanide were used. The radicals in the irradiated samples were observed using a 9.44 GHz Bruker ESP300E spectrometer, with the sample placed in a flow helium cryostat (Oxford Instruments CF935) so that the sample temperature could be cycled between 4 and 250 K. The magnetic field B of the spectrometer 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. The radiationinduced EPR signal from the E'γ center in the Suprasil sample tubes (that frequently overlapped with the resonance lines of organic radicals) is shadowed white in the EPR spectra. The calculations of the hfcc’s and radical structures and energetics (Tables 1S to 3S) were carried out using a density functional theory (DFT) method with the B3LYP functional

60, 61

and 6-31+G(d,p) basis set from Gaussian 03.

62

In the following, a

denotes the isotropic hfcc corresponding to the hfc tensor, and B is the anisotropic part of this tensor with principle axes (a,b,c). Powder EPR spectra were simulated using firstorder perturbation theory. For convenience, the principal values of the g-tensor are reported as δgνν=(gνν-2•1)x104, where ν=x,y,z are the principal axes.


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We stress that radiolytic ionization necessarily generates both the products of solvent reduction and solvent oxidation. For cyclical carbonates, this radiolytic oxidation yields a radical cation that (similarly to other such species) promptly deprotonates, yielding the H loss radicals shown in Scheme 1 (which is very common and extremely rapid reaction that has been overlooked in current [gas-phase] theoretical studies).

63, 64

For PC, this PC(-H)● radical has already been identified by Shaede and Symons. 41

3. RESULTS. 3.1. Dimethyl- and ethylmethyl- carbonates. In order to contrast the behavior of linear and cyclical carbonates, we examined the two simplest carbonates, dimethylcarbonate (DMC) and ethylmethylcarbonate (EMC) which also serve as diluents in the commercial electrolytes. Figure 1 shows EPR spectra obtained in radiolysis of frozen DMC observed at 50 K, and Figure 1S exhibits the changes in these EPR spectra as the irradiated sample is warmed from 50 K to 200 K, at which point the matrix softens and the radicals decay. The quartet of the methyl radical (●CH3) and the doublet of the formyl (H●CO) radical are clearly discernible in these EPR spectra, and the same features are also observed in the EPR spectra obtained in 355 nm laser ionization of d10An in DMC (Figure 1), suggesting that both of these radicals are generated in DEA reactions 9 and 10 (Scheme 3S). e-● + (MeO)2CO → ●CH3 + MeQ

(9)

e-● + MeOC(O)OR → H2CO + H●CO + RO-

(10)

Reaction 9 is analogous to reaction 3 for cyclical carbonates, while reaction 10 is unique for DMC and EMC. In addition to these two radicals, there is also a narrow singlet line that disappears above 125 K (in the EPR spectrum shown at the bottom of Figure 1 it overlaps with the broader singlet from d10An+● cation). There is also a group of lines that become increasingly prevalent as the sample is warmed and methyl radicals disappear (Figure 1S). This group entirely comprises the EPR spectrum obtained at 200 K. It is less prominent in the photoionized samples, and it corresponds to a terminal radical that we identify with ●CH2OC(O)OMe (while other than the parent compound potential H atom


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donors are generated in photolysis and radiolysis, the probability of the corresponding Habstraction reactions is low given the low radical yield and the abundance of the parent compound). The formation of this radical in radiolysis is readily accounted through the oxidation of the solvent: (MeO)2CO → e-● + ●CH2OC(O)OMe + H+

(11)

(with another solvent molecule serving as the proton acceptor). The observations of the same radical in photoionization (Figure 1), albeit at a much lower yield, and through the transformation of the EPR spectra concurrent with the disappearance of the methyl radical (Figure 1S) suggest that it is also formed in a secondary reaction of the released fragment (X●) radicals, X● + (MeO)2CO → ●CH2OC(O)OMe + HX

(12)

●

where X can be either a methyl or formyl radical. It appears that some of the methyl radicals generated in reaction 9 promptly abstract H even at 77 K, whereas most of these radicals become trapped and abstract H after thermal activation. The narrow singlet line observed in the EPR spectra of irradiated DMC is likely to be a matrix-stabilized DMC-● radical anion. The latter species appears to be stable below 150 K. The EPR spectra observed in radiolysis of frozen EMC (Figure 2) are qualitatively similar to the ones observed for DMC. There is a narrow singlet that we attribute to EMC-● and a quartet from the methyl radical (which is much more prominent in the photoionized sample, Figure 2). In radiolyzed samples, there are several other radicals present, whose outer resonances are consistent with either the ethyl radical or MeOC(O)OCH2●CH2. These radicals begin to decay at 150 K (Figures 2S); the residual EPR spectrum is still complex and suggesting of the presence of at least two progenitors, one of which is likely to be the MeOC(O)O●CHMe radical and the other (poorly resolved triplet) is likely to be ●CH2OC(O)OEt. Both of these radicals can be regarded either as the oxidized/deprotonated EMC (reaction 11) or secondary radicals generated through H abstraction from EMC by the methyl or ethyl radicals generated via DEA. Both EMC and DMC, therefore, provide examples of oxidative and reductive channels, yielding the same radical species after the occurrence of secondary radical reactions. We stress that EPR observations of R● radicals in reduction of ROC(O)OR' carbonates are equally consistent with the occurrence of reaction 13


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e-● + ROC(O)OR' → R'O- + CO2 + R●

(13)

as opposed to e-● + ROC(O)OR' → R'Q + R●

(14)

The relative enthalpies of these two reactions depend on the stability of the terminal carbonates, and both reactions 13 and 14 are energetically feasible. Since the R'O- and R'Q anions interconvert through reaction 8, preference for reaction 14 over reaction 13 does not have great bearing on the subsequent chemistry. In the following, we will assume that DEA occurs via reaction 14.

3.2.Propylene carbonate. As was observed in the Introduction, radiolysis of frozen PC has been studied previously. Two radical species, PC-● anion and PC(-H)● radical (Scheme 1) were tentatively identified.

41

The latter radical is formed through oxidative deprotonation that

favors the loss of methine hydrogen (Table 2S); only such an H loss radical is consistent with the EPR spectra (see EPR simulations in Figure 4S). The radical anion can be identified through its

13

C satellites, indicating a radical with a=153 G and Bcc=28 G.

41

These hfcc parameters compare well with our DFT estimate for PC-● anion giving a=167 G and Bcc=27 G (Table 1S). Shaede and Symons

41

observed that the EPR spectrum

cannot be fully accounted for by only these two radicals (which is also consistent with our data), but spectral congestion prevented further analysis. The fact that the PC-● anion is quite stable at 77 K is remarkable given the widespread conviction that ring-opening of this anion is energetically favored and readily occurs in solution (see the Introduction). As the theoretical arguments for such ring-opening depend on the role of lithium ions in solvation of the product, we repeated these EPR experiments in the presence of 1 M Li PF6 (Figures 3a and 3S). The characteristic narrow line of the PC-● anion was still observed. Protracted storage of the irradiated sample at 77 K caused gradual decay of this species, and it completely disappeared in 3-5 days (Figure 3b). The wings of the residual EPR signal were almost exactly the same as the ones observed immediately after radiolysis (Figure 3b). Subtracting the EPR spectrum of an isothermally annealed sample from the EPR spectrum of a freshly irradiated sample yields a Gaussian line with a peak-


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to-peak width of 6.2 G. The PC-● signal also decayed as the freshly irradiated sample was warmed over 125 K (Figure 3S). It follows from these results that the presence of Li+ ion has no effect on the stability of the PC-● anion in a solid matrix. While we observed its gradual decay when the sample was isothermally and/or thermally annealed, our EPR data do not suggest that this decay was specifically due to the ring-opening, as back electron transfer would also account for these observations. Indeed, had the PC-● anion decayed through ring-opening reaction 3 or 4a, spectral signatures of the corresponding secondary radicals would be observed (Figure 5S). Contrary to these expectations, the EPR spectrum of the isothermally annealed irradiated samples remains the same. It follows that the relaxed PC-● anions do not undergo ring-opening, whether in neat solvent or in the presence of Li+ ions. A closer examination of the EPR spectra shown in Figure 3 reveals another oddity. Had the PC(-H)● radicals been generated exclusively through radiolytic oxidation of PC in reaction 11 (Scheme 4) while the reductive channel yielded exclusively PC-● anions, there would be parity in the yields of these two radicals. However, integration of the EPR spectrum obtained at low microwave power (in order to avoid microwave saturation) indicates that the PC(-H)● radicals are in 10X stoichiometric excess over the PC-● anions, i.e., the latter species cannot be the main product of reduction. In other words, while PC-● is a stable anion, the reduction yields some other radical(s), which is (are) "missing" from the observed EPR spectrum. This means either that (i) the PC(-H)● radicals are generated both through reductive and oxidative chemistry and (ii) that some of the EPR lines attributed to the PC(-H)● radical, in fact, originate through another radical that is generated through the reductive channel. As shown in section 3.1, for DMC and EMC generation of H loss radicals occurs both in the reduction and oxidation reaction channels, as the fragment radicals abstract H from the parent molecules. It is reasonable to expect that the same reaction occurs for cyclical carbonates. Since PC has a weak C-H bond at the tertiary carbon, H abstraction from this site is facile. If ring-opening DEA reactions yield the primary QCHMe●CH2 radical (reaction 15a), this radical can readily abstract H from the parent molecule (Scheme 4)


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e-● + PC → QCHMe●CH2

(15a)

QCHMe●CH2 + PC → QCHMe2 + PC(-H) ●

(15b)

Apparently, reaction 15b is so facile, even at 77 K, that the parent terminal radical is never observed. In radiolysis, the electrons are generated with excess energy of a few electron volts, gradually thermalizing through inelastic scattering in the matrix. Most of the electron attachment reactions involve electrons that have sufficient excess energy to initiate reaction 15a; only a fraction of these electrons thermalize and attach to PC forming the stable radical anion. This rationale accounts for the disparity in the radical yields, but it also presents a problem: as secondary radicals are more stable than primary radicals, alternative ringopening reaction (Scheme 4) e-● + PC → QCH2●CHMe

(16a)

should produce a radical which is less reactive in H abstraction reactions, so it could be expected to stabilize. In the gas phase, QCH2●CHMe is 0.14 eV more stable than QCHMe●CH2 (Table 2S), so it is not clear why reaction 15a should be greatly preferred to more exergonic reaction 16a. To address these concerns, we examined the temperature dependence of the EPR spectra for isothermal annealed irradiated PC solutions (Figure 4). The matrix softens at 165 K, so radicals cannot be observed above this temperature. As rotation of the methyl group in the PC(-H)● radical becomes free above 125 K, the EPR spectra observed at 150 K are more amenable to comparison with the simulated EPR spectra. As shown in Figure 5, the wings of the EPR spectrum are well reproduced by the PC(-H)● radical with hfcc parameters given in Table 1 (which are consistent with our DFT calculations and estimates reported in the literature, Table 4S), but there are also discrepancies indicating the presence of other radical species. Addition of 1 M Li PF6 increases the softening temperature of the glass, so the sample can be warmed to 185 K. Above 150 K, the PC(H)● radical decays and the residual EPR spectrum (Figure 6)

reveals a seven-line

multiplet that corresponds to a radical with six equivalent (or nearly equivalent) protons. Among the possible radicals, QCH2●CHMe and Q●CMe2 radicals can yield such a pattern (Figure 6S; the hfcc's for reference radicals are given in Table 5S). Once the rotation of the methyl groups becomes free and the anisotropies average out, the latter radical


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exhibits narrow lines corresponding to six magnetically equivalent protons. The QCH2●CHMe radical yields broader lines as the anisotropies in the methine and methylene protons are not averaged out through the rotation of the methyl group. We remind that alkyl radicals can undergo 1,2-migration of hydrogen in which the locus of the unpaired electron shifts by a C-C bond as a result of internal H atom transfer.

65, 66

EPR observation of the Q●CMe2 radical (that can only be a secondary

radical) would suggest that the primary QCHMe●CH2 radical can rearrange to a more stable tertiary radical QCH(Me)●CH2 → Q●CMe2

(16b)

that is even more stable than the PC(-H)● radical (which is also a tertiary radical). Our DFT calculations (Table 2S) suggest that the Q●CMe2 radical is 0.34 eV more stable than the QCH2●CHMe radical generated in reaction 16a. To establish the presence of the Q●CMe2 radical, we changed the relative yield of the PC(-H)● radical, as the latter (according to our hypothesis) can be generated both through reduction and oxidation. To this end, PC solutions containing 1M Li PF6 and 1 M Li Br were irradiated (Figure 7). The bromide anion is readily ionized, yielding bromine atom that reacts with another bromide and yields the Br2-● anion. The EPR spectrum of this anion is spread over 3.5 kG, so it has negligible overlap with the resonances of organic radicals (Figure 6S). As bromide becomes ionized instead of the PC molecules, the yield of the PC(-H)● radical decreases, and through comparison of the EPR spectra in solutions with and without the bromide (that are normalized by the outer resonance lines to which only PC(-H)● radical contributes) one can contrast resonance lines originating from other radicals (Figure 7). This comparison yields the same seven-line pattern that was observed in Figure 6. More importantly, the lines observed in the 125 K spectrum are narrow, suggesting that the progenitor radical is indeed Q●CMe2. Remarkably, this species is generated even at low temperature, suggesting that 1,2-migration may be concerted with the DEA, as it typically requires thermal activation. 66 Another way to support the reaction scheme shown in Scheme 4 is to use laser photoionization, as it excludes the oxidation of the solvent (as the radical cation of the aromatic molecule is generated instead). Figure 8S(a) exhibits the EPR spectrum obtained in photoirradiated d10An/PC. There is a narrow singlet line at the center (mainly from


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d10An+●) and another narrow line at g=2.01485 (see below). Most importantly, there is also a background line that is clearly from the PC(-H)● radical, proving our conjecture that the latter radical can be generated through electron reduction chemistry (Scheme 4). The g=2.01485 signal decays at 125 K (Figure 8S(b)), whereas the resonance lines from the d10An+● and PC(-H)● radicals persist to the glass transition point. Even higher relative yield of the g=2.01485 line was observed in the d14Pe/PC system (Figure 8a). Subtracting the normalized EPR spectrum observed at 100 K and 75 K, we obtained the EPR spectrum that is shown at the top of Figure 8b, which corresponds to a radical with δg=(156, 63, 63), which is very close to g-tensor parameters reported for CO3-● in other

solid matrices (Table 6S). It appears, therefore, that in this photosystem DEA occurred not only through reaction 15 (that also occurs in radiolysis) but also through more energetic reaction 2, which was not observed in radiolysis. Since d10An+● and d14Pe+● exhibit narrow lines, it is impossible to tell from EPR spectra shown in Figures 8 and 8S whether PC-● was produced in laser photolysis. Since the TMPD+● cation has a broad resonance line, we used a TMPD/PC system to resolve this issue (Figure 9S(a)). As this radical cation (Wurster's blue) is exceptionally stable, the temperature can be increased above the glass transition point (in order to destroy other radicals) and then lowered back, to allow observation of the TMPD+● cation without interference of other radical species (Figure 9S(b)). At 50 K, in addition to the broad signal from TMPD+● cation and the resonance lines from CO3-● and PC(-H)●, there clearly is a narrow resonance line from PC-●. We conclude that biphotonic ionization results in the concurrency of all the electronattachment reactions shown in Scheme 4. We can now provide a consistent account of PC reduction summarized in Scheme 4. The outcome of the reaction depends on the electron energy. At low energy (thermal electrons), stable PC-● anion is formed. There is no evidence that this anion undergoes thermally-activated ring-opening, with or without Li+ present in the solution. At intermediate electron energy, ring-opening occurs with C-O bond dissociation at either one of the carbons. The resulting primary radical promptly abstracts H from the parent molecule or undergoes 1,2-migration, which may occur in concert with C-O bond dissociation during DEA. Since the latter reaction yields a tertiary radical that is


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significantly more stable than a secondary radical that would be generated in the alternative C-O bond dissociation (reaction 16a) the latter reaction does not seem to occur. Finally, if the electron energy is sufficiently high, both of the C-O bonds dissociate, and the carbonate radical is generated. The main product of the reduction is the PC(-H)● radical (which is formally the oxidation product!); in addition, some Q●CMe2 radicals are generated. Both of these species are tertiary radicals. With this knowledge, we turn to the reduction of EC.

3.2. Ethylene carbonate. The EPR spectra observed in irradiated EC and 1 M Li PF6 solutions are indicative of the presence of several radicals (Figure 9). Additional complications arise from arrested rotation in some of these radicals. Some conclusions, however, can be reached even from cursory examination of these EPR spectra. Neither the carbonate nor EC-● radicals are observed, which suggests the prevailing occurrence of ring-opening DEA reaction e-● + EC → QCH2●CH2

(17)

Since EC has only methylene carbons, H abstraction reaction QCH2●CH2 + EC → QEt + EC(-H)●

(18)

should be less facile than the corresponding reaction 15a for PC. The competing reaction is radical 1,2-migration QCH2●CH2 → Q●CHMe

(19)

The EC(-H)● radical generated in reaction 18 can also be generated via oxidation. The QCH2●CH2 and EC(-H)● radicals can be fingerprinted as shown in Figure 10S. Extrapolating from the previous example (section 3.2) we can expect that at least three radicals contribute to the EPR spectra: EC(-H)●, QCH2●CH2, and Q●CHMe. The simulated EPR spectra for these three species (using the hfcc obtained in our DFT calculations and from comparisons to reference systems, see Tables 1S and 5S to 7S in the Supplement) are shown in Figures 10, 11, and 10S. These simulations indicate considerable spectral overlap complicating identification of these radicals. The easiest species to identify is QCH2●CH2, as its resonance lines disappear from the EPR spectrum above 150 K (Figures 9 and 11S). This radical makes the largest


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contribution at 125 K, and in Figure 10 we simulated this spectrum using the hfcc parameters given in Table 1. In Table 7S these estimates are compared to the ones calculated for gas-phase QCH2●CH2 and -OCH2●CH2 radicals and the hfcc's reported for other XCH2●CH2 radicals. It is seen that the -OCH2●CH2 radical provides a poor fit for the observed hfcc's, which is one of the reasons to believe that DEA proceeds via reaction 14 rather than reaction 13 (section 3.1). The closest hfcc parameters are observed for X=OH and OCF3, while a(Hβ) for X=O2CMe, OtBu, and OSO3- is lower (~25 G vs. 31 G). Our DFT calculation yields good estimates for 1Hα proton, but poor estimate for 1Hβ proton (44.8 G), which is not surprising given that the carbonate group is strongly solvated. The calculated anisotropic B tensors are close to the experimental estimates given in Table 1. All of these results point to the occurrence of reaction 17, in agreement with Scheme 2. The occurrence of reaction 18 (which is not present in Scheme 2) is suggested by conservation of the integral of the EPR signal during the disappearance of the lines of QCH2●CH2 at 150 K transition (Figures 9 and 11S). The resulting EPR spectrum observed at 200-250 K only approximately corresponds to the EC(-H)● radical (Figure 11 and Table 1). The extracted hfcc parameters agree with the previously reported estimates for isotropic hfcc's in this radical (Table 8S). Essentially the same EPR spectrum was observed for EC and Li+ salt solutions at 200-250 K (Figure 12S), whereas at lower temperature there are considerable differences (Figure 13S) as well as slow evolution of the spectrum during isothermal anneal (e.g., Figure 14S). In Figure 15S, we juxtaposed the experimental spectrum obtained at 230 K and simulated spectra for EC(-H)● and Q●CHMe (Tables 1 and 5S) radicals to illustrate how their combined spectra account for the features observed. Gradual annealing of the sample at 300 K converts this complex EPR spectrum into a triplet shown in Figure 12. At low temperature this triplet exhibits additional structure that disappears at 150 K and reappears when the sample is cooled back to 50 K (see simulations in Figure 16S with parameters given in Table 1). Just such an EPR spectrum is expected from a methylene radical with broken magnetic equivalency between two 1Hα protons and a single 1Hβ proton. The hfcc parameters are similar to those of the vinoxyl radical (●CH2CHO) in which 1Hβ reversibly moves out of the plane in the low temperature matrix (see the plot of the hfcc's as a function of the dihedral angle


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in Figure 17S). As this radical is derived from either EC(-H)● or Q●CHMe radicals, we suggest that the former species undergoes reaction 20 at room temperature, EC(-H)● → ●CH2CHO + CO2

(20)

In the gas phase, this reaction is exergonic by 0.64 eV (Table 3S). As shown in Part 2, acetaldehyde is one of the products of radiolysis of EC, which supports the occurrence of reaction 20. The latter needs considerable thermal activation (as the C-O bond dissociation is strongly endergonic), which accounts for our inability to observe the analogous reaction for PC (as the PC(-H)● radical decays at 150-180 K before reaction 20 can occur). Overall, EC and PC seem to have rather similar reduction chemistry (Schemes 4 and 4S), with the exception of a more facile ring-opening DEA in the former molecule. We also tried to initiate the reduction of EC using photoionization of aromatic molecules (sections 3.1 and 3.2). While we observed strong EPR signals from the corresponding aromatic radicals (Figure 18S), the lines from EC related radicals were not discernible. Carbonate radicals (that were so prominent for photoionization of PC) were not generated. Some organic radicals were observed in photoexcitation of ferrocyanide in aqueous and anhydrous EC (Figure 19S); the pattern is broadly consistent with the Q●CHMe radical.

4. DISCUSSION. Our studies suggest that the outcome of one-electron reduction strongly depends on its energetics (Scheme 4). Two secondary radical reactions following ring-opening DEA (that is, H abstraction and 1,2-migration) readily occurred even at cryogenic temperature; these reactions certainly occur in room-temperature solutions. The lifetimes of the QCH(R)●CH2 radicals that are envisioned to be the progenitors of dicarbonates in reactions 5 and 6 (Scheme 2) cannot be sufficiently long for these radical reactions to occur, as this lifetime is limited by H abstraction and/or 1,2-migration. The short lifetime of these primary radicals would naturally account for the surprising lack of the postulated long-chain dicarbonates (which is consistent with the experiment), but the formation of short-chain carbonates through this mechanism would require that the recombination of the QH(R)●CH2 with the carbonate radical occurs faster


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than H abstraction, which seems unlikely. These considerations suggest that the reaction path to dicarbonates should be more involved than suggested by Scheme 2. Assuming that QCH2CH2● radicals convert to EC(-H)● before they react with other radicals, the dicarbonates can be formed in another sequence of reactions (Scheme 5), that is Q● + EC(-H)● → EC(-H)Q

(21a)

e-● + EC(-H)Q → Q●CHCH2Q

(21b)

with the resulting Q●CHCH2Q radical abstracting H atom from the solvent Q●CHCH2Q + EC → (CH2Q)2 + EC(-H) ●

(21c)

●

and generating an EC(-H) radical that enters reaction 21a, sustaining the chain reaction (Scheme 5). According to our DFT estimates (Table 2S), despite the steric hindrance in EC(-H)Q, reaction 21a is more exergonic than reaction 5b (3.65 eV vs. 3.52 eV), as there is slight repulsion between the two (lithiated) carbonate groups in the (CH2Q)2. Reaction 21b is analogous to ring-opening reactions for EC and PC. In Part 2,

59

we demonstrate

13

using C NMR that the EC(-H)Q species is one of the main products of radiolysis of EC solutions, which corroborates the mechanism. The same Q●CHCH2Q radical can also be generated via addition of the carbonate radical to QCHCH2 (which is a product of radical disproportionation, see below). Yet another possibility is that the carbonate radical reacts with EC analogously to reaction 7b (Scheme 5S). Q● + EC → (CH2Q)2+●

(22a)

e-● + (CH2Q)2+● → (CH2Q)2

(22b)

It has been suggested (see the Introduction) that the (CH2Q)2 is formed in nucleophilic attack of the carbonate anion, Q- (rather than carbonate radical, Q●) on EC.

20, 58

In this

scenario, the role of radical chemistry is to produce carbonate radicals that are subsequently reduced to carbonate anions. In such a case, it would not matter whether the carbonate radical first reacts with EC and the resulting radical anion is reduced (reactions 22a and 22b) or the carbonate radical is reduced and the resulting dianion reacts with EC (Scheme 5S). Importantly, while EC(-H)● is unstable at room temperature, slowly undergoing reaction 20, this reaction yields the vinoxyl radical that is capable of H-abstraction,


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CH2CHO + EC → MeCHO + EC(-H)●

(23)

sustaining reactions 21. This explains why the formation of (CH2Q)2 is (overall) a two electron process despite the occurrence of side reactions: the latter (e.g., reactions 21c and 23) reintroduce the EC(-H)● radical. Another chemical consequence of the revised reaction scheme is the bias towards radical disproportionation. While primary radicals readily recombine (the yield of radical disproportionation relative to the recombination is typically 0.15:1), secondary radicals disproportionate more readily than they recombine (0.7:0.3), whereas tertiary radicals overwhelmingly disproportionate (Scheme 6).

67

Thus, the reactions of (for example)

Q●CHMe and EC(-H)● radicals would yield olefinic species, such as vinylene carbonate (VC, Scheme 1) and QCHCH2. As shown in Part 2,

59

VC is indeed one of the major

reaction products in radiolysis of EC. Thus, in addition to ethylene (reaction 2), radical reactions can introduce unsaturated products. These olefins can sustain radical polymerization.

68

In Scheme 7, we envision a

simple reaction of this kind involving stepwise growth of a polymer through the addition of QCHCH2 units. Unlike the anionic polymerization (Scheme 3) that internalizes carbonate groups, this polymerization yields pendant carbonate groups. Molecular modeling in Figure 13 suggests that repulsion between such groups will point them into opposite directions. When the polymer chains are placed side by side, Li+ ions "zip" the chains so that each Li+ is shared by two carbonate groups and each Li+ has tetrahedral coordination (Figure 13). A similar motif has been observed by Borodin et al.

69

in

molecular dynamics calculations for Q-terminated molecules (mono- and dicarbonates). Our semi-empirical MNDO calculation supports the existence of this binding motif (Figure 20S). Such "zipping" qualitatively accounts for the formation of "mats" that are impenetrable to solvent molecules, while Li+ conduction can occur through an ionexchange mechanism, with the incoming Li+ ion releasing bound Li+ ions.

69

While we

are not suggesting that the highly regular structure shown in Figure 13 is present in the SEI, even a moderate amount of Q-branch inclusion can account for strong interchain interaction in the polymer leading to 3D network formation. If the secondary radicals were not involved, only weakly-interacting linear polymers would be formed.


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It is now possible to rationalize the differences between EC and PC. As shown in section 3, the initial reactions in EC and PC are, basically, the same; the difference emerges only when the secondary radical reactions are considered. As the stable radicals derived from PC (that is Q●CMe2 and PC(-H)●) are both tertiary, they are much more likely to disproportionate than recombine (Scheme 8). While this disproportionation introduces olefins, it is easy to see that the resulting radical adducts would also tertiary radicals (Scheme 8), so radical polymerization is impeded at every step. Thus, a small change in the chemical structure has dramatic consequences for the efficiency of radical polymerization, facilitating it for EC and impeding it for PC. Without the formation of branched chains (Scheme 7 and Figure 13), "zipping" of these chains does not occur, and the resulting matrix remains permeable to solvent molecules. The occurrence of radical polymerization also rationalizes the beneficial role of vinylene carbonate (VC), which is the common electrolyte additive for stable graphite SEI formation.

46, 48, 70, 71

As noted above, VC is, actually, one of the products of radical

disproportionation. Addition of radicals (X●) to the double bond of VC yields EC(-H)X● adducts (Scheme 6S). Further growth of the polymer chain internalizes the EC unit as shown in Scheme 6S. Electron reduction of such internalized units (in analogy to ringopening DEA reactions for cyclical carbonates) yields a carbonate side group and an internal radical. Both of these centers facilitate cross-linking between polymer chains, by "zipping" in the former case and through C-C bond formation in the second case. The above scenario postulates the occurrence of radical polymerization. What if this polymerization is anionic? As EC(-H)● species are prevalent organic radicals in electrolytic breakdown of EC, EC(-H)X species (where X● is either Q● (reaction 21a) or a Q-terminated radical) are the most common recombination products. When the growing polymer reacts with such reaction products (Scheme 9), the terminal unit becomes doubly Q-terminated, whereas reactions 7 involving EC always yields single Q-termination. Thus, a bifurcation point in the polymer chain is introduced, creating the possibility for the formation of a 3D network. Even if one of the Q-branches in Scheme 9 avoids subsequent reaction 7a or 7b, it can "zip" the polymer chains as shown in Figure 13. For PC, only tertiary radicals are generated whose cross reactions are biased to disproportionation that yield olefins incapable of supporting polymer growth via radical


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polymerization (Scheme 7). Since cross recombination of such radicals is also suppressed, the yield of Q-branched recombination products (analogous to EC(-H)X species shown in Scheme 9) is greatly decreased, so there is no Q-branching of the polymer chains in anionic polymerization, too (that can still occur via reactions 7). The same bias that makes radical polymerization inefficient also makes Q-branching in anionic polymerization inefficient. We reach the following general conclusion: the very fact that the reduction of EC produces secondary radicals as opposed to primary radicals (envisioned in Scheme 2) naturally leads to the formation of branched polymers. This occurs regardless of the polymerization mechanism, which can be either radical or anionic, or a combination of both. Each Q-branch on the growing polymer (Figure 14) has dual function, serving either as a locus for further chain growth (via anionic polymerization) or becoming a site for "zipping" of the polymer chains through lithium ions. A cross-linked 3D dimensional polymer network is formed in this fashion. Without this network formation, only linear chains are present and the resulting matrix remains permeable to solvent molecules. The reduction of PC yields only tertiary radicals that cannot sustain either radical polymerization or Q-branching in anionic polymerization, and this cross-linked 3D network is not formed. Linear polymers and dicarbonates are still generated, but in the absence of cross-linking they do not form a sufficiently cohesive matrix.

5. CONCLUDING REMARKS. In this study, we sought to establish radical chemistry initiated by one-electron reduction of linear and cyclical carbonates using direct spectroscopic observations of reaction products. Our study complements and revises current scenarios (Scheme 2) and provides new insights in SEI formation, which would be difficult to obtain without experimental guidance. One-electron reduction of cyclical carbonates does not follow the single chemical route. The most exothermic reactions dissociate two C-O bonds and yield the carbonate radical; less exothermic reactions cause single C-O bond dissociation and ring-opening; the least exothermic reactions yield stable radical anions. The latter do not undergo thermally activated ring-opening. The radicals generated in ring-opening reactions are


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unstable: primary radicals readily abstract H from the parent molecule, while both primary and secondary radicals undergo 1,2-migration. As a result, the second-generation (more stable) radicals are either secondary (for EC) or tertiary (for PC). As discussed in section 4, this distinction has nontrivial consequences for radical and anionic polymerization. For EC, side carbonate groups are introduced into the growing polymer chain. For PC, this "branching" is suppressed due to the tendency of the tertiary radicals to disproportionate rather than recombine. Had primary radicals been the predominant products of one-electron reduction, as postulated in Scheme 2, only linear oligomers would be formed. With the secondary radicals in play, branching polymers forming a 3D network are generated. This network is connected not only through chain branching (Scheme 8) but also through "zipping" of free carbonate branches through lithium ions (Figures 13 and 14). We suggest that it is this 3D network structure that gives SEI its unique properties. We do not dispute previous results suggesting that dicarbonates and linear oligomers are generated in electrochemical reduction of cyclical carbonates. Rather, we suggest that such molecules play auxiliary roles in SEI. In particular, linear oligomers lack side carbonate groups allowing strong interchain interactions, whereas the dicarbonates are too small and soluble to form a rigid network. We caution the reader that while our conclusions regarding the initial radical chemistry in section 3 are based on direct spectroscopic observations, the scenario for subsequent reactions and SEI formation presented in section 4 is (at this point) an informed guess that is based on our mechanistic findings. In Part 2 of this study this scenario is further supported and strengthened. The main difference between the photolytically and radiolytically induced reactions in the bulk that are examined here and electrochemically initiated reactions on graphite is the influence of the electrode surface and solid inorganic component of the SEI, neither of which was modeled in our studies. Such interfacial effects can only be studied in situ, and require different experimental approaches.


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ACKNOWLEDGMENT IAS thanks P. Fenter, L. Curtiss, P. Zapol, H. Iddir, and A. Gewirth for useful discussions. The work at Argonne was supported by the US-DOE Office of Science, Division of Chemical Sciences, Geosciences and Biosciences under contracts Nos. DEAC02-06CH11357.

ASSOCIATED CONTENT Supporting Information: A PDF file containing a list of abbreviations, synthetic procedures (Section 1S), reaction schemes 1S to 6S, Tables 1S and 8S, and Figures 1S to 20S with captions, including the experimental and simulated EPR spectra, reference data, and DFT calculations. This material is available free of charge via the Internet at http://pubs.acs.org.


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Scheme 1. Structural Formulas for Electrolytes Ethylene Carbonate (EC) and Propylene Carbonate (PC), Their Hydrogen Loss Radicals (EC(-H)● and PC(-H)●, Respectively), Electrolyte Additive Vinylene Carbonate (VC), and Common Electrolyte Diluents Dimethyl Carbonate (DMC) and Ethylmethyl Carbonate (EMC).

R=H: EC R=Me: PC

EC(-H)• PC(-H)•

VC

R=H: DMC R=Me: EMC


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Scheme 2. Current Scenarios for Reduction of Cyclical Carbonates and the Formation of SEI Components. In this Scheme, Q Stands for the -OCO3- Li+ Group.

R

O

-

+e

O

Q

O

x2

R

+ e-

Q

Q R

O Q O

Q

OR

- CO + e-

R

R

R dicarbonates -

O

-

O

R


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Scheme 3. Anionic Polymerization of Ethylene Carbonate.

-CO2 +EC

+EC

further growth

co-polymer of EO and EC units


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Scheme 4. Summary of Radical Products Generated in Photo- and Radiolytically Induced Redox Reactions of Cyclical Carbonates. Hydrogen 1,2-Migration May Occur Either as a Thermally-Activated Secondary Reaction or in Concert with the Ring-Opening.

R

R

Q

Q + e?

O

O

O -

O

O

O

O

+e

O

R

R - e-

- H+

-

O R

+ eCO3-

R

O O

O

-CO2

R


O R

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Scheme 5. Possible Reaction Pathways for the Formation of (CH2Q)2 (see also Scheme 5S).

O

O Q

O

O

O

O Q

+ e-

+ EC Q

Q

Q

Q Q

radical disproportionation

Q


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Scheme 6. Radical Recombination and Radical Disproportionation. "Wavy" Bonds Indicate Either a Chemical Bond or an Aliphatic Spacer, Depending on the Radical Partner.

recombination

Q

2 2

Q

Q Q

Q Q

Q

&

Q

disproportionation

O O

Q

Q O

O

O

O O

Q Q

O

Q

O

Q

Q


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Scheme 7. Growth of the Polymer Chain in Radical Polymerization and the Formation of Pendant Q-Branches (For EC). The Growth-Sustaining Olefin Compound Can Also Be Ethylene and VC.


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Scheme 8. Suppression of Radical Recombination and Polymerization in Reduction of PC that Yields Tertiary Radicals.

O O

O O

Q

reduction

O

RECOMBINATION SUPPRESSED

O

O Q O

O

DISPROPORTIONATION FAVORED

Q Q Q Q

Q

polymerization suppressed


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Scheme 9. Anionic Polymerization Involving (i) Regular Ethylene Carbonate Molecules (see also Scheme 3) and (ii) a Recombination Product EC(-H)X, Where X is a Q-Terminated Radical (see Scheme 6).

(i)

+ EC

+ EC linear polymer growth

(ii) + EC

bifurcation point, Q-branch formation


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Figure Captions.

Figure 1. First-derivative EPR spectra of frozen dimethylcarbonate (DMC) irradiated by 2.5 MeV electrons (at the top) and 355 nm laser light (at the bottom) at 77 K. The latter solution contained 10 mM anthracene-d10. These EPR spectra were obtained at 50 K using a microwave power of 2 mW (solid line) or 0.02 mW (dashed line). The magnified spectrum (x10) indicates the resonance lines of the formyl radical. The resonance lines of the methyl radical are indicated with open circles and vertical lines, and the resonance lines of the ●CH2OC(O)OMe radical are indicated with arrows. The singlet resonance line at the center of the lower spectrum involves the spin transitions in the d10An+● radical cation.

Figure 2. Like Figure 1, for ethylmethylcarbonate (EMC) and anthracene-d10 and perylene-d14 solutions. Dashed lines indicate the EPR spectra obtained at low microwave power (0.02 mW), while the solid lines indicate the spectra obtained at 2 mW (showing some microwave saturation in the central line indicated by the open square). Vertical lines and the open circles indicate the resonance lines of the methyl radical, the filled circles indicate the doublet of the formyl radical, and the arrows indicate the outer resonance lines of the ethyl radical, with the possible additional contribution from the ●

CH2CH2OC(O)OMe and Me●CHOC(O)OMe radicals.

Figure 3. (a) Normalized EPR spectra of frozen PC and PC solution containing 1 M Li PF6 irradiated by 2.5 MeV electrons at 77 K. The spectra obtained at 55 K using the microwave power of 0.02 mW (solid lines) and 2 mW (dashed lines). The narrow saturable singlet is from the PC-● anion. The background signal is mainly from the PC(H)● radical. (b) After 5-day storage of the irradiated PC sample at 77 K, the signal from the PC-● anion disappears. The difference trace shown at the top of the panel is attributed to this radical anion.


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Figure 4. Temperature dependence of the EPR spectrum of the annealed irradiated sample from Figure 3b. The matrix becomes soft at 165 K. As the temperature increases, the rotation of the methyl group in the PC(-H)● radical becomes free (see Figure 5).

Figure 5. Best least squares fit of the EPR spectrum shown in Figure 4 (solid line, 150 K trace) using the simulation parameters given in Table 1 (the simulated trace is shown using dotted line).

Figure 6. At the top: EPR spectrum of PC sample irradiated by 2.5 MeV electrons at 77 K and annealed at 160 K. The spectra were obtained at 125, 100, and 75 K using the microwave power of 0.02 mW. Lack of a temperature effect on the appearance of this spectrum suggests rapid rotational averaging of hfcc anisotropies. At the bottom: EPR spectrum of irradiated 1 M Li PF6 solution both annealed and observed at 175 K. At this temperature the PC(-H)● radical decays and 7-line pattern from another radical species (later shown to be Q●CMe2) is observed (vertical lines). The arrows indicate the resonance lines from a spin center in the irradiated sample tube.

Figure 7. Comparisons of EPR spectra from (i) PC, (ii) PC solution containing 1 M Li PF6 and (iii) PC solution containing 1 M Li Br irradiated at 77 K and annealed fro 3 days at 77 K. The EPR spectra were obtained at (a) 50 K and (b) 125 K at low microwave power (0.02 mW). The difference traces obtained by subtracting trace (ii) from trace (iii) are shown at the top of each panel, and the resonance lines of the Q●CMe2 radical are indicated by vertical lines. These spectra were normalized by the outermost resonance lines of the PC(-H)● radical.

Figure 8.


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(a) Normalized EPR spectra (trace i) obtained from frozen PC solution containing 5 mM perylene-d10 and irradiated by 355 nm laser light at 77 K (0.02 mW, 50 K). The narrow line indicated by the open circle is from PC-● anion and d10Pe+● cation. The resonance lines of the PC(-H)● radical are indicated by the arrows. Trace iii is the EPR spectrum of the same sample annealed at 150 K (observed at 50 K). At this temperature the carbonate radical and PC-● decay and the residual EPR spectrum is from the d10Pe+● and PC(-H)●. Trace iii indicates the EPR spectrum of neat PC irradiated by 2.5 MeV electrons at 77 K and annealed at 77 K. (b) EPR spectra of the same photosystem obtained at 75 K and 100 K. A selective decay of the carbonate radical is observed in this temperature range, and the difference trace iv shows predominantly the resonance line of the carbonate radical. Trace v is the simulated EPR spectrum obtained using the parameters given in Table 1 (cf. Table 6S). The arrows indicate magnetic fields corresponding to the alignment along the principal axes of the g-tensor.

Figure 9. EPR spectra obtained at low microwave power for EC irradiated by 2.5 MeV electrons at 77 K. The arrows indicate the outermost resonance lines of the QCH2●CH2 radical (see Figures 10S and 11S). The vertical lines indicate the outermost resonance lines of the EC(-H)● radical. See Figures 10 and 11 for simulations. There is no indication of the presence of the carbonate radical (cf. Figure 8) or a narrow, saturable line of EC-● in these EPR spectra (cf. Figure 3).

Figure 10. Simulated EPR spectrum of the QCH2●CH2 radical (broken line) with the arrows indicating finger printing resonance lines. The solid line indicates the EPR spectrum of EC solution containing 1 M Li PF6 that was irradiated by 2.5 MeV electrons at 77 K and thermally annealed at 125 K (2 mW). The simulation parameters are given in Table 1.

Figure 11. Simulated EPR spectrum of the EC(-H)● radical (dotted line) compared with the EPR spectrum (solid line) of EC solution containing 1 M Li PF6 that was irradiated by 2.5


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MeV electrons at 77 K and thermally annealed at 230 K (0.02 mW). The simulation parameters are given in Table 1.

Figure 12. The effect of the room temperature anneal on the EPR spectra of EC solutions containing 1 M Li PF6 and irradiated by 2.5 MeV electrons at 77 K and thermally annealed at 230 K (trace i). Following the anneal, the EPR spectrum transformed to trace ii. Both traces i and ii were obtained at 50 K. When the same species was observed at 150 K, the spectrum reversibly changed to trace iii. The solid lines are traces obtained using the microwave power of 2 mW and the broken lines are for the traces obtained using the microwave power of 0.02 mW.

Figure 13. HyperChem view of "mats" composed of (QCHCH2)∞ chains. Side carbonate branches are "zipped" through lithium ions.

Figure 14. The conceptual view of the SEI suggested by our mechanistic study. The dendritic polymer is mainly composed of the EO and EC units with occasional "forks" (Scheme 9) some of which involve free carbonate groups that "zip" the polymer chains. The resulting 3D network is a dense mesh preventing the diffusion of solvent molecules to the anode surface. Inorganic components and small molecules are entrapped in the voids of this polymer mesh.


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Table 1. Estimated hyperfine parameters for selected radicals. a)

radical QCH2●CH2

1

Η

2x α 2x β

EC(-H)●

α

2x β Q●CHMe

PC(-H)●

Q●CMe2

α

CH2CHO,

Baa , G

Bbb , G

Bcc , G

-21

-15

0

15

(-22.8) c

(-13)

(0)

(13)

31

-1.5

-1.5

3

(44.8)

(-2)

(-1)

(3)

-18

-14

0

14

(-25.5)

(-13.4)

(0)

(13.4)

35

-2

-2

4

(38.2)

(-1.8)

(-1.4)

(3.2)

-24

-13

0

13

(-22.8)

(-12.2)

(0.3)

(11.9)

3x

23

βc

(20.9)

2x β

-32.8

-0.4

-0.4

0.7

(37.8)

(-1.7)

(-1.3)

(3)

-5.3

2.6

2.6

(-9.8)

(-0.9)

(10.7)

-16.5

-8

-1.4

9.5

(-19.0) d

(-10.7)

(-0.6)

(11.2)

6.8

-1.3

-1.3

2.6

(-1.9)

(-0.6)

(2.5)

3x

19.4

βc

(23.9)

6x β 21.3 c

●

a,G

α(1)

50 K

(18.8) -22.4 (-18.8)

α(2) β

(0.4) ●

CH2CHO,

150 K

d

d

2x α

-22.4

-9.7

-4.3

14

β

2.8

-1.2

0.3

0.9


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a) the directions of the principal axes for the hyperfine tensors were taken from our DFT calculations (Table 1S). b) estimated from DFT calculations for gas-phase -O2CO- terminated radicals (Table 1S). c) free rotation, isotropic hfcc's only. d) planar radical see Tables 4S, 5S, 7S, and 8S for comparisons with reference radicals.


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TOC stamp graphic (for abstract).

e-•

N

? S

EPR of radical intermediates of SEI formation


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47

rev. 1


Figure 1, Shkrob et al.

2 mW 0.02 mW

H CO

H CO

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

x10 radiolysis

photolysis CH3 CH2OC(O)OMe

3300

DMC, 50 K 3400G


Page 48 of 62

Page 49 of 62


EMC, 50 K 2 mW


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


H CO CH3 CH2Me

radiolysis

photolysis (An)

photolysis (Pe)

3300

3400G



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


(a)

(b)


Page 50 of 62

Page 51 of 62


150 K


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


125 K

100 K

75 K

50 K

PC, 0.02 mW radiolysis at 77 K 5-day isothermal anneal

3300

3400G




experiment simulation


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

experimental dsp

3300

3400G


Page 52 of 62

Page 53 of 62


PC, 0.02 mW radiolysis 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


125 K 100 K 75 K

cool down from 160 K

175 K w 1 M Li PF6

3300

3400G



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


(a) 50 K

(b) 125 K

3300

3400G


Page 54 of 62

Page 55 of 62

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



3300

3400G




300 K -> 200 K 230 K


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

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

EC, 0.02 mW radiolysis

3300

3400G


Page 56 of 62

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 57 of 62

Q EC w 1 M LiPF6 125 K, 2 mW sim. QCH2 CH2

-50G

0

50




experiment simulation


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

3350

3400G


Page 58 of 62

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



EC + 1 M Li PF6


Page 59 of 62

(iii)

150 K

- CO2

(ii)

(i)

3300

50 K

50 K

3400G



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


O

C

Li


Page 60 of 62

Page 61 of 62

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



Li+ Li+



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

N

e

? S

EPR spectroscopy of radical intermediates of SEI formation


Page 62 of 62

Reduction of Carbonate Electrolytes and the Formation of Solid

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