Mechanistic Insight into the Protective Action of Bis (oxalato) borate

Oct 18, 2013 - ... of oxalate (for BOB) or fluoride anions (for DFOB) and the concurrent formation of oxalatoboryl adducts. ..... Michael L. Lazar , B...
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Mechanistic Insight into the Protective Action of Bis(oxalato)borate and Difluoro(oxalate)borate Anions in Li-Ion Batteries. Ilya A. Shkrob,*,† Ye Zhu,† Timothy W. Marin,†,‡ and Daniel P. Abraham† †

Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439 United States ‡ Chemistry Department, Benedictine University, 5700 College Road, Lisle, Illinois 60532 United States S Supporting Information *

ABSTRACT: Lithium bis(oxalato)borate (LiBOB) and lithium difluoro(oxalato)borate (LiDFOB) are uniquely efficient as electrolyte additives in carbonate-based Li-ion batteries. Electrochemical redox reactions of these anions facilitate the formation of robust solid-electrolyte interphases (SEIs) at the graphite and oxide electrodes. We used electron paramagnetic resonance (EPR) spectroscopy to demonstrate that oxidation of these anions causes the elimination of a carbon dioxide molecule and the formation of a stable acyl radical, whereas the reduction of these anions results in the loss of oxalate (for BOB) or fluoride anions (for DFOB) and the concurrent formation of oxalatoboryl adducts. The latter species enters a previously identified radical cycle implicated in SEI formation. Recombination of the acyl radicals at the oxide− electrolyte interface yields difluoroborane dimers that (being strong Lewis acids) form strong B−O bonds with oxygens at the surface, thereby passivating the electrode and preventing oxidation of the electrolyte.

1. INTRODUCTION Li-ion batteries1−3 are based on electrolytes containing lithium salts. Acyclic (e.g., ethyl methyl carbonate) and cyclic [e.g., ethylene carbonate (EC) and propylene carbonate (PC); see Scheme 1] carbonates containing 1−2 M LiPF6 are typically used. Although hexafluorophosphate anion prevents anodic dissolution of Al current collectors,4,5 this anion gradually decomposes to F− and PF5 as cells cycle or age, especially at elevated temperatures, which accelerates electrolyte decomposition.6 Recently, the lithium salts lithium bis(oxalato)borate (LiBOB) and lithium difluoro(oxalato)borate (LiDFOB) (Scheme 1) have been considered as promising electrolyte additives for Li-ion batteries, providing many thermal and electrochemical benefits.7−11 Both LiBOB and LiDFOB improve the long-term cycling performance of cells. Recent data indicate that high-energy cells containing the baseline electrolyte (3:7 w/w ethylene carbonate/ethyl methyl carbonate containing 1.2 M LiPF6) lose 73% of their initial discharge capacity after 200 cycles in the 2.2−4.6 V window. In contrast, such cells containing 1 wt % LiBOB and 2 wt % LiDFOB as electrolyte additives lose only 23% and 36%, respectively, of their initial discharge capacity.12,13 Electrochemically induced redox reactions of the B[ox]2− and F2B[ox]− anions (Scheme 1) have been implicated, as these anions reduce at 1.7 and 1.6 V, respectively, and oxidize at 4.6 and 4.4 V versus Li/Li+, respectively.12,13 Both additives facilitate the formation of a more robust solid-electrolyte © 2013 American Chemical Society

interphase (SEI) at the graphite electrode. This interphase is an organic−inorganic composite that serves as a physical barrier to undesired chemical reactions, including solvent intercalation into the electrode and solvent breakdown. These additives are also known to passivate the oxide electrode (such as Ni- and Mn-substituted LiCoO2) in high-capacity high voltage Li-ion cells.12−17 For example, cycling experiments in cells with a Li− Sn reference electrode showed that the LiDFOB additive reduces impedance rise at the Li1.2Ni0.15Mn0.55Co0.1O2-based positive electrode. It has been speculated that oxidation of F2B[ox]− at the oxide surface yields difluoroboryl (F2B•) radicals (Figure 1, structure i) that attack electrolyte molecules and induce their chain polymerization, leading to the formation of an organic SEI that protects the electrolyte from further oxidation while permitting Li+ conduction and reducing dissolution of transition-metal ions from the electrode.12 The B[ox]2− anion can be oxidized in a similar way, although supporting evidence is presently lacking.13 The conceptual difficulty with this rationale is that the tentative polymer products are also readily oxidizable under the conditions favoring electrolyte oxidation. In this study, matrix-isolation electron paramagnetic resonance (EPR) spectroscopy was used to directly observe radicals generated through radiolytically induced redox Received: August 1, 2013 Revised: October 14, 2013 Published: October 18, 2013 23750

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structures v,a and v,b), whereas one-electron reduction results in the loss of a fluoride anion (for F2B[ox]− anion) or oxalate dianion (for B[ox]2− anion). To save space, additional tables, figures, and schemes are available in the Supporting Information.

Scheme 1. Chemical Structures of Ethylene Carbonate (EC), Propylene Carbonate (PC), Bis(oxalate)borate (B[ox]2−), and Difluoro(oxalate)borate (F2B[ox]−)

2. EXPERIMENTAL AND COMPUTATIONAL DETAILS Heat-treated samples of LiDFOB were sealed in vacuo in Suprasil tubes and frozen by rapid immersion in liquid nitrogen. The samples were irradiated by 2.5 MeV electrons to 3 kGy at 77 K. The radicals were observed using a 9.44 GHz EPR spectrometer at 50−200 K (2 G modulation at 100 kHz). The magnetic field and the hyperfine coupling constants (hfcc’s) are given in the units of gauss (1 G = 10−4 T). The microwave power is indicated in the figures. The calculations of hfcc tensors and radical structures were carried out using a density functional theory (DFT) method with the B3LYP functional18,19 and 6-31+G(d,p) basis set from Gaussian 03.20 In the following discussion, aiso denotes the isotropic hfcc constant, and B denotes the anisotropic part of the hyperfine tensor. Powder EPR spectra were simulated using first-order perturbation theory assuming isotropic g tensors. 3. RESULTS AND DISCUSSION 3.1. Preliminary Considerations. Our DFT calculations indicate that, in the gas phase, elimination of CO2 from oxidized X2B[ox]− anion (X = F or X2 = ox) according to reaction 2 X 2B[ox]− → {X 2B[ox]}• + e−•

(1)

{X 2B[ox]}• → X 2BOC•O + CO2

(2)

is exergonic by 0.27 and 0.36 eV, respectively (see Table 1S, Supporting Information, and Figure 1, structures iv,a, iv,b, v,a, and v,b), whereas the elimination of the second CO2 molecule X 2BOC•O → X 2B• + CO2

(3a)

is endergonic by 1.24 and 1.29 eV, respectively (Figure 1, structures i and iii). Decarbonylation of this radical X 2BOC•O → X 2BO• + CO

(3b)

is also endergonic by 1.06 and 1.09 eV, respectively, suggesting remarkable stability of the X2BOC•O radical as compared to alkyloxycarbonyl radicals that readily eliminate CO2.21 Our calculations favor a planar geometry of this X2BOC•O radical, although the rotation barrier for the X2B group is only 0.16− 0.19 eV (Table 1S, Supporting Information). The reduction of the X2B[ox]− anions is likely to proceed through the stepwise elimination of X− anions (which, for B[ox]2−, amounts to ring-opening; see Scheme 1S, Supporting Information) Figure 1. Geometry-optimized structures [B3LYP/6-31G(d,p) method] of gas-phase radicals: (i) difluoroboryl, (ii) {FB[ox]}−•, (iii) oxalatoboryl, (iv) [ox]BOC•O, and (v) F2BOC•O. Energetics and hfcc constants of these radicals are given in Tables 1S and 2S, respectively, of the Supporting Information. Conformation a refers to a planar X2BOC•O radical, and conformation b refers to a mirror symmetrical out-of-the-plane geometry.

X 2B[ox]− + e−• → {XB[ox]}−• + X−

(4)

{X 2B[ox]}−• → [ox]B• + X−

(5)

(see Figure 1, structures ii and iii). The energetics of these reactions depend on solvation/stabilization energies for eliminated anions; this energy is higher for fluoride than for oxalate. Figure 1 displays geometry-optimized structures for gas-phase X2B•, {FB[ox]}−•, and X2BOC•O radicals (structures i and iii, structure ii, and structures iv,a−v,b, respectively), and Figure 2 shows simulated EPR spectra for these species using

reactions of F2B(ox)− anions. We demonstrate that oneelectron oxidation of this anion results in the loss of a carbon dioxide molecule, yielding the F2BOC•O radical (Figure 1, 23751

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the unpaired electron is coupled only to the 11B nucleus, with aiso ≈ −2.2 G (Table 1S, Supporting Information). The tentative X2BO• radicals (reaction 3b) are similar to type-1 boron oxygen hole centers observed in various oxide materials,24,27−29 having small aiso(11B) values of 10−15 G and a highly anisotropic g tensor. The F2BO• radical would also exhibit aiso(19F) ≈ 14 G. 3.2. EPR Spectroscopy. Figure 3 displays EPR spectra for irradiated frozen LiDFOB. For comparison, we juxtapose EPR

the estimated hfcc tensors given in Table 2S of the Supporting Information.

Figure 2. Simulated first-derivative EPR spectra for radicals shown in Figure 1 (same notations). In panel a, radical vi is {FB[ox]}•. In the upper traces, EPR spectra of F2BOC•O radical depend on the F−B− O−C dihedral angle ϕ, as aiso(19F) varies with this angle. The spectra for (a) ϕ = 0° and (b) ϕ = 90° are shown, as well as the EPR spectrum corresponding to free rotation of the F2B group. The arrows indicate the M(219F) = ±1 components of the triplet due to coupling to the two 19F nuclei. In panel b, simulated EPR spectra of boranyl radicals are shown for 11B isotopomers. The quartet of the resonance lines for the oxalatoboryl radical corresponds to the four projections of nuclear spin on the magnetic field of the spectrometer for the 11B (spin-3/2) nucleus.

Figure 3. First-derivative EPR spectra obtained for irradiated frozen P14 BOB and crystalline LiDFOB (50 K). In both panels, traces i and ii were obtained for LiDFOB using microwave powers of 2 and 20 mW, respectively. Trace iii in panel b is from P14 BOB (note the magnification factor). The solid squares indicate resonance lines from the formyl radical, and the open circles indicate the quartet of radical I (cf. Figure 2).

Boron has two naturally occurring magnetic isotopes, 10B (spin-3, 20%) and 11B (spin-3/2, 80%). Recall that 12C and 16O are nonmagnetic nuclei, whereas 19F is a spin-1/2 nucleus with a large magnetic moment. The [ox]B• σ radical (Figure 1, structure iii) is a boron dangling-bond center, in which aiso(11B) ≈ 290 G. This estimate compares favorably to a B E′ center [that is, a (−BO)2B• radical] observed in irradiated vitreous B2O3 and borosilicate glasses.22−25 For the 11B isotopomer, the corresponding EPR spectrum is a quartet (Figure 2b). The related 19F211B• radical (reaction 3a), shown in Figure 1, structure i, would have a readily recognizable EPR spectrum (Figure 2b) because of its large aiso(19F) value; see Tables 1S and 2S (Supporting Information). For F2B• in Xe matrix at 4 K, Nelson and Gordy26 reported isotropic hfcc values of 297 and 190 G for 11B and 19F nuclei, respectively. Other radicals shown in Figure 1 have much smaller or negligible aiso(19F) and aiso(11B) values; see Tables 1S and 2S (Supporting Information) and Figure 2a. For 19F211BOC•O (Figure 1, structures v,a and v,b), aiso(19F) depends on the F− B−O−C dihedral angle, being smallest when the F2B group is orthogonal to the C σ orbital and largest when the F2B group is in the symmetry plane of the radical (Figure 1), which is the energetically favored conformation in the gas phase. The [ox]11BOC•O radical (Figure 1, structures iv,a and iv,b) has a much narrower resonance line than this F2BOC•O radical, as

spectra observed from irradiated P14 BOB in ref 30 (where P14+ is 1-butyl-1-methylpurrolydinium cation). The narrow singlet in Figure 3a has been previously attributed to the [ox]BOC•O radical (Figure 1, structures iv,a and iv,b).30 In agreement with Figure 2a, the singlet line in the EPR spectrum of LiDFOB is wider. The tentative F2BO• and F2B• radicals are not observed (Figure 3a,b), whereas both LiDFOB and P14BOB exhibit identical quartets of resonance lines from radical I (which is indicated by the open circles in the plot). The observed pattern corresponds to a singly coupled 11B nucleus with aiso ≈ 77 G; there are no coupled 19F nuclei. This radical I cannot be the [ox]11B• dangling-bond radical, which would have a significantly greater aiso(11B) value, as discussed above. As seen from Table 2S (Supporting Information) and the simulations in Figure 2a, both X2BOC•O and {FB[ox]}−• radicals (Figure 1) can potentially contribute to the singlet line shown in Figure 3a. To resolve this issue, we annealed the sample (Figure 4a,b). As the temperature increases, the EPR signal decreases, as radicals decay at higher temperatures, and the rotation of the F2B group becomes less hindered, so side lines (Figure 4a) from the two fluorine-19 nuclei with Azz∼12 G (cf. Figure 2a) become more clearly observed. This implicates the presence of the F2BOC•O radical (Figure 1, 23752

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2S and Table 3S, Supporting Information). In the resulting pyramidal Bδ‑•O3 group, the dangling bond has an increased B 2p character, and this distortion considerably reduces the isotropic hfcc that depends on the B 2s character of the σ orbital. The same effect can qualitatively account for the observed properties of radical I, as our DFT calculations indicate that [ox]B−•OC(O−Li+)-terminated radicals with pyramidal boron atoms have reduced aiso(11B) values; see Figure 1S and Table 3S (Supporting Information). For B[ox]2−, such a species can form through a reductive ring-opening reaction 4 that yields the [ox]Bδ‑•OC(O−Li+)CO2Li radical shown in Scheme 1S (Supporting Information). This possibility was considered by Amine et al.15 DFT calculations indicate that the B−OC bond dissociation energy in this radical would be ∼1.6 eV (vs 1.24 eV for the difluoro analogue; Table 3S, Supporting Information), suggesting that the elimination of oxalate from reduced X2B[ox]− is energetically prohibitive. However, this open-ring radical cannot be radical I, as the analogous species for F2B[ox]− would have large hfcc’s (∼113 G) in the 19F nuclei (Table 3S, Supporting Information), which is inconsistent with EPR observations. Therefore, we suggest that X2B[ox]− anions are reduced to boranyl radicals (reaction 5) rather than such intermediate radicals (reaction 4), but the former species does not exist as a free radical, adding to another X2B[ox]− anion (Figure 1S and Scheme 2S, Supporting Information). This addition can involve a carbonyl oxygen, yielding a C-centered radical, or a bridging oxygen and involve ring-opening with the formation of a B-centered radical (Scheme 2S, Supporting Information). In the gas phase, the B−O bond in the carbonyl oxygen adduct is exergonic by 2−2.5 eV (Table 3S, Supporting Information). These energetics suggest that the tentative [ox]B• radical would be unstable in solid LiDFOB, just as the B E′ centers are unstable in alkali borate glasses.22,24 However, the negligible aiso(11B) value in this carbonyl oxygen adduct (Table 3S, Supporting Information) excludes it as a progenitor for radical I. In contrast, an open-ring [ox]Bδ‑•OC(O−Li+)CO2BX2 species with pyramidal boron (with an O−B− O−O angle of ∼126°) has a comparably low aiso(11B) value. In the gas phase, the bridging oxygen addition is nearly thermoneutral (Table 3S, Supporting Information). As the stabilization of such open-ring radicals depends on interactions with several Li+ cations in the matrix, these gas-phase calculations might significantly underestimate binding energetics. Given the precedent of boron electron center formation in related systems,24,25 we suggest that radical I is a variant of such a center with the structure shown in structures iii of Scheme 2S and Figure 1S (Supporting Information). Thus, radiolytically induced one-electron reduction occurring in crystalline LiDFOB should be written as (X = F)

Figure 4. Progression of EPR spectra from irradiated LiDFOB as the sample warms from 50 to 200 K. Traces i and ii correspond to EPR spectra for samples annealed at 300 K and observed at 50 K for microwave powers of 2 and 0.02 mW, respectively. The arrows indicate the same resonances as in Figure 2a. Panel b shows the magnified wings of the EPR spectra with resonances from radical I indicated by vertical lines. The dashed 50 K trace corresponds to the sample annealed at 300 K. The solid squares indicate the resonance lines from the formyl radical.

structures v,a and v,b). Another indication favoring this attribution is the approximate parity in radiolytic yields of radical I and F2BOC•O (as estimated by double integration of the resonance lines). This parity is expected provided that the former radical is a trapped electron center and the latter radical is a trapped hole (electron-deficiency) center, as the ionization yields equal numbers of electron and hole centers. Thermal annealing of the irradiated sample causes gradual decay of radical I, as shown in Figure 4b. When the annealed sample is subsequently cooled to 50 K, the doublet of the formyl (HC•O) radical is observed. The same doublet (indicated by solid squares in Figure 3b) can also be observed at high microwave power before annealing, as the formyl radical is less saturable than radical I. We believe that this is an impurity radical that originates from residual moisture in the sample. The yield of this radical is very low compared to those of the other two radicals. We conclude that one-electron reduction of X2B[ox]− anions yields B-centered radical I (rather than C2O3−•, F2BO−•, or other alternatives examined in ref 15), whereas one-electron oxidation yields the X2BOC•O radical (rather than the X2B• radical, as was previously suggested).12 The relatively small aiso(11B) value in B-centered radical I makes it analogous to the so-called boron electron centers in irradiated alkali (Alk+) borate glasses.22,24 Shkrob et al.24 suggested that these centers are (−BO)2Bδ‑•···O(Alk+)−B< radicals, in which the spin-bearing boron couples to a terminal Alk+O−B< group. This radical can also be thought of as an adduct of the boranyl radical to nonbridging oxygen (Scheme

e−• + 2Li+X 2B[ox]− → [ox]Bδ −•OC(O−Li+)CO2 BX 2 + Li+ + 2X−

(6)

Reaction 6 is driven not only by elimination of smaller anions (X−) but also through B−OC bonding in the adduct radical. 3.3. One-Electron Reduction on Graphite Electrode. These EPR observations have ramifications for the electrochemical reduction of X2B[ox]− anions in Li-ion batteries. On the graphite electrode, the release of F− from F2B[ox]− and oxalate from B[ox]2− contributes to the formation of the inorganic inner layer of the SEI.1,31 The tentative oxalatoboryl radical can react with carbonate molecules, such as EC (Scheme 23753

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1), either by B−O addition or by H-abstraction. DFT calculations (Scheme 3S and Figure 2S, Supporting Information) suggest that the reaction [ox]B• + EC → [ox]BOCH 2CH 2OC•O → [ox]BOCH 2CH 2• + CO2

(7)

is favored energetically, with a B−O bond energy of ∼2.16 eV and an overall enthalpy of 2.86 eV, yielding radicals a and b shown in Figure 2S (Supporting Information). We emphasize that (as in reaction 6), reactions 5 and 7 must be concerted, as the release of free [ox]B• radical would be energetically prohibitive; that is, the net reaction is e−• + X 2B[ox]− + EC → [ox]BOCH 2CH 2• + CO2 + 2X−

(8)

The resulting terminal radical [ox]BOCH2CH2• can abstract hydrogen from the EC,32,33 yielding the EC(−H)• radical (Hloss radical at carbon-4, Scheme 1), so the eventual outcome of these radical reactions might not depend on the specific chemical pathway (Scheme 1S, Supporting Information). Both the −OCH2CH2• and EC(−H)• radicals have been implicated in stepwise formation of dicarbonates and polymer components of the outer SEI,1,32,33 which might account for the benign role of X2B[ox]− anions in SEI formation. 3.4. One-Electron Oxidation on Positive Electrode. We now turn to reactions on the positive electrode. As argued above, the X2BOC•O radical (Figure 1) is the main product of one-electron oxidation of F2B[ox]−, so further chemistry depends on the fate of this radical. The F2BO termini in this radical and related difluoroborane products are similar to BF3, a strong Lewis acid, in that the (tetragonal) boron strongly interacts with oxygen atoms in other molecules (including alcohols, ethers, and carbonyls in ketones and esters; see ref 34). BF3 also strongly interacts with bridging and nonbridging oxygens on the metal oxide surfaces (e.g., ref 35). As the oxidation of F2B[ox]− occurs near the surface (Figure 5a,b), it can be expected that the radical stays at this surface, being anchored to a bridging oxygen. Similar bonding would also occur in the electrolyte solution. Our DFT calculations (Scheme 4S, Supporting Information) suggest that X2BOC•O radical forms a B−O bond with the carbonyl oxygen of the EC (∼0.37 eV). Although this interaction weakens the B−O bond in the radical, the CO2 elimination is thermoneutral unless there is ring-opening in the EC unit of the resulting adduct radical (Scheme 4S, Supporting Information). The X2BOC•O radical can also abstract H from the EC, yielding the corresponding formate ester, HCO2BX2, and EC(−H)• radical, that is, H-loss radical at carbon-4 (Schemes 1 and 4S, Supporting Information). The C−H bond energies in the latter are 4.42 and 4.44 eV for X = F and X2 = ox, respectively, compared to C4−H bond energies of 4.48 and 4.42 eV for EC and PC, respectively (Scheme 1). This implies that H abstraction from EC is nearly thermoneutral, similarly to CO2 elimination from the EC−X2BOC•O complex (see above). The resulting formate ester would be as readily oxidized as the electrolyte itself 2EC + h+• → H+(EC) + EC( −H)•

Figure 5. Suggested reaction sequence on the positive electrode in Liion batteries (where M represents a transition-metal ion). (a) Oxygen hole centers at the surface react with adsorbed F2B[ox]− anions. (b) The resulting F2BOC•O radicals, being Lewis acids, strongly interact with bridging and nonbridging oxygens at the surface and recombine with each other. (c) The product of recombination of these radicals, (CO2BF2)2, remains chemisorbed at the surface, serving as the foundation for the protective interface layer.

carbonyl oxygen. That is, one can expect regeneration of these X2BOC•O radicals through the reaction EC + HCO2 BX 2 + h+• → H+(EC) + X 2BOC•O

(10)

These considerations suggest low reactivity of the X2BOC•O radicals toward the electrolyte; this reactivity is still lower for the surface-bound radicals shown in Figure 5b. We suggest that this chemical peculiarity results in preferential decay through cross recombination (Figure 5c) 2F2BOC•O → (CO2 BF2)2

(11)

with the formation of dimers with a C−C bond energy of 3.75 eV (Figure 2S, Supporting Information). According to our DFT calculations, the gas-phase ionization potential for this dimer is 10.73 eV compared to 11.08 eV for EC. For gas-phase EC, proton-transfer-coupled reaction 9 decreases the overall enthalpy of oxidation to 9.34 eV (in solution, additional stabilization energy is provided by polarization effects), whereas this lowering cannot occur for the (CO2BX2)2 dimer, which lacks deprotonation sites. Compared to the electrolyte and the X2B[ox]− anions, this product is difficult to oxidize. In the gasphase dimer, the boron atoms are internally bound to carbonyl oxygens (structure c in Figure 2S, Supporting Information), but the B−O energy is only 0.25 eV, which is comparable to the bonding energy (∼0.3 eV) to EC molecules in the open-chain conformer (structures d−f in Figure 2S, Supporting Informa-

(9)

where h+• is the reactive hole center at the oxide surface and H+(EC) is a proton adduct of the ethylene carbonate at the 23754

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tion). It is more likely, however, that this dimer remains at the oxide surface, bound to the bridging oxygens, as shown in Figure 5c. Structural analysis (Figure 3S, Supporting Information) indicates that no steric hindrance is involved in such B−O binding and that the energy of the corresponding bonds is at least 0.2−0.4 eV higher than that of the B−O bonds involving the carbonyl oxygens of electrolyte molecules. We suggest that this oxidation-resistant dimer adsorbs on the oxide surface and prevents the diffusion of electrolyte molecules toward reactive hole centers at the surface that are formed during high-voltage operation (Figure 5c). Potentially, even more of such (CO2BF2)2 dimers can pile up at the surface, as boron atoms in one molecule can form B−O bonds with the carbonyl oxygens of another molecule. Such difluoroboranes have been shown to be effective additives in polymer Li-ion batteries,36 and we suggest that the unusual oxidation chemistry of LiDFOB provides a path for electrochemical synthesis of such agents, which account for its protective action.

AUTHOR INFORMATION

Corresponding Author

*Tel.: 630-252-9516. E-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank S. Chemerisov, R. Lowers, and D. Quigley for technical support and P. Zapol for helpful discussions. The work at Argonne was supported by the U.S. Department of Energy Office of Science, Division of Chemical Sciences, Geosciences and Biosciences, under Contract DE-AC0206CH11357. Programmatic support from the DOE SISGR grant “An Integrated Basic Research Program for Advanced Nuclear Energy Separations Systems Based on Ionic Liquids” is gratefully acknowledged.

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4. CONCLUDING REMARKS Although the mechanisms discussed in sections 3.3 and 3.4 rationalize some of the recent observations of Li-ion battery performance in the presence of LiBOB and LiDFOB additives, there are also observations that still await explanation. In particular, (i) LiBOB is known to form thicker and more resistant SEIs on graphite anodes than LiDFOB;15,37 (ii) electrolyte with LiBOB and LiDFOB in pure propylene carbonate can be cycled on graphite;17,38,39 and (iii) LiBOB can also passivate the metal oxide positive electrodes, although much less efficiently than LiDFOB.13,40 Unfortunately, insufficient knowledge of the details of SEI morphology and formation on graphite electrodes prevents us from linking the initial redox chemistry with these end points; however, we believe that these observations, too, can be rationalized through the suggested model, albeit in a more speculative fashion. With regard to SEI thickness on the graphite electrode, this can be the effect of fluoride versus oxalate release in oneelectron reduction of the corresponding anions (see above). The formation of a more robust SEI for propylene carbonate can be rationalized by the formation of secondary (as opposed to tertiary) radicals through ring-opening reactions analogous to reaction 8 for ethylene carbonate. Such secondary radicals can be subsequently involved in radical polymerization reactions, whereas tertiary radicals can only disproportionate.33 The oxidation of LiBOB yields [ox]BOC•O radicals that (being weak Lewis acids) are even more prone to the formation of (CO2B[ox])2 dimers (that are also weak Lewis acids) and more stable to decarboxylation. It seems unlikely that these dimer molecules would adsorb strongly on oxide surfaces or interact with the solvent (as the corresponding B−O bond energy for the carbonyl oxygens is only 0.1 eV). Thus, it appears that such molecules would have poor solubility and can form insulating coats on the oxide surface. Although such coats would provide less robust of a barrier than the (CO2BF2)2-dressed oxide surfaces shown in Figure 5c, they can still hinder solvent oxidation.

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S Supporting Information *

List of abbreviations, Schemes 1S−4S, Tables 1S−3S, and Figures 1S−3S. This material is available free of charge via the Internet at http://pubs.acs.org. 23755

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