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Reductive Decomposition Mechanism of Prop-1Ene-1,3-Sultone in the Formation of a Solid-Electrolyte Interphase on the Anode of a Lithium-Ion Battery Young-Kyu Han, Jaeik Yoo, and Jaehoon Jung J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07525 • Publication Date (Web): 02 Dec 2016 Downloaded from http://pubs.acs.org on December 11, 2016

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The Journal of Physical Chemistry

Reductive Decomposition Mechanism of Prop-1-ene1,3-sultone in the Formation of a Solid-Electrolyte Interphase on the Anode of a Lithium-Ion Battery Young-Kyu Han,*,† Jaeik Yoo,† and Jaehoon Jung‡,* †Department

of Energy and Materials Engineering, Advanced Energy and Electronic Materials

Research Center, Dongguk University-Seoul, Seoul 100-715, Republic of Korea

‡Department

of Chemistry, University of Ulsan, 93 Daehak-ro, Nam-gu, Ulsan 44610, Republic

of Korea

1 ACS Paragon Plus Environment


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ABSTRACT A novel electrolyte additive, prop-1-ene-1,3-sultone (PES), has recently attracted great attention due to its formation of effective solid-electrolyte interphase (SEI) films and remarkable cell performance in lithium-ion batteries. Herein, the reductive decomposition of PES is investigated through density functional calculations combined with a self-consistent reaction field method using which the bulk solvent effect is accounted for the geometry optimization and transition-state search. We examine three ring-opening pathways, namely O–C, S–C, and S–O bond-breaking processes. Our calculations reveal that Li+ ion plays a pivotal role in the reductive decomposition of PES. While the most kinetically favored process—the S–O bond breaking—is effectively blocked via the formation of an intermediate structure, namely the Li+-participated seven-membered ring, the other decomposition processes via O–C and S–C bond breaking lead to stable decomposition products. The constituents of SEI observed in previous experimental studies, such as RSO3Li and ROSO2Li, can be reasonably understood as the decomposition products resulting from O–C and S–C bond breaking, respectively.


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INTRODUCTION Lithium-ion batteries (LIBs) represent one of the most popular power sources not only for portable electronic devices but also for electric vehicles, because of their high energy and power density, and long-cycle durability.1,2 Significant efforts have been devoted to improving the performance and safety of LIBs via the development of new electrolyte additives, as the cell performance is considerably enhanced with only small quantities, usually less than 5% of total electrolytes.3-9 In particular, the additive to achieve effective solid-electrolyte interphase (SEI) films on anode surfaces is of great importance in improving the cyclability, stability, and power density of LIBs, because this not only prevents the further decomposition of electrolytes and the degradation of electrodes but also plays a pivotal role as a route of Li+-insertion/extraction.10-14 Recently, a novel SEI-forming additive, prop-1-ene-1,3-sultone (PES, see Scheme 1), has been intensively studied15-33 due to its high potential as an alternative to vinylene carbonate (VC), the most widespread SEI-forming additive in commercial LIBs. The usefulness of PES as an SEI-forming additive was first demonstrated by Li et al. in comparison with 1,3-propane sultone (PS), a similar sulfur-containing additive, where the use of PES provided superior cycle performance compared to PS for a prototype propylene carbonate (PC)-based LiCoO2/graphite Li-ion cell.15-17 The PES-containing cell was also reported to present lower gas production and higher capacity retention than the VC-containing cell, especially at elevated temperatures.18-21 Furthermore, recent experimental studies have indicated that the characteristics of LIB can be controlled by a combination of PES with VC by varying their relative amounts22-26; thus, there is still extensive scope to improve the battery performance with a variety of blending of additives and electrolytes.27-32



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Understanding the detailed decomposition process of SEI-forming additives is fundamental in terms of interpreting experimental observations; it is also a prerequisite for designing novel compounds or identifying the effective combinations of additive blends. For that purpose, a computational approach based on density functional theory (DFT) has been widely applied to investigate the reductive decomposition mechanism of various additives, such as VC,34 PS,35 and ethylene sulfite (ES).36,37 For PES, a fascinating DFT study was carried out to obtain insight into the gas production via reductive decomposition,21 in which the formation of C3 gas products, such as propene and propane, accompanied with the formation of solid Li2SO3 was reasonably examined through O–C bond breaking in five-membered ring (a in Scheme 1). Herein, we examined the three decomposition pathways, namely O–C, S–C, and S–O bond breaking in a five-membered ring (a, b, and c, respectively, in Scheme 1) to provide fundamental insights into the reductive decomposition processes of PES in LIBs. The bulk solvent effect is accounted for in all of the computations concerning molecular geometric and electronic structures using the self-consistent reaction field (SCRF) method. The computationally obtained molecular properties of PES, such as the lowest unoccupied molecular orbital, Gibbs free energies for reduction, and binding energies with Li+ ion, indicate that PES has a prominent potential as an SEI-forming additive. Our calculations reveal that Li+ ion plays a pivotal role in the reductive decomposition of PES. While the most kinetically favored process, the S–O bond breaking in the five-membered ring (c in Scheme 1), is effectively blocked via the formation of an

intermediate

structure,

namely

Li+-participated

seven-membered

ring,

the

other

decomposition processes via O–C and S–C bond breaking lead to the formation of stable decomposition products with highly π-conjugated geometric and electronic structures. The constituents of SEI observed in previous experimental studies, such as RSO3Li and ROSO2Li,


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can be reasonably understood by the decomposition products resulted from O–C and S–C bond breaking, respectively.

COMPUTATIONAL DETAILS A computational study based on Kohn–Sham DFT was extensively performed to explain the reductive decomposition processes of PES. All of the molecular structures were optimized with no symmetry constraints using the B3LYP functional38,39 and 6-311++G(d,p) basis set implemented in the Gaussian09 software package,40 which was reported to provide a good valence between accuracy and cost in the computational study on PES.21 The conductor-variant polarized continuum model (CPCM),41 a kind of SCRF approach, was employed to carefully reflect the influence of dielectric medium on the molecular geometric and electronic structures; in principle, this is useful for describing the interaction between a highly positive Li+ ion and anionic PES in a polar medium. We adopted a dielectric constant (ε) of 31.9, which corresponds to the weight-averaged value for EC(89.2):EMC(2.9) = 1:2 medium.10 The other ε value (20.0) was also tested for some calculations, especially for the barrier heights, because that value has been experimentally obtained for the 3:7 EC/EMC medium.42 However, we did not observe a significant deviation due to the change in dielectric constant (see Table S1 in Supporting Information). To confirm both transition states and local minima, frequency calculations were carried out with the same functional and basis set as those used in the geometry optimization. All potential energy surfaces for reductive PES decomposition were constructed with zero-point energy (ZPE) correction, and thermal corrections to evaluate Gibbs free energy were performed



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at 298.15 K and 1 atm. We used the ultrafine grid in numerical integration and the tight selfconsistent field convergence criterion in all calculations.

RESULTS AND DISCUSSION Recent robust computational approaches to develop a promising SEI-forming additive have suggested that the molecular properties, such as frontier molecular orbitals, i.e., highest occupied and lowest unoccupied molecular orbital (HOMO and LUMO), redox potential (including bulk solvent effect), and binding energy with Li+ ion, are useful in the selection of potential candidate additives.17,43-45 Because this study focuses on the reductive process on an anode, the computationally evaluated LUMOs, Gibbs free energies for reduction (ΔGred), reduction potential (RP), and binding energies with Li+ ion (BE(Li+)) for PES and other commonly used electrolyte components, VC, ethylene carbonate (EC), and propylene carbonate (PC), are compared in Table 1. The LUMO, ΔGred, and RP values for the electrolyte molecules binding with a Li+ ion are also presented in the parentheses. The lowest LUMO value of PES indicates that the reduction of PES is more favorable compared to other molecules, especially VC, which coincides with the results of previous reports.18,43 The frontier molecular orbitals should be used carefully in judging the relative priority of reduction process when significant geometric deformation is involved during reduction process in a solvent medium.46-48 Therefore, we evaluated ΔGred including the bulk solvent effect as a more reasonable indicator for the reduction priority compared to LUMO. The evaluated ΔGred values of PES are 0.29 and 0.21 eV lower than those of the widely used VC for the cases of the molecule itself and of binding with a Li+ ion, respectively, which also strongly supports the feasibility of PES as an SEI-forming additive. Note that the experimentally-reported reduction potentials of PES and VC are ~1.1 and ~0.9 V vs.


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Li/Li+, respectively,22,26 in which the difference of ~0.2 V vs. Li/Li+ is quantitatively wellmatched with the difference in their ΔGred values of 0.29 (or 0.21) eV. Another qualification factor for an SEI-forming additive is the low BE(Li+), which is closely related to easy desolvation of the additive during SEI formation.43 As shown in Table 1, PES shows weaker interaction with Li+ ion than other molecules for both neutral and anionic forms. The BE(Li+) of neutral PES is similar to that of VC, with only a difference of 0.14 kcal/mol, but the BE(Li+) difference increases considerably by 2.95 kcal/mol in anionic form. Therefore, PES would be more effective than VC in the SEI-forming process, especially on the anode surface. Table 1 also implies the importance of Li+ ion in the reductive decomposition process. For neutral molecules, although the BE(Li+) values on the ZPE-corrected potential energy surface are all exothermic, the Gibbs free energy–corrected BE(Li+) values indicate that none of the molecules bind with Li+ ion at room temperature due to the significant loss of entropy. However, the reduced molecules spontaneously bind with Li+ ion, mainly due to a strong electrostatic interaction; this implies that Li+ ion may have an influence on the reductive decomposition process, and thus should be taken into account in studying the reductive decomposition mechanism. Figure 1 shows the optimized geometries of anionic PES binding with Li+ ion as an initial state of the reductive decomposition process, in which 1m and 1b are the local minima and 2T corresponds to transition state between them. 1m and 1b interact with Li+ ion in a monodentate and bidentate manner, respectively, as shown in Figure 1. It is shown that 1m is slightly more stable than 1b by 0.18 (0.86) kcal/mol (hereafter, ZPE-corrected energy (Gibbs free energy– corrected energy)); the figure also shows a shorter O–Li+ distance (dO-Li) of 1.88 Å. The barrier



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height from 1m to 1b is only 0.44 (1.20) kcal/mol, which implies that the Li+ ion may easily move around the electronegative SO3 group. We investigated the three reductive decomposition processes of PES beginning from 1m, i.e., (i) O–C, (ii) S–C, and (iii) S–O bond-breaking processes in a five-membered ring, as presented in Figure 2 (see also Scheme 1). The optimized geometries of the transition states and local minima are presented in Figure 3. First, the decomposition of PES via O–C bond breaking requires the activation barrier of 7.31 (7.29) kcal/mol, which is the highest energy compared with the other processes. The dO–C value increases from 1.49 Å (1m) to 1.80 Å at the transition state 3T as shown in Figure 3. The decomposition product 5 becomes extremely stable from the initial state 1m by 46.18 (46.85) kcal/mol. The high stability of 5 can be understood according to the stabilization of an unpaired radical electron through the π-conjugated network, as represented in its chemical structure (Figure 2). The Li+ ion maintains the monodentate manner of interaction with PES (dO–Li = ~1.88 Å) during the decomposition process, although its orientation varies quite freely, as expected from the shallow barrier of less than 1 kcal/mol between 1m and 1b through 2T (see Figure 1). Second, the PES decomposition by S–C bond breaking is kinetically more favorable than that by the O–C bond-breaking process. The corresponding activation energy is 3.46 (3.47) kcal/mol and thus is smaller than that required in O–C bond breaking by 3.85 (3.82) kcal/mol. Note that we could not find the transition state for S–C bond breaking with the monodentate Li+ ion, and only the transition state 6T with the bidentate Li+ was obtained. The dS–C at 6T lengthens to 2.23 Å, which is longer than that at 1m (1.72 Å) by 0.51 Å. The larger change in dS–C (ΔdS–C) compared to that observed in O–C bond breaking (ΔdO–C = 0.31 Å) is also reflected in the smaller imaginary frequency of 6T (–391 cm–1) compared to that of 3T (–969 cm–1). However,


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the decomposition products 7 and 8 are extremely unstable compared to 5 formed though the O– C bond-breaking process. Indeed, 5 is about three times more stable than 8, with the energy difference between 5 and 8 reaching 29.08 (28.05) kcal/mol. The relative instability of 8 compared to 5 can be explained by the hybridization of the terminal carbon atom, which mainly takes an unpaired radical electron, because the stability of a free radical located at sp3-hybridized carbons atom is superior to that at sp2-hybridized ones. In addition, the resonance effect can also contribute to stabilizing the radical species, which will be discussed later. It is remarkable that the instability of decomposition product 8 can be overcome by hydrogen migration from α to γ position, which leads to the formation of 8¢, as depicted in Figures 2 and 3. 8¢ becomes drastically more stable than 8 by 33.12 (33.25) kcal/mol; thus, it acquires high stability that comparable to that of 5, with an energy difference of 4.04 (5.20) kcal/mol. Previous experimental studies using X-ray photoelectron spectroscopy (XPS) suggested that sulfur-containing species, such as RSO3Li and ROSO2Li, in addition to Li2SO3, exist in SEI film constructed in PEScontaining cells.21,26 The decomposition products formed through the O–C and S–C bondbreaking processes are directly related to the formation of RSO3Li and ROSO2Li, respectively. In particular, although it is difficult to thermodynamically explain the formation of ROSO2Li through the decomposition process (Figure 2), further hydrogen migration leads to a greatly stabilized product, as mentioned above, which may be facilitated by the catalytic effect of the anode surface.3 Finally, we investigated the decomposition of PES via S–O bond breaking, where the ringopening process exhibits quite different mechanism from the previous O–C and S–C bondbreaking processes. As presented in Figure 2, the activation barrier is only 1.65 (2.44) kcal/mol, and the vibrational motion of the imaginary frequency (–108 cm–1) mode corresponds to the



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migration of Li+ ion toward the oxygen atom in the five-membered ring, thereby resulting in the formation of additional electrostatic O–Li+ interaction (see 9T, Figure 3). The barrier height required for S–O bond breaking is much lower than those of O–C and S–C bond-breaking processes by 5.66 (4.85) kcal/mol and 1.81 (1.03) kcal/mol, respectively. Thus, S–O bond breaking is the most kinetically favored decomposition route in the five-membered ring of PES (see Table S1 for the superior kinetic priority of the S–O bond-breaking process examined by the additional means of other PBE049 and M06-2X50 functionals). The intermediate state 10 formed through the transition state 9T presents an interesting geometric feature. The monodentate Li+ ion in 1m comes to interact with two oxygen atoms, that is, in a bidentate manner, and the fivemembered ring readily opens due to S–O bond breaking; this leads to the formation of a Li+participated seven-membered ring, as shown in Figure 3. In addition, the dS–O increases from 1.65 (1m) to 2.39 Å (10). Although 10 is less stable than the intermediate 4 formed through O–C bond breaking, it is more stable by ~15 kcal/mol than the products formed from S–C bond breaking because the decomposed oxygen is stabilized by the strong electrostatic interaction with Li+ ion. After the formation of 10, interestingly, the reaction energy diagram indicates that the subsequent steps lead to the gradual destabilization of decomposition products, in contrast to the other two processes. While the relaxation processes are greatly favorable in the O–C and S–C bond-breaking decompositions (Figure 2), the decomposition products 11 and 12 are less stable than 10 by 5.06 (4.10) and 8.95 (6.59) kcal/mol, respectively. Therefore, based on our results, it is feasible to suggest that the S–O bond decomposition process can be effectively blocked by the formation of a stable intermediate structure, that is, the Li+-participated seven-membered ring. In LIBs, Li+ ions can interact with more than one electrolyte molecule, which may lead to complexity in the electrolyte decomposition process.36,51 Therefore, we extended our


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investigation to the influence of an additional electrolyte molecule on the reductive decomposition process of PES in order to verify the computational results obtained using a single PES molecule with the SCRF method (Figure 2). We employed EC as a one explicit electrolyte molecule because the amount of solvents is much greater than that of additives, and EC is one of the most popular electrolytes used in LIBs. The obtained reaction energy profiles (Figure 4, see also Figure S1 for the optimized geometries) for three ring-opening pathways of PES involving one explicit EC molecule are virtually identical to those for a single PES molecule. In comparing two reaction energy diagrams (Figures 2 and 4), the largest deviation in corresponding potential energies is only 0.52 (1.86) kcal/mol, which is the energy difference between 5 and 16, the products of the O–C bond-breaking process. Therefore, this sophisticated computation that takes one explicit EC into account leads to the same observation mentioned above, i.e., the blocking of the S–O bond decomposition process is due to the formation of intermediate species 21, which corresponds to 10. This remarkable coincidence between Figures 2 and 4 can be explained by the much lower ΔGred value of PES (–2.24 eV) or Li+–PES (–2.50 eV) compared to that of EC (– 1.66 eV) or Li+–EC (–2.02 eV), as presented in Table 1, which means that the reduction can be dominantly occurred in PES from a spatial viewpoint. Our computational results indicate that the influence of an additional dipole moment from explicit EC on the decomposition of reduced PES is not significant, and thus the SCRF method is an effective approach for investigating the decomposition process of additive species with a high reduction priority compared to abundant electrolyte species. Our findings may also provide a rational explanation for why there has been no report regarding the production of oxygen-containing gas species on an anode surface when PES was employed as an SEI-forming additive. It has been mostly reported that C3 gas species, such as



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propane and propene, were produced with the formation of solid Li2SO3.16,17,21,25 The detailed reaction mechanism of the formation of C3 gas species and Li2SO3, in addition to a variety of routes to form RSO3Li via additional alkylation, was already well-examined through computational study.21 Our results also show that C3 gas species may have stemmed from the further dissociation (i.e., the dissociation of hydrocarbon moiety from the decomposition products formed by O–C and S–C bond-breaking processes), as shown in Figure 2. However, because the existence of 10 in SEI is not yet experimentally observed, it is important to examine how the dissociation of intermediate 10 proceeds. The main reaction mechanism producing C3 gas species and Li2SO3 has been suggested to proceed through two-electron reduction.21,35,37,52 Self et al. intensively studied the two-electron reduction mechanism, mainly focusing on O–C bond breaking, in which 4 (or 5) is reduced to 24 with a second electron and then forms 25 binding with a second Li+ ion (see Figure 5).21 The formation of C3 gas and Li2SO3 is initiated from the intermediate species 25. Therefore, we considered a similar scenario for intermediate 10, in which 10 is reduced to 26 and then forms 27 binding with a Li+ ion (see Figure 5). The ΔGred values for 4 and 10 are evaluated to be –3.72 and –4.76 eV, respectively, both much lower than that of PES (–2.24 eV), indicating that the second reduction process is much easier than the first reduction process. The binding energies with a second Li+ ion of 24 and 26 are –12.85 (–5.78) and –15.14 (–8.14) kcal/mol, respectively. Therefore, 10 is not only more easily reduced but also more strongly binding with Li+ ion compared to 4, implying that the formation of 27 is more feasible than that of 25. As previously reported, the dissociation of 25 is highly exothermic; the energy released during the production of propene and Li2SO3 is –155.56 (–155.39) kcal/mol in the presence of hydrogen atoms.21 27 also releases a large amount of energy (–142.13 (–141.51) kcal/mol) during the dissociation process, which implies that 27 can participate in the formation


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of dissociation products and can thus contribute to play a role as an effective SEI-forming additive. To manifest the stability of the decomposition products presented in Figure 2, we elucidated their spin density in Figure 6, because the stabilization of unpaired radical electrons is crucial in assessing the overall stability of decomposition products. The spin density of the most stable decomposition product 5 formed through O–C bond breaking well represents the ideal πconjugated nature; that is, the spin density is evenly distributed along the carbon chain. In contrast, the spin density in the decomposition product 8 resulting from S–C bond breaking is highly localized at the terminal carbon atom. Therefore, the relative spin density distribution between 5 and 8 is well correlated with their stability order, as shown in Figure 2. However, if 8¢ can be formed by the hydrogen migration from α to terminal γ carbon atom, the intermolecular πconjugated network can be created and further extended to an oxygen atom. In the sevenmembered ring including Li+ in 10, the spin density can be divided into two electronegative terminal moieties, i.e., –SO2 and –O, in which an unpaired radical electron can be effectively stabilized by strong electrostatic interaction with a positive Li+ ion. Therefore, it can be expected that the remarkable performance of PES as an SEI-forming additive can be attributed to the formation of stable decomposition products via O–C and S–C bond breaking due to the πconjugated electronic structure (5 and 8¢, respectively), while the S–O bond-breaking process is blocked by the formation of Li+-participated seven-membered ring due to the strong electrostatic interaction with Li+ ion (10).

CONCLUSIONS



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To conclude, our computational study based on the hybrid DFT method with a full account of the bulk solvent effect for molecular geometric and electronic structures using the SCRF method revealed the proper molecular properties of PES as an SEI-forming additive and the reaction mechanism for the reductive decomposition processes of PES. The computationally obtained molecular properties of PES, such as the lower values of LUMO and ΔGred and the weaker interaction with Li+ ion than that of VC, indicates that PES has a remarkable potential as an SEIforming additive. Three decomposition pathways, O–C, S–C, and S–O bond-breaking processes in the five-membered ring of PES, are investigated for both cases with and without one explicit EC molecule. The most kinetically favored decomposition process with the lowest barrier (S–O bond breaking in a five-membered ring) could be blocked via the formation of a stable Li+participated seven-membered ring, which means that Li+ ion plays an important role in the decomposition of PES, that is, the stabilization of an unpaired radical electron with strong electrostatic interaction. For the other two decomposition processes via O–C and S–C bond breaking, it was elucidated that the formation of highly stable decomposition products were formed due to π-conjugated geometric and electronic structures, which also led to the stabilization of an unpaired radical electron. The decomposition products through the O–C and S–C bond-breaking processes were directly related to the formation of RSO3Li and ROSO2Li, respectively. It is also expected that the stable intermediate formed by S–O bond breaking might be dissociated through two-electron reduction process, considering low Gibbs free energy for second reduction and high binding energy with second Li+ ion. This work not only provides a rationale for interpreting the experimental observations for PES as an SEI-forming additive but also improves our understanding of the effective development of novel electrolyte additives.


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ASSOCIATED CONTENT Supporting Information Barrier heights for the three reductive decomposition processes of PES, optimized geometries of transition states and local minima, and Cartesian coordinates of optimized geometries. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.xxxxxxx.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]; Phone: +82 2 2260 4975. *E-mail: [email protected]; Phone: +82 52 259 2339.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant (2016R1A2B4013374 and 2015R1C1A1A01052947).



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Xia, J.; Ma, L.; Aiken, C. P.; Nelson, K. J.; Chen, L. P.; Dahn, J. R. Comparative Study on Prop-1-ene-1,3-sultone

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Carbonate

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Ma,

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Xia,

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Dahn,

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Scheme 1. Chemical structure of 1-prop-1-ene-1,3-sultone (PES). The chemical bonds corresponding to the reductive decomposition pathways, O–C (a), S–C (b) and S–O (c) bond breaking processes, are highlighted with red-colored lines.


9 2 .0

2.5

5 2.0

1.94

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3

Page 25 of 32

1.88

1m

2T

1b

Figure 1. Optimized geometries (in Å) of anionic PES binding with Li+ ion. 1m and 1b are the local minimum structures and 2T corresponds to transition state between 1m and 1b. Selected interatomic distances are presented in Å. (S, yellow; O, red; C, gray; Li, purple; H, white)



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Page 26 of 32

Figure 2. Reaction energy diagram of the three reductive decomposition processes of PES, O–C (black line), S–C (blue), and S–O (red) bond breaking in a five-membered ring. All potential energy surfaces are constructed with ZPE-corrected values (in kcal/mol). The Gibbs free energy corrected values (in kcal/mol) at 298.15 K and 1 atm are presented in parentheses. For the transition states, the imaginary frequencies are presented in italics.


Page 27 of 32

1.88

1.88

1.88

1.80

5

3T 4 2.04

1.87 β

1.87

2.07

α

γ

8 2.23 1.89 γ

6T

7

β

α

8’ 1.89

1.97

1.99

2.35

7 2.3

9 2.3

7 1.6

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


1.92

9T

10

11

1.84

12

Figure 3. Optimized geometries (in Å) of transition states and local minima formed during the reductive decomposition of PES through O–C (3T, 4, and 5), S–C (6T, 7, 8, and 8¢), and S–O (9T, 10, 11, and 12) bond-breaking processes. Selected interatomic distances are presented in Å. (S, yellow; O, red; C, gray; Li, purple; H, white)



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Page 28 of 32

Figure 4. Reaction energy diagram of the three reductive decomposition processes of PES, O–C (black line), S–C (blue), and S–O (red) bond breaking in five-membered ring, involving one explicit EC molecule. All potential energy surfaces are constructed with ZPE-corrected values (in kcal/mol), and Gibbs free energy corrected values (in kcal/mol) at 298.15 K and 1 atm are presented in parentheses. For the transition states, the imaginary frequencies are presented in italic.


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2.04

1.97

1.94 2.01

24

25

1.97 1.90

2.00 1.92 1.83

1.82

26

27

Figure 5. Optimized geometries (in Å) of local minima formed during the two-electron reductive decomposition of PES, O–C (24 and 25) and S–O (26 and 27) bond-breaking processes. Selected interatomic distances are presented in Å. (S, yellow; O, red; C, gray; Li, purple; H, white)



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5

Page 30 of 32

8

8’ 10

Figure 6. Spin density plots for 5, 8, 8¢, and 10 (iso-surfaces at 0.005 e/Å3) created using the VESTA program.53


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Table 1. LUMO (in eV), Gibbs free energy for reduction (ΔGred, in eV), reduction potential (RP, in V vs Li/Li+), and binding energy with Li+ ion (BE(Li+), in kcal/mol) of PES, VC, EC, and PC. LUMO, ΔGred, and RP values for the electrolyte molecules binding with a Li+ ion are presented in parentheses. The BE(Li+) values were evaluated with ZPE-correction for both neutral and anionic molecules (Gibbs free energy corrected values are in parentheses). The bulk solvent effect was involved in all calculations, and Gibbs free energy was computed at 298.15 K and 1 atm. BE(Li+)a LUMO

ΔGred

RP neutral

anion

PES

–1.29 (–1.51)

–2.24 (–2.50)

0.87 (1.13)

–4.58 (+2.17)

–10.22 (–3.81)

VC

–0.24 (–0.48)

–1.95 (–2.29)

0.58 (0.92)

–4.72 (+1.34)

–13.17 (–6.57)

EC

–0.14 (–0.48)

–1.66 (–2.02)

0.29 (0.65)

–5.60 (+0.68)

–14.31 (–7.66)

PC

–0.10 (–0.41)

–1.61 (–1.99)

0.24 (0.62)

–5.70 (+0.80)

–14.48 (–7.95)

aBE(Li+)

= E(Mn(a)–Li+) – {E(Mn(a)) + E(Li+)}, Mn(a) = neutral (anionic) molecule. Each energy values are corrected with ZPE or Gibbs free energy.



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