Computational Studies of Polysiloxanes: Oxidation Potentials and

ARTICLE pubs.acs.org/JPCC

Computational Studies of Polysiloxanes: Oxidation Potentials and Decomposition Reactions Rajeev S. Assary,†,|| Larry A. Curtiss,*,†,‡ Paul C. Redfern,§ Zhengcheng Zhang,§ and Khalil Amine§ Materials Science Division, ‡Center for Nanoscale Materials, and §Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States Chemical & Biological Engineering, Northwestern University, Evanston, Illinois 60208, United States

)

†

bS Supporting Information ABSTRACT: Silicon-containing solvents have tremendous potential for application as electrolytes for electrical energy storage devices such as lithium-ion (air) batteries and supercapacitors. Quantum chemical methods were employed to investigate trends in oxidation potentials and decomposition reactions of a series of polysiloxanes. Various electron-donating and -withdrawing substituents can be used to tune the oxidation potential in shorter chain siloxanes but not in longer ones. Decomposition reactions of siloxanes in their oxidized states were investigated and compared against their carbon analogues. These studies suggest that the Si
O group provides added stability for siloxanes over their carbon analogues. Computational studies have also been performed for various disiloxanes and siloxanes with spacer groups to understand their thermochemical stability and oxidation potentials.

1. INTRODUCTION Recently, silicon-containing solvents for electrolytes have received much interest for electrical energy storage devices such as lithium batteries and supercapacitors due to their nontoxic, nonflammable, and biocompatible nature.1
5 Also, they have a potential role to play as electrolytes in high energy density lithium-air batteries, where conventional organic carbonates undergo decomposition in the presence of oxygen reduction products.6,7 It has been reported that polysiloxanes containing oligo(ethylene oxide) groups have very good properties for electrolytes, such as low glass transition temperatures, effective ionic transport, low viscosity, and good conductivity in the presence of lithium ions,1
3,8
12 although the conductivity needs improvement. The polysiloxanes, shown in Scheme 1, are referred to as 1NMx, where x is the number of ethylene oxide (EO) units. An important aspect of the silicon-containing solvents is their greater stability compared to carbon analogues due to their higher oxidation potentials.13 This greater electrochemical stability over conventional organic electrolytes is probably due to the presence of the trimethylsilyl group. A previous study by Zhang et al.14 has examined the oxidation potentials of organic solvents and discussed possible fragmentation patterns. Due to this interest in silicon-containing solvents as electrolytes, we have carried out an investigation into trends of the oxidation potentials and decomposition reaction energies for a series of polysiloxanes. In this paper we report on the trends in oxidation potentials for polysiloxanes containing oligo(ethylene oxide) groups compared to their carbon analogues as well as commonly used electrolytes such as ethylene carbonate and r 2011 American Chemical Society

propylene carbonate. We have performed computations to understand the effects of various electron-donating and electron-withdrawing substituents in tuning the oxidation potential for the purpose of designing electrolytes with higher oxidation potential. In this paper we also report an investigation into the thermochemical stability of siloxanes containing oligo(ethylene oxide) groups against their carbon analogues by calculating the energies of various decomposition reactions. Studies have also been performed on decomposition energies for longer chain siloxanes and disiloxanes; both are potential electrolytes for lithium-ion or lithium-air batteries. The methods used in this study are described in section 2. The results and discussion are given in section 3, and conclusions are presented in section 4.

2. COMPUTATIONAL METHODS We have employed B3LYP15 density functional theory (DFT) to investigate the structure and oxidation potentials. The 6-31G (BS1), 6-31þG(d) (BS2), 6-311þþG(d) (BS3), and 6-311þG(2df,p) (BS4) basis sets were used, and zero-point vibration energies, enthalpies, and Gibbs free energies (at 298.15 K, 1 atm) were calculated at the corresponding level of theory. The conductorlike polarizable continuum model16,17 (CPCM) at the level of B3LYP/BS2 was used to calculate the free energies of solvation for the reaction species at the optimized structures. United atom for Hartree
Fock (UAHF) atomic radii and a dielectric constant (ε) of 5.67 (average of the experimental Received: March 1, 2011 Revised: April 13, 2011 Published: June 01, 2011 12216

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Table 1. Calculated Oxidation Potentials for both 1NM1 and Substituted 1NM1a oxidation potential (V)

this work

dielectric values 3.6
7.5) were used in all the CPCM calculations for siloxanes and their carbon analogues, while experimental dielectric constants were used for conventional electrolytes.18 Similar to previous studies,19,20 the oxidation potential (E0) of the electrolytes relative to Li/Liþ reference electrode is determined by using E0 = [(
ΔG298solution )/F]
1.46 V, where ΔG298solution is the free energy change during the oxidation process and F is the Faraday constant. When the predicted and experimental oxidation potentials are compared, it should be noted that the latter includes kinetic effects not included in the calculations.14 Calculations of the fragmentation energies were done at various levels of theory including the ones described above for the oxidation potentials. Accurate bond-breaking reactions often require high-level calculations such as the Gn methods.21 We used this method to calculate selected reaction energies to assess the B3LYP method, which generally gave good agreement with the G4 results. The Gaussian 0322 computational package was used for all calculations.

published

experimental

theor value

lit.value

(R)3SiO(CH2)2OCH3 -CH3 [=1NM1]

4.11

-CH2CH3

3.79

-CH2CF3

4.15

-H

4.27

-Cl

4.75

-CF3

5.02

-CN

5.03

-NO2 (CH3)3CO(CH2)2OCH3

5.14 4.14 Organic Solvents

EC PC

5.25

5.58b

5.17

b

BC

4.81

GBL

4.61

VC

3.92

VEC

4.62

5.61

6.70c 5.2
5.6d, 5.98c

b

5.51

4.06b

a

3. RESULTS AND DISCUSSION 3.1. Oxidation Potentials. 3.1.1. Monosiloxanes. Computed oxidation potentials of 1NM1 siloxane [(CH3)3SiO(CH2)2OCH3] and the effect of silyl group substitution [(R)3SiO(CH2)2OCH3] are shown in Table 1. The methyl groups of the silyl fragment of the 1NM1 siloxane were substituted by both electron-donating and electron-withdrawing substituents to understand the effect on the oxidation potential. The calculated oxidation potential of 1NM1 is 4.12 V. The substitution of an electron-donating group (R = -CH2CH3) decreased the oxidation potential to 3.79 V. Upon substitution by electron-withdrawing groups (R = -CF3, -NO2, -CH2CF3, -H, -CN), the oxidation potential increases to as high as 5 V. On the basis of orbital analysis, the highest occupied molecular orbital (HOMO) for 1NM1 is the 2pz orbital of the oxygen adjacent to the Si atom. Stabilization and destabilization of the HOMO by the electron-donating and -withdrawing groups, respectively, explains the relative trends in our calculations. The results in Table 1 suggest that electron-withdrawing substituents could be used to make a 1NMx siloxane species with very high oxidation potentials. Synthesis with the Cl substitution is most likely possible, as there are difficulties in synthesizing the 1NM1 siloxane species with CF3, CN, and NO2 substituents. Additionally, due to the problems with poor solubility of lithium salts in 1NM1, both 1NM2 and 1NM3 are more probable candidates for Cl substitution. We have computed the oxidation potential of 1NMx (x = 1, 2, 3) and their Cl-substituted analogues. The results are given in Table 2. From the table it is found that the substitution effect is predominant in 1NM1 and negligibly small in 1NM3. There is 0.2 V increase in the oxidation potential of 1NM2 upon chlorination. The small effect is because, in both 1NM2 and 1NM3, the highest occupied molecular orbital is the 2pz orbital of the second oxygen from the silicon

Calculations were done at the B3LYP/BS2 level of theory with respect to Li/Liþ reference electrode potential. Results are also given for the carbon analogue of 1NM1 and several organic solvents. EC, ethylene carbonate (ε = 90.0); PC, propylene carbonate (ε = 65.5); BC, butylene carbonate (ε = 56.0); GBL, γ-butyrolactone (ε = 39.0); VC, vinyl carbonate (ε = 45.0); VEC, vinyl ethylene carbonate (ε = 45.0). Experimental dielectric constants are taken from ref 16. b Computed at a dielectric of 78 (ref 12). c Glassy carbon electrode with scan rate of 10 mV/s (ref 23). d Activated carbon electrode with scan rate of 5 mV/s (ref 23).

atom, not the oxygen adjacent to the silicon atom. Therefore, the electron-withdrawing inductive effect of Cl will have little effect on tuning the oxidation potential of 1NMx where x is 2 or larger. The use of Cl as a substituent on 1NMx for increasing the oxidation potential will be of value in the 1NM1 species only if it can be modified to make conventional lithium salts more soluble. We have also computed the oxidation potentials of commonly used organic solvents, such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), γ-butyrolactone (GBL), vinyl carbonate (VC), and vinylidene ethylene carbonate (VEC) for comparison (see Table 1). Computed oxidation potentials were also compared with published literature values from previous experimental and theoretical studies.14,23 Additionally, we have investigated the effect of adding more spacer groups -CH2- in the oxidation potential (Scheme 2). These groups are introduced between the Si and the first oxygen atom. Depending on the number of these groups introduced in 1NM1, we have labeled these species as 1SxM1. We have investigated species with x = 1, 2, and 3. The calculated oxidation potentials reveal that the introduction of a single spacer group reduces the oxidation potential of 1NM1 by 0.74 V, due to active participation of the 2p orbital of carbon in formation of the HOMO, which destabilizes the HOMO. A similar destabilizing effect, the R-effect, is commonly found in species containing 12217

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Table 2. Comparison of Oxidation Potentials of 1NMn (n = 1
3) with Those of Chlorinated Analoguesa

Scheme 3. Disiloxanes

oxidation potential (V) siloxane

(CH3)3

(CH3)2Cl

(CH3)1Cl2

Cl3

1NM1

4.12

4.33

4.58

4.75

1NM2 1NM3

3.96 4.03

4.04 4.03

4.11 4.02

4.15 4.10

Calculated at the B3LYP/BS2 level of theory with respect to Li/Liþ reference electrode potential. a

Scheme 2 : Monosiloxanes with Spacer Groups

Table 3. Calculated Oxidation Potentials for 1NM1 and 1SxM1 Monosiloxanes and for Disiloxanesa siloxane

oxidation potential (V) Monosiloxanes

1NM1

4.11

1S1M1

3.38

1S2M1

4.09

1S3M1

4.01 Disiloxanes

2NM3

4.05

2SM3 2SD3

3.76 3.75

2ND3

3.92

Calculated at the B3LYP/BS2 level of theory with respect to Li/Liþ reference electrode potential. Disiloxanes were modeled at the B3LYP/ BS2//B3LYP/BS1 level of theory. 1NM1= (CH3)3SiO(CH2)2OCH3; 1SxM1 = (CH3)3Si(CH2)xO(CH2)2OCH3. a

tertiary carbon or silyl groups. However, two or more spacer groups do not seem to affect the oxidation potential significantly (Table 3). 3.1.2. Disiloxanes. In addition to the monosiloxanes, the oxidation potentials of various disiloxanes (Scheme 3) have been calculated. The disiloxanes that were investigated have potential for use as electrolytes due to their ability to dissolve electrolyte salts at optimum concentration and the capacity to transport lithium ions.12,24 The calculated oxidation potentials of the disiloxanes are also shown in Table 3. The calculated oxidation potentials for 2NM3 and 2ND3 are 4.05 and 3.92 V, respectively, while both 2SM3 and 2SD3 have a lower oxidation potential (3.75 V). Therefore, the introduction of the spacer group CH2)3- causes a slight decrease in the oxidation potential, similar to that for 1S3M1 (Table 3). The introduction of an Si
C bond through spacer groups takes away the thermal stability provided by the strong Si
O bond in the siloxanes and this explains the lower oxidation potential for both 2SM3 and 2SD3 compared to 2NM3 and 2ND3. If it is assumed that the decomposition of electrolytes follows their oxidation, both 2SM3 and 2SD3 will

undergo decomposition at lower potential than 2NM3 and 2ND3. Among the disiloxanes investigated here, 2NM3 has the highest oxidation potential. 3.2. Decomposition Reactions. Decomposition reaction energies have been calculated for cationic species resulting from oxidation. We have considered the following: (1) fragmentation of 1NM1 and 1NM2, including the carbon analogue of 1NM1; (2) increase in chain length for species from 1NM1 to 1NM5; (3) effect of introduction of spacer groups -CH2- between silicon and oxygen atoms in 1SnM1, where n is the number of spacer groups introduced between the Si and O atom in 1NM1; and (4) a disiloxane, 2NM3. 3.2.1. Monosiloxanes (1NM1, 1NM2) and Carbon Analogues. The possible fragmentation patterns (I
V) that we have considered for the decomposition of 1NM1 siloxane are shown in Figure 1. Fragmentation of the cationic species can lead to one cationic species and one neutral species. Thus, two sets of products are considered for each fragmentation process. We have investigated the energetics of two selected fragmentation patterns of 1NM1, namely, patterns II and IV, using the B3LYP/ BS2, B3LYP/BS3, and MP2/BS4//B3LYP/BS3 levels of theory and compared these results with the accurate G4 method. The computed energetics [change in electronic energy (ΔEe), enthalpy (ΔH298K), and free energy (ΔG298K)] are shown in Table 4. From the results, it is clear that the density functional methods provide more accurate energies than the MP2 level of theory when compared to G4 theory results. Therefore, all of the energy interpretations are performed on the basis of either the B3LYP/BS2 or B3LYP/BS3 levels of theory. We have calculated the thermochemistry [change in electronic energy (ΔEe), enthalpy of the reaction at 298 K (ΔH298), and change in free energy at 298 K (ΔG298)] of all the fragmentation patterns at the B3LYP/BS3 level of theory for 1NM1 and its carbon analogue. The results for decomposition of the cationic species are tabulated in Table 5; results for the neutral species are given in Table S1 (Supporting Information). The calculated free energy for fragmentation pattern I of 1NM1, which describes the breaking of the (CH3)3Si
O bond, is compared to that of the (CH3)3C
O bond. Cleavage of the Si
O bond is computed to be endothermic and thermodynamically uphill in the two possibilities considered in pattern I. However, breaking of C
O is found to be thermodynamically favorable. The fragmentation reaction leads to [(CH3)3
Si
H]1þ for siloxane and 12218

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Figure 1. (Left) Decomposition patterns investigated for 1NM1, where R = CH3, and (right) decomposition products considered for decomposition pattern I.

Table 4. Performance of MP2/BS4//B3LYP/BS3, B3LYP/ BS3, and B3LYP/BS2 Levels of Theory in Predicting Energetics of Selected Decompositions against G4 Level of Theorya level of theory

ΔEe

ΔH298K

ΔG298K

(kcal/mol)

(kcal/mol)

(kcal/mol)

[(CH3)3SiOCH2CH2OCH3]1þ f [(CH3)3SiOH]1þ þ [CH2dCHOCH3]0 G4 MP2/BS4//B3LYP/BS3

24.3 17.2

21.6 14.0

9.3 1.8

B3LYP/BS3

27.8

24.7

12.5

B3LYP/BS2

27.5

24.2

12.6

[(CH3)3SiOCH2CH2OCH3]1þ f [(CH3)3SiOH]0 þ [CH2dCHOCH3]1þ G4 MP2/BS4//B3LYP/BS3

11.2

9.0


3.2

3.8

1.4


10.8

B3LYP/BS3

14.1

11.7


0.5

B3LYP/BS2

13.6

10.8


0.7

[(CH3)3SiOCH2CH2OCH3]1þ f [(CH3)3SiOCHdCH2]1þ þ [CH3OH]0 G4 MP2/BS4//B3LYP/BS3

1.5


0.6


12.6


4.5


6.8


18.7

B3LYP/BS3

3.1

0.8


11.0

B3LYP/BS2

2.2


0.2


11.5

[(CH3)3SiOCH2CH2OCH3]1þ f [(CH3)3SiOCHdCH2]0 þ [CH3OH]1þ G4

62.9

58.7

MP2/BS4//B3LYP/BS3

64.5

59.3

45.8 46.6

B3LYP/BS3

64.8

59.6

47.0

B3LYP/BS2

63.5

58.8

46.2

a

Patterns II and IV were investigated; see Figure 1 for decomposition patterns.

[(CH3)3
C
H]1þ for carbon analogue. The computed free energy of fragmentation suggests that the former is thermodynamically uphill by þ17.2 kcal/mol and the latter is downhill by
19.7 kcal/mol. This difference is clearly depicted in Figure 2. Thus, from our calculations, the Si
O bond is much stronger (by 24
38 kcal/mol) than the C
O bond. The Si
O bond is found

to be more than a single bond, in fact partially a double bond. For pattern II we have addressed the energetics of dissociation of the O
C bond, where oxygen is adjacent to the silicon group. The free energies of fragmentation suggest that the breaking of O
C bond for both siloxane (
0.5 kcal/mol) and its carbon analogue (
2.9 kcal/mol) are similar and feasible. For pattern III, the breaking of the C
C bond in the ethylene oxide moiety was considered, and the decomposition reaction was calculated as highly endothermic in both 1NM1 and its carbon analogue. The decomposition to produce methanol (pattern IV) and methane (pattern V) are calculated to be slightly exothermic and thermodynamically downhill processes for both 1NM1 and its carbon analogue. Quantitatively, for patterns IV and V the free energy for fragmentation of carbon analogue was found to be 3 kcal/mol more favorable than for the siloxane. Figure 2 summarizes the comparison of free energy of bond-breaking patterns for 1NM1 and its carbon analogue. From these calculations, breaking of the Si
O bond is less likely in 1NM1 compared to its carbon analogue. Thus the tertiary silyl group brings added stability with respect to decomposing the molecule compared to a tertiary carbon analogue. However, both molecules could decompose to give either methanol or methane if the molecule gains sufficient activation energy. We have further investigated the energetics of these decomposition patterns in 1NM2 to find out the relative trends upon increase in the chain length of siloxanes, and these results are summarized in Table 6 for cationic species (see Table S2 inSupporting Information for neutral species). The decomposition reactions were found to be more exothermic and thermodynamically more favorable in 1NM2 compared to in 1NM1. In patterns II and IV, the fragmentation leads to the formation of an organic cationic fragment with a terminal double bond that can rearrange to a more stable delocalized form. The thermodynamic stability is achieved by delocalization of positive charge over the π-system and further assisted by the oxygen lone pairs. Formation of such tautomers would require additional energy for the hydrogen shift; however, these reactions could happen at a high potential. 3.2.2. 1NMn, n = 1
5. Longer chains of siloxanes, 1NMn (n = 2
5), are currently used to replace 1NM1 due to their ability to dissolve lithium salts at moderate concentrations because of their relatively larger dielectrics. We have investigated the effect of longer chains in the stability of the Si
O bond of various siloxanes. The structure and energetics were evaluated at the 12219

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Table 5. Energetics of Decomposition of Cationic Species of 1NM1 and Its Carbon Analogue at the B3LYP/BS3 Level of Theorya 1NM1 species

ΔEe (kcal/mol)

ΔH298K (kcal/mol)

ΔG298K (kcal/mol)

Pattern I [(CH3)3
Si
H]1þ

[CH3
O
CH2
CHdO]0

62.9

57.9

44.3

[(CH3)3
Si
H]0

[CH3
O
CH2
CH=O]1þ

55.0

50.9

38.0

[(CH3)3
Si
H]1þ

[CH3
O
CHdCH
OH]0

31.4

28.9

17.2

[(CH3)3
Si
H]0

[CH3
O
CHdCH
OH]1þ

50.9

46.9

36.5

Pattern II [(CH3)3-Si
OH]

[CH3
O
CHdCH2]

27.8

24.7

12.5

[(CH3)3
Si
OH]0

[CH3
O
CHdCH2]1þ

14.1

11.7

-0.5

[(CH3)3
Si
O
CH3]1þ

cyclic

[CH2
O
CH2]0

40.5

37.7

25.5

[(CH3)3
Si
O
CH3]0

cyclic

[CH2
O
CH2]1þ

71.8

68.5

55.7

[(CH3)3
Si
O
CHdCH2]1þ

[CH3
OH]0

3.1

0.8


11.0

[(CH3)3
Si
O-CHdCH2]0

[CH3
OH]1þ

64.8

59.6

47.0

[(CH3)3
Si
O
CH2
CHdO]1þ [(CH3)3
Si
O
CH2
CHdO]0

[CH4]0 [CH4]1þ

5.5 93.8

1.33 86.2


10.7 74.4

ΔEe (kcal/mol)

ΔH298K (kcal/mol)

ΔG298K (kcal/mol)

1þ

0

Pattern III

Pattern IV

Pattern V

Carbon Analogue of 1NM1 species Pattern I [(CH3)3
C
H]1þ

[CH3
O
CH2
CHdO]0

37.8

33.8

20.4

[(CH3)3
C
H]0 [(CH3)3
C
H]1þ

[CH3
O
CH2
CHdO]1þ [CH3
O
CHdCH
OH]0

16.2
7.4

14.3
7.7

1.1
19.7

[(CH3)3
C
H]0

[CH3
O
CHdCH
OH]1þ

15.4

12.9

0.7

[(CH3)3
C
OH]1þ

[CH3
O
CHdCH2]0

32.5

29.3

18.4

[(CH3)3
C
OH]0

[CH3
O
CHdCH2]1þ

10.4

8.9


2.9

[(CH3)3
C
O
CH3]1þ

cyclic

[CH2
O
CH2]0

38.8

34.8

25.4

[(CH3)3
C
O
CH3]0

cyclic

[CH2
O
CH2]1þ

67.8

65.1

54.2

[(CH3)3
C
O
CHdCH2]1þ [(CH3)3
C
O
CHdCH2]0

[CH3
OH]0 [CH3
OH]1þ


0.3 61.1


2.7 56.6


13.0 45.5

Pattern II

Pattern III

Pattern IV

Pattern V [(CH3)3
C
O
CH2
CHdO]

[CH4]


1.9


1.5


13.2

[(CH3)3
C
O
CH2
CHdO]0

[CH4]1þ

91.2

84.1

73.3

1þ

a

0

See Figure 1 for fragmentation patterns.

B3LYP/BS2 level of theory (see Table S3 in Supporting Information). The computed enthalpies and free energies of fragmentation predict that the breaking of the Si
O bond is highly endothermic and thermodynamically uphill regardless of the decomposition of cationic or neutral species with the endothermicity in the range of 38
69 kcal/mol. 3.2.3. Disiloxanes. Disiloxanes are another class of siloxane electrolytes increasingly used as electrolytes for lithium batteries due to their low viscosity and higher ionic conductivity.12,25 The disiloxane backbone includes a Si
O
Si moiety, and furthermore,

one or both silicon atoms are bonded with the ethylene oxide chain. Decomposition reactions of the organic chains and the Si
O bond were investigated in the earlier sections. We have particularly studied various decomposition patterns of a disiloxane, 2NM3, with a Si
O
Si headgroup. The fragmentation patterns we have investigated are shown in Figure 3 along with energies (see also Table S4 in Supporting Information). The computed energetics of patterns I
III suggest that the bond breaking of the possible three Si
O bonds (see Figure 3) is strongly endothermic (about 70 kcal/mol) and thermodynamically 12220

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Figure 2. Comparison of free energy of fragmentation of 1NM1 cation versus its carbon analogue at B3LYP/BS3 level of theory at 298 K.

Table 6. Energetics of Selected Decompositions of 1NM2 at the B3LYP/BS3 Level of Theory ΔEe (kcal/mol)

decomposition products

ΔH298K (kcal/mol)

ΔG298K (kcal/mol)

Pattern II [CH3
O
CH2
CH2
O
CHdCH2]1þ

[(CH3)3
Si
OH]0

5.4

3.0

[CH3
O
CH2
CH2
O
CHdCH2]0

[(CH3)3
Si
OH]1þ

35.0

31.5


9.0 17.4

[CH3
O
CHdCH
O
CH2
CH3]1þ [CH3
O
CHdCH
O
CH2
CH3]0

[(CH3)3
Si
OH]0 [(CH3)3
Si
OH]1þ


16.5 34.9


19.6 30.2


30.3 18.5

[(CH3)3
Si
O
CH2
CH2
O
CHdCH2]1þ

[CH3
OH]0

5.9

3.7


8.5

[(CH3)3
Si
O
CH2
CH2
O
CHdCH2]0

[CH3
OH]1þ

70.7

65.6

52.3

[(CH3)3
Si
O
CHdCH
O
CH2
CH3]1þ

[CH3
OH]0


20.4


21.8


35.8

[(CH3)3
Si
O
CHdCH
O
CH2
CH3]0

[CH3
OH]1þ

67.9

62.8

49.5

[(CH3)3
Si
O
CH2
CH2
O
CH2
CHdO]1þ

[CH4]0

5.9

2.1


11.6

[(CH3)3
Si
O
CH2
CH2
O
CH2
CHdO]0 [(CH3)3
Si
O
CHdCH
O
CH2
CH2
OH]1þ

[CH4]1þ [CH4]0

98.3
27.4

90.7
29.2

77.3
41.3

[(CH3)3
Si
O
CHdCH
O
CH2
CH2
OH]0

[CH4]1þ

111.5

104.1

90.4

Pattern IV

Pattern V

uphill. The energetics of pattern IV suggests that such decomposition is thermodynamically feasible (
10.3 kcal/mol). The free energy change is quantitatively more negative than the corresponding free energy change of fragmentation of 1NM1 (pattern II,
0.5 kcal/mol), indicating that the O
C bond in disiloxane is marginally less stable than in monosiloxanes. Meanwhile, the energetics of pattern V of 2NM3, indicating the strength of the SidO bond (ΔG298K = 37.2 kcal/mol), is similar to that in 1NM1 (ΔG298K = 38.0 kcal/mol), with both of them being strongly endothermic. Overall, it can be concluded that disiloxanes show similar stability as monosiloxanes. 3.2.4. Siloxanes with Spacer Groups. The molecular structures of the siloxanes with spacer groups (1S1M1, 1S2M1, 1S3M1) and their detailed decomposition patterns investigated here are shown in Figure 4 along with the reaction energies See also Table S5 in Supporting Information). We have considered one, two, and three bond-breaking patterns for 1S1M1, 1S2M1, and 1S3M1, respectively (Figure 4). The enthalpy change for decompositions where a SidCH2 bond is

formed is found to be endothermic (40
43 kcal/mol) for all of the siloxanes. One exception is pattern III of 1S3M1, where the bond-breaking process is thermodynamically feasible. This is probably due to the formation of cationic species stabilized by the π-bond near to the oxygen atom. Similarly Si
C bond-breaking pattern II of 1SM2 is only slightly uphill (2.3 kcal/mol). In addition to decomposition of the oxidized species, we have also investigated the decomposition of neutral siloxane species compared to some carbonate solvents. The results are summarized in Table 7 and indicate that the siloxane species are more stable than the carbonates for likely decomposition reactions. This is consistent with experimental results for the thermal stability of the siloxanes compared to EC.13

4. SUMMARY In this paper, the oxidation potentials and decomposition reactions of silicon-containing electrolytes were investigated by 12221

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Figure 3. Decomposition reactions investigated for 2NM3. Free energies (kilocalories per mole) of decomposition at the B3LYP/BS2 level of theory (298 K) are shown. Values outside and inside parentheses correspond to the fragmentation patterns that give (Aþ, B) and (A, Bþ), respectively.

Figure 4. Fragmentation patterns investigated for 1S1M1, 1S2M1, and 1S3M1. Free energies (kilocalories per mole) for fragmentation at the B3LYP/ BS2 level of theory (298 K) are shown. Values outside and inside parentheses correspond to the fragmentation patterns that give (Aþ, B) and (A, Bþ), respectively.

quantum chemical methods. The following conclusions can be drawn from this study. (1) Oxidation potentials of various siloxanes were computed and compared against commonly used electrolytes

(various carbonates). Various electron-donating and -withdrawing substituents are found to significantly change the oxidation potential in 1NM1. This suggests that substituted analogues of 1NM1 can be mixed with 12222

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Table 7. Computed Energetics at the B3LYP/BS3 Level of Theory Corresponding to Decomposition of ECa and 1NM1b

EC 1NM1 a

ΔEe (kcal/mol)

ΔH298K (kcal/mol)

ΔG298K (kcal/mol)


15.8


19.1


32.0


2.8


6.5


16.8

EC f CH3CHO þ CO2. b 1NM1 f (CH3)3SiOCH2dCHO þ CH4.

other siloxanes to use as potential electrolytes with high oxidation potential. Four disiloxanes studied here have comparable oxidation potentials with the monosiloxanes such as 1NM3, and their ability to dissolve lithium salts significantly highlights the possibility of their application in future energy storage devices. (2) We find that the main difference in decomposition reactions of siloxanes compared to their carbon analogues is due to the Si
O group. Prediction of accurate bond-breaking barriers in addition to the thermochemistry explained in this investigation is necessary to understand the bond-breaking pattern in detail for siloxanes. (3) Detailed fragmentation patterns were investigated for one disiloxane compound, 2NM3. The results suggest that the silicon headgroup does provide high thermodynamic stability, and breaking up to two silicon units is found to be thermodynamically uphill. (4) Finally, the neutral siloxanes species are found to be more stable to thermal decomposition compared to carbonates, which is consistent with experiment.

’ ASSOCIATED CONTENT

bS

Supporting Information. Five tables, showing fragmentation energetics for neutral 1NM1 and 1NM2, Si
O fragmentation energetics for 1NMn (n = 1
5), selected fragmentation energetics for 2NM3, and Si
C fragmentation energetics for 1SnM1 (n = 1
3). This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the U.S. Department of Energy under Contract DE-AC02-06CH11357. We gratefully acknowledge grants of computer time from EMSL, a national scientific user facility located at Pacific Northwest National Laboratory, ANL Laboratory Computing Resource Center (LCRC), and Center of Nanoscale Materials (CNM).

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