Theoretical Insight into Oxidative Decomposition of Propylene

J. Phys. Chem. B 2009, 113, 5181–5187

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Theoretical Insight into Oxidative Decomposition of Propylene Carbonate in the Lithium Ion Battery Lidan Xing,† Chaoyang Wang,† Weishan Li,*,†,‡,§ Mengqing Xu,†,‡ Xuliang Meng,† and Shaofei Zhao† School of Chemistry and EnVironment, South China Normal UniVersity, Guangzhou 510006, China, College of Materials Science and Engineering, South China UniVersity of Technology, Guangzhou 510641, China, and Key Lab of Electrochemical Technology on Energy Storage and Power Generation in Guangdong UniVersities, Guangzhou 510006, China ReceiVed: NoVember 22, 2008; ReVised Manuscript ReceiVed: February 6, 2009

The detailed oxidative decomposition mechanism of propylene carbonate (PC) in the lithium ion battery is investigated using density functional theory (DFT) at the level of B3LYP/6-311++G(d), both in the gas phase and in solvent. The calculated results indicate that PC is initially oxidized on the cathode to a radical cation intermediate, PC•+, and then decomposes through three pathways, generating carbon dioxide CO2 and radical cations. These radical cations prefer to be reduced on the anode or by gaining one electron from PC, forming propanal, acetone, or relevant radicals. The radicals terminate by forming final products, including trans-2-ethyl-4-methyl-1,3-dioxolane, cis-2-ethyl-4-methyl-1,3-dioxolane, and 2,5-dimethyl-1,4-dioxane. Among all the products, acetone is most easily formed. The calculations in this paper give detailed explanations of the experimental findings that have been reported in the literature and clarify the role of intermediate propylene oxide in PC decomposition. Propylene oxide is one of the important intermediates. As propylene oxide is formed, it isomerizes forming a more stabile product, acetone. 1. Introduction The lithium ion battery has been an irreplaceable power source for electronic equipment required by today’s high-tech society and is believed to be the most competitive power source for electric vehicles needed in future, thus attracting extensive interest.1-6 A typical lithium ion battery system usually comprises a transition metal oxide cathode, a graphite anode, divided by a separator, and an electrolyte solution, which enables ion transfer between the two electrodes.7,8 The performance of a lithium ion battery depends to a great extent on the stability of electrolyte solution, because the high voltage of the battery may cause the decomposition of lithium salt or organic solvents.9-12 Especially, gaseous products are formed from the decomposition of electrolyte solution, which increase the inside pressure of the battery and lead to a safety risk.13 To improve the stability of an electrolyte solution, it is necessary to understand the possible mechanisms on the decomposition of the components in the electrolyte solution. Currently, the most suitable solvents for lithium ion battery remain the mixtures of cyclic alkyl carbonates, such as propylene carbonate (PC) and ethylene carbonate (EC), and linear alkyl carbonates, such as dimethyl carbonate (DMC) and diethyl carbonate (DEC).6 PC is a most valuable solvent component of electrolytes for lithium ion battery because its high permittivity (64.4) provides the electrolytes with high ionic conductivity and its low melting point (-49 °C) provides a battery with better performance under low temperature.14-16 Therefore, the stability of PC has attracted much attention. * Corresponding author. Tel: +86 20 39310256. Fax: +86 20 39310256. E-mail address: [email protected]. † South China Normal University. ‡ South China University of Technology. § Key Lab in Guangdong Universities.

The reduction of PC on the anode in the lithium ion battery has been well understood.16-19 PC is easy to reduce on the anode and co-intercalated into the graphite with lithium ions in the electrolyte, resulting in the exfoliation of the graphite accompanied by the formation of gaseous products such as propene and hydrogen.20-24 Fortunately, the reduction and the cointercalation of PC can be suppressed by an SEI film formed by additives such as vinylene carbonate and butyl sultone.25 The oxidation of PC on the cathode in the lithium ion battery is also well investigated experimentally.26-29 Carbon dioxide, ethane, acetone, propanal, and both cis- and trans-2-ethyl-4methyl-1,3-dioxolane have been identified as the products from PC oxidation. However, the mechanisms that have been proposed by different researchers to explain the experimental phenomena are inconsistent. For example, Krtil et al.30 identified acetone, propanal, and carbon dioxide as the products for PC oxidation and proposed a mechanism involving the intermediates cationic propyl and isopropyl carbonates, as shown in eq 1. Arakawa et al.31 identified carbon dioxide, propanal, and both cis- and trans-2-ethyl-4-methyl-1,3-dioxolane as the products for PC oxidation and proposed a mechanism involving the intermediate propylene oxide. Ufheil et al.28 believed that propylene oxide was one of the final products of PC oxidation.

With the aim of understanding the oxidative decomposition mechanism, high-level density functional theory (DFT) calculations were carried out to identify the initial, transition state, and final products for PC oxidation in this paper. The calculations

10.1021/jp810279h CCC: $40.75  2009 American Chemical Society Published on Web 03/23/2009

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Figure 1. The optimized geometries of the various reactants, intermediates, transition states, and initial oxidative decomposition products of PC at the B3LYP/6-311++G(d) level. Bond lengths are in angstroms.

give detailed explanations on the experimental findings that have been reported in the literature and clarify the role of intermediate propylene oxide in PC decomposition. 2. Computational Details All the calculations have been performed using the Gaussian 03 package.32 The equilibrium and transition state structures are fully optimized by the B3LYP method33 in conjunction with the 6-311++G(d) basis set.34 To confirm each optimized stationary point and make zero-point energy (ZPE) corrections, frequency analyses are done with the same basis set. For each transition state, intrinsic reaction coordinate (IRC) calculation is also performed in both directions to connect these corre-

TABLE 1: Calculated Structural Parameters and Relative Energies (in kJ/mol) for PC and PC•+a) PC•+

PC C1-O5 C1-C2 C2-O3 C2-C7 C4-O3 C4-O5 C4dO6 ∆E a

1.44 1.54 1.45 1.52 1.36 1.36 1.19

1.44 1.54 1.45 1.52 1.36 1.36 1.20 0.0

1.49 1.53 1.54 1.50 1.28 1.28 1.29 1023.4

1.50 1.54 1.54 1.51 1.28 1.29 1.29 1003.3

Bond lengths are in angstroms. The italic data is taken from ref 36, which was calculated at the B3LYP/6-31+G(d) level.

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TABLE 2: Atomic Charges Based on NPA for PC and PC•+ PC PC•+

C1

C2

O3

C4

O5

O6

C7

-0.05 -0.05

0.09 0.11

-0.56 -0.44

1.03 1.02

-0.56 -0.42

-0.63 -0.05

-0.60 -0.61

TABLE 3: Relative Energies, Enthalpies, and Free Energies (in kJ/mol) of the Stationary Points and Imaginary Frequency (ω, cm-1) of the Transition States for the Oxidative Decomposition of PC•+a ∆E

structure •+

0.0 44.7 23.1 34.3 30.4 105.0 5.1 128.0 120.0 145.8 0.4 191.1 120.0 342.8 302.0

PC TS1 PC-1 TS2 M1 TS3 PC-2 TS4 M2 TS5 PC-3 TS6 M3 TS7 PC-4 a

∆E+∆ZPE

∆H

∆G

0.0 29.0 -1.7 20.9 21.7 83.7 -11.8 111.7 106.0 129.3 -15.9 176.4 106.0 321.1 278.0

0.0 29.3 6.4 20.9 23.1 88.9 -5.0 113.7 109.7 132.6 -9.4 177.8 109.7 326.9 286.4

0.0 27.4 -19.4 20.4 20.1 71.9 -26.6 108.3 99.5 123.1 -29.2 174.9 99.5 308.3 258.3

(0.0) (19.0) (4.9) (13.5) (10.0) (94.3) (4.1) (105.6) (107.5) (142.5) (3.6) (180.8) (107.4) (332.3) (310.1)

ω 392i 571i 308i 166i 587i 441i 733i

The data in parentheses refer to those in solvent.

TABLE 4: Atomic Charges Based on NPA for PC-1, PC-3, PC-2, and M1 PC-1 PC-2 PC-3 M1

C1

C2

O3

C4

O5

O6

C7

-0.26 -0.08 -0.08 -0.06

0.39 0.09 0.09 0.11

-0.56 0.00 -0.53 -0.31

1.06 1.04 1.04 1.04

-0.25 -0.53 -0.02 -0.46

-0.47 -0.48 -0.47 -0.47

-0.72 -0.62 -0.62 -0.44

TABLE 5: Relative Energies, Enthalpies, and Free Energies (in kJ/mol) of the Stationary Points and Imaginary Frequency (ω, cm-1) of the Transition States for the Conversion of the Radical Cationsa ∆E

structure P1•+ TS-1 P3•+ TS-2 P2•+ TS-3 a

23.9 30.7 -125.6 30.0 0.0 100.2

(4.4) (10.6) (-137.9) (19.0) (0.0) (94.3)

∆E + ∆ZPE

∆H

∆G

16.0 22.3 -126.3 24.3 0.0 97.0

17.5 21.9 -125.6 23.3 0.0 96.4

12.1 22.3 -128.3 24.3 0.0 97.1

ω 659i 141i 435i

The data in parentheses refer to those in solvent.

sponding intermediates at the above level. Enthalpies and Gibbs free energies are calculated at 298.2 K. Charge distribution is analyzed by the natural bond orbital (NBO) theory. To investigate the role of solvent effects, the bulk solvent effect is estimated by single point calculations using the polarized continuum models (PCM).35 A dielectric constant of 64.4 for PC is used for all PCM calculations. 3. Results and Discussion 3.1. Initial Oxidation of PC. The initial oxidation of PC involves a one-electron transfer from one PC molecule to the cathode of the lithium ion battery, resulting in radical cation PC•+, which has been detected by in situ FT-IR.36 The optimized geometries and the calculated structural parameters of PC and PC•+ are presented in Figure 1. The structural parameters of PC and PC•+ are also presented in Table 1 with a comparison of the values obtained from the calculation at the B3LYP/631+G(d) level.36 The optimized geometry indicates that the

initial oxidation is accompanied by the cleavage of the C2-O3 bond. With the cleavage of the C2-O3 bond in PC, the C4-O3 and C4-O5 bonds are shortened from 1.36 Å in PC to 1.28 Å in PC•+, and the C4-O6 bond is elongated from 1.19 Å in PC to 1.29 Å in PC•+, which is close to C4-O3 or C4-O5 in PC•+. Table 2 presents the charge distribution on atoms in PC and PC•+ obtained by natural population analysis (NPA). Charge distributes mainly on C and O atoms in PC and PC•+. It can be seen from Table 2 that the charge on O atoms in PC•+ (O3 ) -0.44, O5 ) -0.42, O6 ) -0.61) is more positive than that on O atoms in PC (O3 ) -0.56, O5 ) -0.56, O6 ) -0.63), while the charge on the C atoms in PC and PC•+ are almost the same. This indicates that the electron is taken from O atoms during the initial oxidation of PC. 3.2. Decomposition Mechanism of PC•+. Based on the geometry of PC•+, it can be known that there are four possible pathways for its decomposition, involving four initial products (PC-1, PC-2, PC-3, and PC-4), three intermediates (M1, M2, and M3), and seven transition states (TS1, TS2, TS3, TS4, TS5, TS6, and TS7), as shown in Scheme 1. The geometry of all the intermediates, transition states and initial products involved in the decomposition of PC•+ is presented in Figure 1. The calculated relative energies (∆E), enthalpies (∆H), free energies (∆G), and the imaginary frequencies (cm-1) for transition states (TS) are presented in Table 3. Figure 2 presents the potential energy profile for the decomposition process of PC•+ in gas phase and solvent. In order to confirm the geometry of transition states, frequency analyses and IRC calculations are carried out. Each transition state (TS1, TS2, TS3, TS4, TS5, TS6, and TS7) corresponds to one imaginary frequency (392, 571, 308, 166, 587, 441, and 733i cm-1 in gas phase, respectively), and IRC results indicate that all the transition states are connected with the relevant reactants and products. As shown in Table 3, in the gas phase, the ∆E of the initial products, PC-1, PC-2, PC-3, and PC-4 is 23.1, 5.1, 0.4, and 302.0 kJ/mol, respectively, indicating that the order of stability is PC-3 ≈ PC-2 > PC-1 > PC-4. The data of ∆E + ∆ZPE, ∆H, and ∆G show the same results. Accordingly, in solvent, the ∆E of the initial products, PC-1, PC-2, PC-3, and PC-4 is 4.9, 4.1, 3.6, and 310.1 kJ/mol, respectively, indicating that the order of stability is PC-3 ≈ PC-2 ≈ PC-1 > PC-4. Obviously, the initial products PC-1, PC-2, and PC-3 are far more stable than PC-4. Therefore, PC-4 is unstable and less probable to be the initial product from the oxidation of PC•+. Carbon dioxide is included in PC-1, PC-2, and PC-3 and has been observed by many researchers.3-5 The calculated standard electrode potential for paths 1, 2, and 3 in the gas phase is 4.51, 4.43, and 4.41 V (vs Li/Li+), respectively. On the other hand, the possibility for a reaction to take place depends on its ∆E or the activation energy of its transition state. In path 1, PC•+ converts into PC-1 via only one transition state TS1, with the cleavage of C2-O3 and C4-O5 bonds, the corresponding activation energy is 44.7 and 19.0 kJ/mol in the gas phase and in solvent, respectively. In path 2, PC•+ converts first into intermediate M1 via transition state TS2 with the cleavage of C2-C7 and C4-O3 bonds; the corresponding activation energy is 34.3 and 13.5 kJ/mol in the gas phase and in solvent, respectively. Then M1 dissociates to PC-2 via the transition state TS3 with the direct cleavage of the C1-O5 bond and the formation of the C1-C7 bond, the corresponding activation energy is 105.0 and 94.3 kJ/mol in the gas phase and in solvent, respectively. In path 3, PC•+ converts first into intermediate M2 via transition state TS4 with the cleavage of

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Figure 2. Potential energy profile for the decomposition process of PC•+ in gas phase and in solvent calculated with B3LYP/6-311++(d) and PCM-B3LYP/6-311++(d).

SCHEME 1: Possible Pathways for the Decomposition of PC•+

C2-O3 bond; the corresponding activation energy is 128.0 and 105.6 kJ/mol in the gas phase and in solvent, respectively. Then M2 dissociates to PC-3 via the transition state TS5 with the direct cleavage of the C4-O5 bond; the corresponding activation energy is 145.8 and 142.5 kJ/mol in the gas phase and in solvent, respectively. In path 4, PC•+ converts first into intermediate M3 via transition state TS6 with the cleavage of C2-O3 and C1-O5 bonds and the formation of C1-O3 bond; the corresponding activation energy is 191.1 and 180.8 kJ/mol in gas phase and solvent, respectively. Then M3 dissociates to PC-4 via the transition state TS7 with the direct cleavage of the C1-O3 bond; the corresponding activation energy is 342.8 and 332.3 kJ/mol in the gas phase and in solvent, respectively. Obviously, path 1 is the most favorable pathway, followed by path 2 and path 3 (Figure 2). Path 4 is infeasible because of the high activation energy for transition state TS7 and the instability of PC-4. Since the relative energy of M1 (10 kJ/mol in solvent) is close to that of PC-1, PC-2, or PC-3 and the activation energy for its formation (13.5 kJ/mol in solvent) is quite approximate to that for the formation of PC-1 and lower than that the formation of PC-2 or PC-3, M1 is one of the possible initial products from the oxidation of PC•+. Based on the activation

energy of the formation of the possible initial products, it can be inferred that the content of the initial products should be in the order PC-1 > M1 > PC-2 > PC-3. 3.3. Charge Distribution of the Initial Products. The charge distributions on atoms in PC-1, M1, PC-2, and PC-3 in solvent, obtained by using natural population analysis (NPA), are shown in Table 4. The sum of the charge on O3, C4, and O6 of the formed CO2 in PC-1, PC-2, and PC-3 is -0.03, -0.03, and 0.04, respectively. It is almost zero, indicating that after the decomposition of PC•+, the formed CO2 maintains electrical neutrality and the positive charge mainly concentrates on the residual structure in PC-1, PC-2, and PC-3. The residual structure is radical cation. The radical cation in PC-1 is denoted as P1•+ and the radical cation in PC-2 is similar to that in PC3, denoted as P2•+. For M1, the sum of the charge on C7, H11, H12, and H13 of the formed methyl CH3• radical in M1 is 0.29. Thus the positive charge mainly concentrates on the residual structure in M1, which is a cation and denoted as P4+. One cation similar to P4+ has been detected during the galvanostatic oxidation of PC.3 The CH3• terminates forming ethane, which has been observed experimentally.8,37

Decomposition of PC in Li Ion Battery

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Figure 3. The optimized geometries of the various reactants, intermediates, transition states, and final oxidative decomposition products of PC at the B3LYP/6-311++G(d) level. Bond lengths are in angstroms. The underlined data in DD/DD+ are the parameters of DD+.

•+

•+

Figure 4. Potential energy profile for conversion of P1 , P2 , and P3•+ in the gas phase and in solvent calculated with B3LYP/6311++(d) and PCM-B3LYP/6-311++(d).

3.4. Termination Reactions of Radical Cations. Figure 3 presents the optimized geometry of radical cations P1•+ and P2•+ and the possible conversion pathways of them are shown in Figure 4, together with the potential energy profile. The geometry of all the intermediates, transition states, and initial products involved in the conversion of P1•+ and P2•+ is also

Figure 5. Singlet and triple potential energy surface for the reduction reaction of P1•+, P2•+, and P3•+ in solvent calculated with PCM-B3LYP/ 6-311++(d).

presented in Figure 3. The calculated relative energies, enthalpies, free energies, and the imaginary frequencies (cm-1) for transition states are presented in Table 5. As shown in Figure 4, P1•+ converts into P3•+ via transition state TS-1 with a activation energy of 30.7 and 10.6 kJ/mol in the gas phase and in solvent, respectively, and into P2•+ via transition state TS-2 with a activation energy of 44.7 and 19.0 kJ/mol in the gas phase and in solvent, respectively. This

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TABLE 6: Relative Energies, Enthalpies, and Free Energies (in kJ/mol) of the Stationary Points and Imaginary Frequency (ω, cm-1) of the Transition States for the Termination of the Radical Cationsa ∆E

structure •+

P1 t-P1• P3•+ t-P2• P2•+ TS-4 propylene oxide propanal (t-P1• + t-P2• ) CEMD)/2 [(t-P1• + P3•+) or (t-P2•+ P1•+) ) CEMD+]/2 t-P1• + t-P2• ) TEMD [(t-P1• + P3•+) or (t-P2•+ P1•+) ) TEMD+]/2 (t-P1• + t-P1• ) DD)/2 (t-P1• + P1•+ ) DD+)/2 a

149.5 -621.9 0.0 -646.6 125.6 -577.3 -850.9 -951.4 -959.1 -529.7 -958.5 -530.3 -953.7 -544.1

∆H

∆G

142.3 -626.8 0.0 -647.3 126.3 -583.8 -840.8 -943.9 -940.8 -514.8 -940.2 -515.5 -934.4 -526.4

143.1 -626.1 0.0 -647.4 125.6 -585.6 -843.1 -944.5 -944.4 -517.9 -943.9 -518.4 -938.8 -530.5

140.4 -629.6 0.0 -648.0 128.3 -579.5 -835.8 -942.2 -910.9 -486.1 -910.5 -486.9 -902.4 -495.2

ω

888i

The data in parentheses refer to those in solvent.

TABLE 7: Relative Energies, Enthalpies, and Free Energies (in kJ/mol) of the Stationary Points of PC and the Oxidative Decomposition Productsa ∆E + ∆ZPE

∆E

structure PC CO2 + propylene oxide CO2 + acetone CO2 + propanal CO2 + TEMD/2 CO2 + CEMD/2 CO2 + DD/2 a

(142.4) (-378.1) (0.0) (-401.6) (137.9) (-370.7) (-610.3) (-713.2) (-708.4) (-374.8) (-707.9) (-375.3) (-703.3) (-389.1)

∆E + ∆ZPE

∆H

∆G

0.0 73.3

(0.0) (62.3)

0.0 38.3

0.0 42.8

0.0 -5.3

-57.8 -27.2 -34.3 -34.9 -29.5

(-71.5) (-40.6) (-35.3) (-35.8) (-30.8)

-97.4 -64.9 -61.2 -61.7 -55.4

-90.4 -58.6 -58.0 -58.5 -52.9

-145.5 -111.8 -80.1 -80.5 -71.9

The data in parentheses refer to those in solvent.

suggests that P1•+ converts into P3•+ more easily than into P2•+. It seems that P2•+ cannot be converted into P3•+ via transition state TS-3 due to its high activation energy, 100.2 and 94.3 kJ/mol in the gas phase and in solvent, respectively. Thus, there are three radical cations available during the PC oxidation, P1•+, P2•+, and P3•+. The radical cation may polymerize by itself or with another radical cation forming a divalent cation or gain one electron from the anode or a solvent molecule forming a radical. The optimization in this work fails to locate the divalent cations formed from P1•+, P2•+, and P3•+. Therefore these radical cations cannot polymerize but prefer to gain one electron from the anode forming a radical or from PC forming a radical and a new radical cation, PC•+. The final products are the same for the radical cations to gain electron on the anode and from PC. There are two possible electronic configurations for the reduction products of P1•+, P2•+, and P3•+, singlet and triplet. The prefix “t” is used to denote the structure in the triplet electronic state. The optimized structures of various species on the singlet and the triplet state potential energy profiles are presented in Figure 3, and the relative energies of various species and energy diagram along singlet and triplet reaction pathways in solvent are shown in Figure 5. As shown in Figure 5, for the single potential energy surface, P2•+ is reduced to propylene oxide, and the formation of propylene oxide is exothermic with 748.2 kJ/mol. This suggests that propylene oxide is highly active and easy to isomerize forming a more stable product, acetone, via transition state TS4, the corresponding activation energy is 239.6 kJ/mol. This gives an explanation why some researchers believed that propylene oxide was one of the products of PC oxidation but could not provide any evidence for the existence of propylene oxide during the oxidation of PC.3,4 Thus, the product from P2•+

reduction is acetone, which has been observed as the oxidative decomposition product of PC, 8 and the product from P1•+ or P3•+ reduction is propanal, which has also be observed.5,8 For the triplet potential energy surface, P1•+ and P2•+ are reduced to t-P1• and P3•+ is reduced to t-P2•. Subsequently, radical t-P1• or t-P2• terminates with each other or with the unreduced radical cations, forming the products shown in Table 6 with the optimized structures presented in Figure 3. The products of termination between radicals t-P1• and t-P2• are trans-2-ethyl-4-methyl-1,3-dioxolane (TEMD), cis-2-ethyl4-methyl- 1,3-dioxolane (CEMD), and 2,5-dimethyl-1,4-dioxane (DD). Among them, TEMD and CEMD have been observed by Arakawa and Yanmaki.5 DD has not yet been reported as the product of PC oxidation. This may be due to the lower amount of DD in the products compared with the amount of TEMD and CEMD. The products of the termination between radical t-P1• or t-P2• and radical cation P1•+ and P2•+ are cations TEMD+, CEMD+, and DD+. These cations are reduced on the anode forming TEMD, CEMD, and DD or gain one electron from PC forming TEMD, CEMD, DD, and PC•+. To further confirm the products of PC oxidation, the relative energies, enthalpies, and free energies of PC and its oxidative decomposition products are summarized in Table 7. From Table 7, it can be noticed that the pathway for PC oxidation to form CO2 and propylene oxide has the lowest thermodynamic possibility. Among the possible products of PC, the most thermodynamically favorable product is acetone, followed by propanal, CEMD, TEMD, and DD. 4. Conclusions The detailed mechanism for the oxidative decomposition of PC in the lithium ion battery has been investigated in the present work by DFT calculation at the B3LYP/6-311++G(d) level. Based on the calculated results, PC•+ is generated after PC transfers one electron to the cathode. Subsequently, PC•+ converts into the initial products PC-1, M1, PC-2, and PC3. Radical cations P1•+ and P2•+ are generated after CO2 released from the initial products, and P1•+ is easily converted into a more stable radical cation, P3•+. The radical cations P1•+, P2•+, and P3•+ are reduced on the anode or by gaining one electron from PC, forming propanal, acetone, or relevant radicals. The radicals terminate by forming final products, including trans-2-ethyl-4-methyl-1,3-dioxolane, cis-2-ethyl4-methyl-1,3-dioxolane, and 2,5-dimethyl-1,4-dioxane. Acknowledgment. This work is supported by the National Natural Science Foundation of China (Grant No. NS-

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