Why Do Sulfone-Based Electrolytes Show Stability at High Voltages

Letter pubs.acs.org/JPCL

Why Do Sulfone-Based Electrolytes Show Stability at High Voltages? Insight from Density Functional Theory Yating Wang,† Lidan Xing,*,†,‡,§ Weishan Li,*,†,‡,§ and Dmitry Bedrov∥ †

School of Chemistry and Environment, South China Normal University, Guangzhou 510006, China Key Laboratory of Electrochemical Technology on Energy Storage and Power Generation of Guangdong Higher Education Institutes, South China Normal University, Guangzhou 510006, China § Engineering Research Center of Materials and Technology for Electrochemical Energy Storage (Ministry of Education), South China Normal University, Guangzhou 510006, China ∥ Department of Materials Science and Engineering, University of Utah, 122 South Central Campus Drive, Salt Lake City, Utah 84112, United States ‡

S Supporting Information *

ABSTRACT: Sulfone-based electrolytes have attracted a great attention due to their high oxidation stability comparing to conventional carbonates. However, the ab initio calculated oxidation potentials (Eox) of isolated sulfones are lower than those for carbonates. To understand this contradiction, the oxidations of three carbonates and eleven sulfones in a presence of anions and other solvent molecules have been investigated by the density functional theory calculations with a polarized continuum model. Importantly, we found that the Eox of some of the sulfones show surprisingly high stability toward the presence of anions and another solvent, which is the key factor of high oxidation stability of these electrolytes compared to carbonates. Finally, the way to design new high oxidation stability sulfones by modifying their functional groups is discussed. SECTION: Energy Conversion and Storage; Energy and Charge Transport

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Density functional theory (DFT) calculations have been widely used to investigate the oxidation stability and reaction mechanisms of carbonate solvents. In our previous works, we demonstrated that the oxidation stability and decomposition mechanism of propylene carbonate (PC) and ethylene carbonate (EC) were significantly influenced both by the presence of salt anions and neighboring solvent molecule(s).39−42 For example, recent DFT studies showed that the oxidation decomposition reaction energies of sulfolane (SL) and ethylmethyl sulfone (EMS) have been lowered by the presence of PF6−, BF4− or another neighboring solvent molecule.43−45 Nevertheless, the oxidation stability of sulfones has not been well-understood. In this work, the oxidation stability of sulfones and the influence of an explicit presence of anions and solvent molecules in the first coordination shell were studied by DFT calculations and compared with previous results for carbonates, such as PC, EC, and dimethyl carbonate (DMC). Eleven sulfones were investigated in this work, including SL,29,30,32,33,46 EMS,29,30,33 1-fluoro-2-(methyl-sulfonyl) benzene (FS),33 trimethylene sulfone (TriMS),30 1-methyltrimethylene sulfone (MTS),30 ethyl-iso-butyl sulfone (EiBS),30 ethyl-iso-propyl sulfone (EiPS),30

ithium ion batteries and supercapacitors are the most wellknown energy and power storage devices that have attracted much interest in application for electric vehicles.1−6 Further improvements of energy and power density are still needed to meet the requirements of longer load range and higher charge/ discharge rate.7−11 To increase the energy density, major efforts have been devoted to developing high voltage batteries and electrical double layer capacitors, which require electrolytes with high electrochemical stability.12−20 The oxidation potential of traditional organic solvent based electrolytes are around 4.3− 4.5 V (vs Li/Li+).21,22 Hence, cycling of batteries/capacitors at such a high voltage can be accompanied by oxidation decomposition of the electrolyte and leading to capacity fade.23−28 So far, sulfones,29−38 as one of the high anodic stability solvents, have been reported to be stable at around 5 V (vs Li/Li+). For example, ethylmethyl sulfone (EMS)-based electrolyte has been reported to offer an oxidation potential as high as 5.6 V. Unfortunately, due to high viscosity and melting point, the practical applications of these sulfone-based electrolytes are limited. The search for new solvents with high oxidation stability, melting point, and low viscosity still continues, and therefore an understanding of why sulfone-based electrolytes exhibit higher oxidative stability is necessary to facilitate the design of a new generation of electrolytes with high voltage stability. © 2013 American Chemical Society

Received: August 13, 2013 Accepted: November 7, 2013 Published: November 7, 2013 3992

dx.doi.org/10.1021/jz401726p | J. Phys. Chem. Lett. 2013, 4, 3992−3999

The Journal of Physical Chemistry Letters

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Figure 1. Optimized structures and geometric parameters (bond length in Å, dihedral angle in degree) of the investigate sulfones, carbonates, and anions before and after oxidation.

3,3,3-trifluoropropylmethyl sulfone (FPMS),30 ethyl methoxyethyl sulfone (EMES),31,34 methoxyethyl methyl sulfone (MEMS),34 and ethylmethoxyethoxyethyl sulfone (EMEES).31 Optimized structures and geometric parameters of investigated isolated sulfones, carbonates and lithium salt anions as well as their oxidized states (denoted as “-e”) are given in Figure 1, while the NPA charge distributions before and after oxidation are listed in Figure S-1 (shown in the Supporting Information (SI)). Note that all calculations were conducted using polarized continuum model (PCM), and therefore the influence of the surrounding solvent is taken into account implicitly. As shown in Figure S-1, in carbonates the electron is primarily taken from the carbonyl O (CO), resulting in an increase of the CO and a decrease of the C−O (ether) bond lengths (see Figure 1). However, the initial oxidation of sulfones is different compared to carbonates. For SL, FPMS, and EMS, the electron is mainly subtracted from the O atoms and one of the alkyl groups (especially the alkyl C) that connect to the SO2, leading to an increase of the corresponding C−S bond length as shown in Figure 1 and S-1. However, for TriMS, MTS, and EiPS, the electron is taken from the SO2 group and one of the alkyl C connected to it, leading to the breakage of the corresponding C−S bond (bond length >2.2 Å). Interestingly, for TriMS we

observe a spontaneous formation of a C−O bond after the breakage of the C−S, leading to the formation of a five-member ring. Although the structure of EiBS only has one more primary carbon group than EiPS, the charge distribution and the structure after oxidation are very different from the latter. As can be seen from Figure 1 and S-1, EiBS prefers to vacate the electron from the SO2 group and the two C atoms connected to it, resulting in the increase of both of the C−S bonds. Furthermore, we also note that after oxidation, charge distribution on the primary carbon connected to S atom (such as in EiBS) changes less than the tertiary carbon (such as in EiPS), indicating that the electrophilic ability of C in the former compound is higher. This might be relevant for observed increased oxidation stability Eox of EiBS by 0.3V compared to EiPS. Sulfones with ether (such as MEMS, EMES, and EMEES) or phenyl (such as FS) substitutions showed a similar behavior. Once oxidized, the charge distributions change slightly on S atom and its connected alkyl group, while a decrease of about 0.4 e on ether O and 0.9 e on phenyl group. As a result, the structure of C−SO2−C group stays almost the same, while the dihedral angle of the ether group changes from 180° to 119°, 113° and 86°, for MEMS, EMES and EMEES respectively. For FS, the variation of the structure is negligible. 3993



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loses H) radical, and finally leading to the decrease of Eox. A comparison of Eox of the investigated solvents with and without the presence of anion or additional solvent molecule is given in Figure 2. It is obvious that the presence of anions or another

The calculated Eox of the above investigated solvents using single molecule calculations ranks from high to low as follows: EC = PC (7.1) > DMC = FPMS (6.9) > EMS (6.5) > SL (6.3) > EiBS (6.2) > MEMS = EMES (6.1) > EiPS = FS (5.9) > EMEES (5.6) > MTS (5.5) > TriMS (5.0). This order is in a good agreement with the results obtained by previous high-level DFT calculations.37,45 However, it is clear that the calculated Eox values for sulfones are lower than the corresponding numbers for carbonates, except for FPMS, which is equal with DMC. Obviously, the Eox obtained from single molecule calculations cannot explain the experimentally observed trends showing that most of sulfones have a better anodic stability and cycling at high voltages compared to carbonates and therefore have been considered as a promising alternative solvent for high-voltage lithium ion batteries.8 It was noted before in experiments that oxidation potential of PC decreases in the presence of electrolyte salts.47 Our recent DFT studies where we considered molecular complexes in a PCM shell (instead of single molecules in a PCM shell) have confirmed this observation and showed that explicit inclusion of anions and additional solvent molecules alter the thermodynamics and kinetics oxidation reaction of carbonates.39−41,44,45 Similar results have been also obtained for SL (referred as TMS in ref 45) and EMS.45 Hence, the influence of anion and additional solvent molecules on the oxidation stability of the investigated solvent must be taken into account. In order to understand how and why anions affect the oxidation stability of a solvent, the initial oxidation reactions of isolate PF6−, BF4− and ClO4− ions were also investigated in this work and are presented in Figure 1. The calculated Eox of the above anions is 8.6, 7.9, and 6.1 V, respectively, which is in good agreement with the experimental observations (conducted at 5 mVs−1 scan rate using glassy carbon as working electrode and electrolyte compositions 0.65 M lithium salt/PC) that the oxidation stability of lithium salts is in the order LiPF6 > LiBF4 > LiClO4.26,48,49 It is important to note that the calculated oxidation potentials of PF6− and BF4− are higher than all of the investigated solvents. However, for ClO4−, the potential is lower than that of the carbonates and most of the sulfones. So, one can expect that the different anions will have a different influence on the oxidation stability of solvent. Recent molecular dynamics simulations showed that increasing of electrode potential on the cathode results in significant accumulation of anions and certain types of solvent molecules on the electrode surface.50,51 Hence, the influence of anion and surrounding solvent must be taken into account when the oxidation reactions are considered on these surfaces. Therefore, we have conducted DFT calculation on molecular clusters where anions or neighboring solvent molecules (same type, referred as S−S, or DMC, referred as S-DMC) have been conducted for all considered solvent molecules. Inclusion of DMC molecule explicitly was motivated by the fact that it is one of the most common cosolvents in electrolytes and has been widely used due to its low viscosity and freezing point. All calculations for the clusters were also conducted using the PCM method to implicitly represent the remaining electrolyte environment. Figure S-2 presents the optimized structures of the investigated carbonate-anion (S-anion) and carbonate-solvent clusters before and after oxidation, together with the calculated Eox and NPA charge distribution. H transfer reactions occurred during oxidation reaction can be found for all of the investigated carbonate clusters, generating PF5, HF, HClO4, BF3, carbonate +H (carbonate captures H) cation, and carbonate−H (carbonate

Figure 2. Calculated oxidation potentials (V vs Li+/Li) of the investigated isolate solvents, anions, and clusters.

solvent lowered the Eox of isolate carbonates to a various extent. Importantly, we found that the Eox of carbonate−anion clusters has the order: carbonate-PF6− > carbonate-BF4− > carbonateClO4−, which is consistent with the order of anion oxidation stability discussed above, therefore indicating that an increase of salts anodic stability can significantly improve the oxidation stability of the solvent in the electrolyte. This is in good agreement with experimental results of Hayashi et al. (using the scan rate 0.1 mVs−1, glassy carbon as working electrode, and electrolyte composition 1 M lithium salt/carbonate), who observed the same trend for monosolvent (includes EC, DMC and PC) electrolytes.52 Furthermore, the explicit presence of another solvent molecule also shows great influence on decreasing the Eox of carbonates. Eox of PC−PC cluster is 0.1 V lower than that of PC−ClO4−. For DMC, the effect of additional solvent molecule is not as great as for the other two carbonates. Eox of DMC−DMC cluster is 0.2 V higher than for DMC−ClO4− and 0.5 V lower than for DMC−PF6−. The calculated Eox of isolate sulfones and sulfone-anion/ solvent clusters are also given in Figure 2, while the optimized structures of the clusters before and after oxidation are shown in Figures 3 and 4 (1 to 2). Figure 2 shows that explicit presence of anions and other solvent molecules significantly lowers the oxidation potentials of SL, TriMS, MTS, FPMS, and EiPS, which is consistent with the results for carbonates discussed above. Surprisingly, explicit inclusion of anions and additional solvent molecules do not affect the anodic stability of EiBS, MEMS, EMES, EMEES, FS, and EMS sulfones, and their oxidation stability remains high. The latter is consistent with the experimentally observed trend of high oxidation stability of sulfones compare to carbonates. Specifically, the calculated Eox values of FPMS, EiBS, MEMS, EMES, FS and EMS are higher than for PC and EC. As we mentioned above, calculated Eox of isolate PC is much higher than that of isolated SL. However, the calculated Eox of PC−PC drops to 5.5 V, which is only 0.1 V higher than that of SL−BF4− cluster, and is consistent with the withstand voltage test results of 3994



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Figure 3. Optimized structures and calculated oxidation potentials (V vs Li+/Li) of the sulfone-anion/solvent clusters before and after oxidation. Sum of NPA charge of the oxidized clusters presented in red, gray, and blue circle is relative to +1, 0, and −1 e, respectively.

Naoi et al. containing salts with BF4− anion (scan rate 5 mVs−1, glassy carbon as working electrode, and electrolyte compositions 1.5 M BF4/solvent).53 The calculated Eox of linear sulfones has the order EMS > MEMS ≈ EMES > EMEES, indicating that the oxidation stability increases with the decrease of oligoether chain length. This is also in a good agreement with recent experiment reports31,34 (scan rate 1 mV s−1, working electrode Pt, electrolyte compositions LiTFSI/sulfone). By comparing the optimized structures of the oxidized sulfone clusters and their Eox, it is easy to find that anions and additional solvent molecules lower the oxidation stability of sulfones by getting involved in the oxidation and decomposition reactions, such as SL, TriMS, MTS, and EiPS clusters, shown in Figure 3. While those sulfones show high oxidation stability in the presence of anions and other solvent molecules, they oxidize without charge sharing between molecules or involving in subsequent oxidation driven reactions, as can be seen for FPMS, EiBS, MEMS, EMES, EMEES, FS, and EMS clusters in Figure 4. But the FPMS−ClO4− and EMS−ClO4−clusters are the exception and will be discussed below. Anions and neighboring solvent molecules participate in all of the oxidation reaction of sulfone clusters shown in Figure 3 except for SL−PF6− and SL−SL. Importantly, it can be noted from Figures 3 and 4 that the oxidation of sulfone-anion clusters does not spontaneously generate HF, which agrees with the

results reported by Borodin et al.44,45 and HClO4. It is known that those compounds can poison cathode materials,26 and therefore an absence of these compounds during sulfones oxidation might improve the cyclic performance of the lithium ion batteries. The charge distribution of the oxidized sulfone clusters (Figure 3) shows that the electron is taken from sulfone (the solvent), resulting in an increase or breakage of the S−C bond of sulfones and generating one positively charge C, a mechanism that is similar to oxidation of isolate sulfones discussed above. Then the neighboring anion or the most negative charge atom in the neighboring solvent molecule attacks the positively charged C by nucleophilic reaction. For the PF6− and BF4− anions, this reaction leads to the break of P−F and B−F bonds and generation of PF5, BF3, and fluorate sulfones. In the presence of ClO4− anion and other solvent molecules, the nucleophilic reaction generates perchlorate sulfones and oligomer, respectively. For the SL−PF6− and SL−SL clusters, the charge distribution on the C of SL-e is −0.2 (see Figure S-1), which makes the nucleophilic reaction more difficult. Moreover, the bond length of the related S−C is 2.09 (shorter than the S−C breakage of oxidized sulfones), indicating that the interaction between S and C is stronger than the other oxidized sulfones. Hence, the presence of PF6− and SL does not provide strong enough influence to break the S−C bond in the oxidized SL. 3995



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Figure 4. (a,b) Optimized structures and calculated oxidation potentials (V vs Li+/Li) of the sulfone-anion/solvent clusters before and after oxidation. Sum of NPA charge of the clusters presented in gray and blue circles is relative to 0 and −1 e, respectively.

stability toward the neighboring anion and solvent. However, FPMS−ClO4− and EMS−ClO4− are exceptions. The optimized structure and charge distribution of FPMS−ClO4− and EMS− ClO4− indicate that ClO4− is oxidized instead of sulfone, which is

Presented in Figure 4a,b are the optimized structures of FPMS, EiBS, MEMS, EMES, EMS, FS and EMEES clusters before and after oxidation. Contrary to sulfone clusters presented in Figure 3, the calculated Eox and structure of sulfones in Figure 4 show high 3996



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using PCM (acetone) confirm the preferentially oxidized ClO4− in EMS-ClO4−-e complex are also given. This material is available free of charge via the Internet http://pubs.acs.org.

in agreement with the results shown in Figure 2 indicating that the oxidation stability of ClO4− is lower than FPMS and EMS. While the Eox of ClO4− anion is higher than for investigated sulfones, it still has the most influence on decreasing the oxidation stability of these sulfones. Based on Eox of all the investigated solvent-anion clusters, we can conclude that among LiPF6, LiBF4, and LiClO4, the LiPF6 is the most suitable lithium salt for high-voltage electrolyte, while the LiClO4 is harmful for the oxidation stability of the electrolyte. In conclusion, we demonstrated that in DFT calculations, the oxidation stability of eleven investigated sulfones is lower than carbonates if a single molecule calculation in implicit solvent shell is conducted. The trend obtained from such calculations is inconsistent with experimental observations that show a higher anodic stability for sulfones-based electrolytes. However, explicit inclusion of anions or additional neighboring solvent molecule in consideration during DFT calculations showed that the molecules in the first coordination shell strongly influence the oxidation potential and stability of electrolyte molecules. These calculations on molecular clusters provided trends that are consistent with experimental observations and revealed mechanisms of a higher oxidative stability of sulfones. Specifically, similar to carbonates, the calculated oxidation potentials of SL, TriMS, MTS, and EiPS are dramatically lowered by the presence of anions (PF6−, ClO4−, and BF4−) or neighboring solvent molecules (e.g., DMC or the solvent itself). However, FPMS, EiBS, MEMS, EMES, EMEES, FS, and EMS showed surprisingly high stability toward the presence of anions and neighboring solvents, resulting in overall higher calculated oxidation potentials than those of carbonates. Importantly, we found that ClO4− anion has the most influence on lowering the oxidation stability of investigate solvents. We also found that the oxidation reaction of sulfone−anion clusters does not generate HF, which agrees with the results reported by Borodin et al.,44,45 and HClO4. All calculations were performed using the Gaussian 09 package.54 The geometry of molecules and their dimers were optimized using the B3LYP level of theory in conjunction with the 6-311++G (d) basis set. Frequency analyses were done with the same basis set to confirm the obtained optimized stationary point. The Gibbs free energy was obtained at 298 K. The atomic charge distributions were computed form natural population analysis (NPA) by using natural bond orbital (NBO) theory. To investigate the role of the environment, the bulk solvent effect was estimated using the polarized continuum models (PCM).55 The acetone dielectric constant (20.5) was used to represent the solvent for PCM calculations. The oxidation potential (Eox) was converted from the absolute oxidation potential of Li+/Li by subtracting 1.4 V from the former, as shown in eq 1.56 Eox (Li+/Li) = [G(M+) − G(M)]/F − 1.4 V

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Corresponding Authors

*Tel.:+86 20 39310256; fax: +86 20 39310256; e-mail address: [email protected] (L.D.X.). *E-mail: [email protected] (W.S.L). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (21273084, 21303061), the Joint Project of the National Natural Science Foundation of China and the Natural Science Foundation of Guangdong (No.U1134002), the Natural Science Foundation of Guangdong Province (10351063101000001, S2011040001731), and the key project of Science and Technology in Guangdong Province (Grant No. 2012A010702003). D.B. would like to acknowledge the support sponsored by the Army Research Laboratory under Cooperative Agreement Number W911NF-12-2-0023. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the Army Research Laboratory or the U.S. Government. The U.S. Government is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright notation herein.

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(1)

+

where G(M) and G(M ) is the free energy of the solvated complex M and its solvated oxidized form M+ at 298.15 K, respectively, and F is the Faraday constant.

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AUTHOR INFORMATION

ASSOCIATED CONTENT

S Supporting Information *

NPA atomic charges distribution of the selected atoms in sulfones, carbonates, and anions before and after oxidation, NPA and ChelpG charge distribution of FPMS-ClO4− and EMS-ClO4− complexes before and after oxidation, and partial geometric parameters (includes bond length and dihedral angle) of the structures shown in Figures 3 and 4 are summarized in the Supporting Information. Calculation results from G4MP2 levels 3997



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