Novel Binary Room-Temperature Complex System Based on LiTFSI

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J. Phys. Chem. C 2007, 111, 5184-5194

Novel Binary Room-Temperature Complex System Based on LiTFSI and 2-Oxazolidinone and Its Characterization as Electrolyte Renjie Chen,†,§ Feng Wu,*,‡,§, Li Li,‡,§ Bin Xu,‡,| Xinping Qiu,† and Shi Chen‡ Department of Chemistry, Tsinghua UniVersity, Beijing 100084, China, School of Chemical Engineering and The EnVironment, Beijing Institute of Technology, Beijing, 100081, China, National DeVelopment Center of High Technology Green Materials, Beijing 100081, China, and Research Institute of Chemical Defense, Beijing 100083, China ReceiVed: September 30, 2006; In Final Form: January 15, 2007

Binary room-temperature complex system has been prepared based on lithium bis(trifluoromethane sulfonyl) imide (LiN(SO2CF3)2, LiTFSI) and 2-oxazolidinone (C3H5NO2, OZO). Both LiTFSI and OZO are solid at room temperature, but their mixture is a liquid with a liquidus temperature about -58.4 °C at a molar ratio of 1:4.0 characterized by differential scanning calorimetry. Thermogravimetry analysis shows that the complex system possesses high thermal stability over a wide range of temperature. Infrared and Raman spectroscopic studies have been carried out to understand the interactions between the precursors, LiTFSI and OZO, of the complex system. It is shown that the OZO can coordinate with the Li+ cation and the TFSI- anion via their polar groups (the CdO and NH groups). Such strong interactions lead to the dissociation of LiTFSI and the breakage of the hydrogen bonds among the OZO molecules, resulting in the formation of the complex. To have a comprehensive understanding to the above interactions, quantum chemistry calculations with nonlocal density function theory have also been performed on the free ions or organic molecule by determining their Mulliken charges, equilibrium configuration, binding energy, and the coordination number of Li+ ion. The calculations indicate that the ionic conductivity of the LiTFSI-OZO complex with different molar ratios depends strongly on the ionic species in the complex system. Electrochemical performances of the complex electrolyte are evaluated with ac impedance spectroscopy, cyclic voltammetry (CV), and in a test electric double layer capacitor, respectively. The complex at a molar ratio of 1:4.5 exhibits the highest ionic conductivity due to the relatively large amount of “free” ions at room temperature. The analysis for the CV behavior indicates that the electrochemical stability window of the electrolyte is about 3V. This kind of complex system proves to be a promising candidate of electrolytes for supercapacitor and other electrochemical devices.

I. Introduction Ionic compounds are usually solid with a high-melting point, boiling point, and rigidity at room temperature due to their strong electrovalent bonds. However, these properties will be drastically changed when the big and asymmetric anions and/or cations are introduced because of the steric hindrance of these anions and/or cations.1,2 Room-temperature molten salts (RTMS) are also known as ionic liquids because they are liquid at ambient or even lower temperatures and are composed entirely of ions. RTMSs have attracted considerable interest in many fields because of their unusual physical and chemical properties, such as wide liquid-phase range, high thermal stability, negligible vapor pressure, nontoxicity and nonflammability, rather high ionic conductivity, high electrochemical stability, etc.3-11 In recent years, these properties have found applications in a number of electrochemical fields including electrochemical mechanical actuators, high-energy density batteries, photoelectrochemical solar cells, supercapacitors, and electrodeposits.12-22 Most of the reported RTMS systems typically contain organic cations represented by the alkyl-substituted nitrogen-containing * Corresponding author. (F.W.) E-mail address: [email protected]. Tel: + 86 10 68912508. Fax: +86 10 68451429. † Tsinghua University. ‡ Beijing Institute of Technology. § National Development Center of High Technology Green Materials. | Research Institute of Chemical Defense.

cations, such as pyridinium,23 triazolium,24 pyrrolidinium,25,26 tetraalkylammonium,27 and particularly imidazolium,28-30 combined with a variety of anions, mainly chosen from the perfluorinated series of molecules, such as BF4-, PF6-, CF3COO-, and CF3SO3-. However, the synthesis and purification of most RTMSs are difficult and costly due to their complicated structures. Therefore, it would be of practical importance to find a simpler and cheaper alternate complex system. Since the 1980s, some complex systems formed with amide and alkali metal nitrates or ammonium nitrates were studied as ion or proton-transfer media.31-36 To understand the interaction between anions and cations in the complex system, some papers used the Monte Carlo method or quantum chemistry calculation to study extensively the kinetics and the related properties of ionic liquids.37-41 In our previous work, we reported a class of complex system based on lithium bis(trifluoromethane sulfonyl) imide (LiN(SO2CF3)2, LiTFSI) and solid organic compounds with acylamino group, acetamide, ethyleneurea, for example.42-47 These complex systems are very easy to prepare and some samples possess superior physical and electrochemical properties. Further studies indicate that the structure (chain or loop type) and the substitute (methyl) in the organic molecules and the activities of their hydrogen bonds strongly influence the charge density and coordination strength of the carbonyl oxygen as well as

10.1021/jp066429f CCC: $37.00 © 2007 American Chemical Society Published on Web 03/13/2007

RT Electrolyte Based on LiTFSI and 2-Oxazolidinone

Figure 1. Molecular structures of LiTFSI (a) and 2-oxazolidinone (b) calculated at the BLYP/DNP level.

the hydrogen bonding interaction between the organic molecules. These factors determine the physicochemical performances of different complex systems.48 In addition, a compromise has to be made between the salt dissociation and ion migration in the complex system to prepare complex electrolytes with lowliquidus temperature and high-ionic conductivity. In this paper, 2-oxazolidinone (C3H5NO2, OZO) is chosen because of its dipolar nature and special structure. OZO organic molecules possess the stable five-membered rings carbamate structure. Because of the electronegative differences among various atoms, the symmetry of molecular structure debases and the area of the charge delocalization extends. As a result, it can form a liquid system with a number of organic and inorganic compounds due to its “waterlike” physical properties (e.g., high dielectric constant and dissociation constant).49,50 Lately, bis(trifluoromethane sulfonyl)imide (TFSI-) is also found particularly effective in lowering the melting point of the molten salts due to its low lattice energy. 25,51 Consequently, the binary complex system based on LiTFSI and OZO (see Figure 1) is prepared and its thermal and electrochemical performances are evaluated. To make a comprehensive study on the cationsolvent interaction, the ionic association and the ionic conduction behavior in the complex system, Fourier transform infrared (FTIR) and Raman spectroscopy, as well as density functional theory (DFT) calculations are applied. II. Experimental Section LiTFSI (3M Inc., 99%) and OZO (Acros Inc., 99%) were dried at 140 and 65 °C for 12 h in vacuum, respectively. All samples were prepared by simply mixing LiTFSI and OZO at various molar ratios in an argon-filled MBraun LabMaster 130 glovebox (H2O < 5ppm). The water content in the complex electrolyte was determined to be less than 42 ppm by the Karl Fischer titration method on DL37KF coulometer, Mettler Toledo. The melting points of the complex system were measured on a DSC 2010 differential scanning calorimeter (TA Inc.) by sealing ca. 10 mg of the sample in an aluminum pan. The pan and the sample were first cooled to about -100 °C with liquid nitrogen and then heated to 100 °C at a rate of 5 °C/min. The thermal stability of the sample was evaluated using a SDT 2960 thermogravimetric analysis (TA Inc.). The sample was heated from room temperature to 550 °C at a rate of 5 °C/min. Special attention was paid to avoid exposing the hygroscopic samples to moisture by continuous nitrogen flowing around the sample during measurement. The FTIR spectra of the samples were recorded on a Nicolet Magna 750 FTIR spectrometer between 4000 and 400 cm-1 with a resolution of 2 cm-1. The solid sample was mixed with dry KBr and pressed into a pellet while a droplet of the liquid sample was spread on a dry KBr pellet for the infrared (IR) spectroscopic measurements. The Raman spectra of the electrolyte sealed in a test tube were recorded on a Nicolet 950

J. Phys. Chem. C, Vol. 111, No. 13, 2007 5185 Raman spectrometer between 3700 and 100 cm-1 with a resolution of 2 cm-1. Each FTIR and Raman spectrum was scanned 50 and 400 times, respectively. All the calculations have been performed using the DMol3 module of the Cerius2 program. The geometry structures of the precursors and different ionic species in the complex system were fully optimized by BLYP method using DNP basis set. The Mulliken charges, the total energy, and frontier molecular orbital energy are calculated at the same level. The sizes of the DNP basis sets are comparable to the Gaussian 6-31G** basis sets, giving the p polarization functions on hydrogen apart from the d functions on the heavy atoms. In particular, the numerical basis set is much more accurate than a Gaussian basis set of the same size.52 Ionic conductivity measurements were carried out in an electrochemical cell with Pt electrode. The cell constant was determined with standard KCl solution (0.01M) at 25 °C. The alternating current impedance of the samples was measured on a CHI660a electrochemical workstation (1 Hz∼100 kHz, 0∼80 °C). The electrochemical window of the complex system was measured with cyclic voltammogram (CV) on the electrochemical workstation. The microelectrode cell was employed with a platinum wire (Φ ) 0.1 mm) as the working electrode and Li foil (99.9%) as the reference and counter electrodes at a scan rate of 0.2mV/s at 25 °C.53 The cells were assembled in the glovebox. Carbon electrodes were prepared by mixing 87 wt % activated carbon (surface area 1200 m2 g-1), 10 wt % acetylene black, and 3 wt % PTFE binder and pressing the mixture into a wafer. Then the electrodes were dried under vacuum at 120 °C for 12 h. A coinlike electric double layer capacitor (EDLC) was built with two carbon electrodes, separated with polypropylene film, all soaked with LiTFSI-OZO electrolyte and locked in a test cell. All the operations were done in the glovebox. The galvanostatic charge-discharge experiment was carried out on a Land Celltester. The CV measurements of capacitors were performed on a CHI1000 electrochemical workstation. III. Results and Discussion When mixing the LiTFSI with OZO by the molar ratio between 1:1.5 and 1:6.5, the contact part of the lithium salt and OZO became wet immediately and liquid drops can be observed on the wall of the container. Thoroughly stirring the mixture leads to the formation of a homogeneous and stable liquid at room temperature. In contrast, the samples with other recipes were obtained after slightly heating and then were cooled down to room temperature. However, most of these mixtures become a waxy solid or an unstable supercooled liquid at ambient temperature. Tiny crystallites are observed in LiTFSI-OZO complex at a molar ratio of 1:7.0 in a few days. Raman spectroscopic study demonstrates that these crystallites are actually pure OZO. 3.1. Thermal Analysis. Figure 2 shows the typical differential scanning calorimetry (DSC) trace of the phase transitions for LiTFSI-OZO complex with a molar ratio of 1:4.0 and 1:5.5. Note that there is a difference in the DSC diagrams of the two LiTFSI-OZO complexes. Only one endothermic peak related to the glass transition temperature is observed in the DSC trace of the LiTFSI-OZO complex at molar ratio 1:4.0 in the measurement temperature range in Figure 2a. However, the glass transition of the sample with a molar ratio 1:5.5 occurs at -62.6 °C. Then the complex crystallizes at -8.2 °C and remelts at 15.5 °C when further heated, corresponding to the devitrification temperature and liquidus temperature, respectively, (see Figure 2b). The sample decomposes until 243.1 °C.

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Chen et al.

Figure 4. TG curves for LiTFSI-OZO complex at molar ratio 1:4.0.

Figure 2. Typical DSC diagram of LiTFSI-OZO complex at molar ratios 1:4.0 (a) and 1:5.5 (b) for determining the glass temperature (Tg), devitrification temperature (Tc), liquidus temperature (Tl), and decomposition temperature (Td).

Figure 3. Various temperatures for LiTFSI-OZO complex at different molar ratios.

These phase transition temperatures (Tg, Tc, Tl, and Td) for LiTFSI-OZO complexes at various molar ratios have been summarized in Figure 3. The glass transition temperature of the complex decreases with increasing OZO concentration (see Table 1), and so do their Tc and Tl values. However, there are absences of the thermal reaction change corresponding to Tc and Tl for these samples except the ones at molar ratio 1:4.5

and 1:5.5. The decomposition temperature of the LiTFSI-OZO system ranges between 200 and 260 °C. Experimental results in Table 1 reveal that the liquidus temperature of the complex system is lower than the melting point of LiTFSI (234 °C) or OZO (88 °C) and the thermal properties of the complex system depend on the LiTFSI content. Figure 4 shows the thermogravimetry (TG) curve for LiTFSIOZO complex at molar ratio 1:4.0. It is clearly indicated that the thermal stability of this sample can be higher than 180 °C. The curve shows two-step weight loss processes: 180∼314 °C (relative weight loss is ca. 43.8%) and 314∼480 °C. With a view to the thermal properties of pure LiTFSI and OZO, it is estimated that the initial loss is corresponding to the dissociation of coordination between Li+ and OZO and the pyrolysis of OZO, and the final step is correlative with the thermal decomposition of LiTFSI. The phenomena observed during sample preparation and the DSC measurement results indicate that a liquid system can be easily formed by mixing LiTFSI with OZO in the range of appropriate molar ratio. It is a common feature for the acylamino group to take an important role in the interaction between LiTFSI and the OZO molecule and the formation of a complex system. Furthermore, the electronegative differences among oxygen and nitrogen atoms in the OZO molecule directly results in the debasement for the symmetry of molecular structure and the aggrandizement of the charge delocalization area. In the present case, it is reasonable to assume that OZO works as a complexing agent due to its polar groups (CdO and NH) being capable of coordinating with cations and anions, respectively, weakening the Coulombic interaction between ions in LiTFSI. The strong interaction between the O atom of the CdO group and the Li cations lead to the breakage of the hydrogen bonds between OZO molecules. In this way, a homogeneous roomtemperature complex electrolyte can be obtained from two solid components. This assumption is supported by the following IR and Raman spectroscopic studies. 3.2. Spectroscopic Characterization and Quantum Chemistry Calculations. In recent years, LiTFSI has been popularly used as the source of lithium in various electrolytes for lithium ion batteries and other electrochemical devices. The assignments of the vibration modes of LiTFSI are very abundant but often

TABLE 1: Thermal Properties for the LiTFSI-OZO Complex System

system

melting point of LiTFSI Tm/°C

1:1.5

LiTFSI-OZO

234

-36.2

a

Room temperature.

glass temperature at various molar ratios Tg/°C 1:2.5 1:3.5 1:4.0 1:4.5 -50.2

-54.8

-58.4

-60.9

1:5.5

melting point of OZO Tm/°C

range of liquid phasea

-62.6

88

1:1.5∼1:6.5


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Figure 7. Resonance forms of 2-oxazolidinone.

Figure 5. Crystal structure of OZO calculated at the BLYP/DNP level.

Figure 6. FT-IR spectra of the CdO stretching mode (a) and the NH stretching mode (b) of pure 2-oxazolidinone and LiTFSI-OZO RTMSs at various molar ratios.

conflict with each other in the literature due to its complicated structure and vibration spectrum.25,27,54-59 It seems that the assignments of the IR and Raman bands by Rey et al.60 are more reasonable than others’ attribution and will be adopted in this paper. The X-ray diffraction analysis for OZO and the IR spectra of the hydrogen bond in OZO crystals have been reported.49,50,61,62 The crystals are monoclinic with space group P21/C and Z ) 4. Correlative unit cell parameters are a ) 7.318 Å, b ) 5.673 Å, c ) 9.987 Å, and β ) 110.79°, respectively. OZO molecules are joined by the hydrogen bond (N-H···O) and spread along the “c” crystal axis.(Figure 5) The recognition to the vibration spectra of OZO in this work are based on the above-mentioned papers and the reports of molecules with similar structures. 3.2.1. Influence of Lithium Bis(trifluoromethane sulfonyl) Imide on the Structure of 2-Oxazolidinone. Figure 6a shows the IR spectra of the LiTFSI-OZO complex with different molar ratios between 1620 and 1900 cm-1. The strong band at 1751 cm-1 in the IR spectrum of solid OZO is attributed to its CdO stretching, which is mainly affected with the neighboring

nitrogen atom and oxygen atom. The nitrogen atom with free electrons pairs is very prone to be polarized and form p-π conjugation with the carbonyl group due to its (sp2 + p) configuration in the solid state. Moreover, the strong electrondrawing oxygen possesses induction effect. Consequently, the frequency of this band is higher than that of the chain-structured acetamide and urea due to the tension effect of the fivemembered rings of solid OZO.63 Verbist et al. 64 studied the crystalline structure of the LiI2OC(NH2)2 complex. They found that there are four O atoms around each Li+ ion in the system. The urea molecules bind with the alkaline atoms and form metal-urea complexes via the O-M bonding because the carbonyl oxygen holds the strongest electronegativity.65 These studies clearly indicate that the Li+ ions tend to coordinate with the carbonyl oxygen of the organic molecule. Similarly, the CdO stretching band changes significantly upon introducing lithium salt into the OZO in Figure 6a. When the resonance forms of OZO are considered (Figure 7), the position of the CdO stretching band shifts from 1751 to 1740 cm-1 and are broadened with increasing LiTFSI content in the complex owing to the influences of the Li+oxygen coordination. The results of the quantum chemistry calculation in the following text show that the CdO bond length of geometry optimization of OZO molecule coordination with the Li+ ion is higher than one of OZO molecule. The molecules of solid OZO are all associated with hydrogen bonding (N-H···O) due to the coordination between the O atom on the CdO group and the H atom on the NH group. Obvious spectral changes are also observed from the spectra when solid OZO is mixed with solid LiTFSI and a homogeneous liquid is formed. Figure 6b shows the IR spectra of the LiTFSI-OZO complex with different molar ratios in the region where the NH vibrations of OZO are expected. The spectrum of pure OZO consists of a less intense, very broad feature centered at roughly 3142 cm-1, and a relatively sharp, strong band at 3271 cm-1, which are ascribed to NH symmetric stretching and asymmetric stretching vibrations of OZO. Both bands blue-shift gradually with increasing LiTFSI content and reach 3300 and 3400 cm-1, respectively, at LiTFSI-OZO ) 1:6.5. This demonstrates the breaking of the hydrogen bonds among the OZO molecules and the coexistence of the associated and nonassociated (free) NH groups in the complex system. It is known that weak hydrogenbonding results in more free NH groups corresponding to stronger NH asymmetric stretching band.66 Accordingly, the NH asymmetric stretching band is broadened and its intensity increases with increasing LiTFSI concentration. These results suggest that the hydrogen bonding in OZO is weakened and even broken down due to the competitive Li+-oxygen interaction in concentrated complex. Figure 8a shows the Raman spectra of the LiTFSI-OZO complex with different molar ratios between 900 and 950 cm-1. The band at 924 cm-1 is assigned to the C-O stretching mode. This band blue-shifts to 931 cm-1 for LiTFSI-OZO ) 1:1.5 with increasing LiTFSI concentration. Considering the resonance forms of OZO (Figure 7), the position of the C-N stretching band at 1256 cm-1 in pure OZO shifts downward slightly in


Chen et al.

Figure 10. Optimized molecular structure and partial bond lengths of OZO (a) and OZO(Li+) (b) at the BLYP/DNP level.

Figure 8. Raman spectra of the C-O stretching mode (a) and FT-IR spectra between 1160 and 1290 cm-1 (b) of LiTFSI-OZO RTMSs at various molar ratios.

Figure 9. Raman spectra of the SO2 stretching mode (a) and the outof-plane SO2 wagging mode (b) of pure LiTFSI and LiTFSI-OZO RTMSs at various molar ratios.

frequency (Figure 8b) owing to the influences of the Li+oxygen coordination and the breaking of the hydrogen bonds. With increasing LiTFSI content in the complex system, the interaction between the Li ions and the O atom on the CdO group becomes more obvious and the C-N stretching band keeps blue-shifting. This will lead to the increase of the amount of OZO with resonance forms (C). As a result, the C-N bonding is more like a double bond in the liquid complex than in solid OZO. The spectral variation of the C-N band is in good agreement with that of the CdO band and the C-O band. 3.2.2. Influence of 2-Oxazolidinone on the Structure TFSIAnion. The Raman band at 1128 cm-1 is assigned to the SO2 stretching mode of solid LiTFSI in Figure 9a. It shifts to higher frequencies with the decrease of LiTFSI concentration. Similarly, the band at 387 cm-1 for the out-of-plane SO2 wagging

mode of solid LiTFSI red-shifts to 409 cm-1 upon the introduction of LiTFSI (Figure 9b). It has been reported that these bands are sensitive to the introduction of the salt and that the negative charges of LiTFSI are delocalized between the N and the O atoms.60 This means that the above spectral variations may be related to the strong interaction between the SO2 group of TFSI- anion and the NH group of OZO. It is understandable that part of the O atoms on the SO2 group possess negative charges when the Li ions coordinate strongly with the CdO group of OZO. Considering the resonance forms of OZO, these O atoms tend to interact with the partially positive-charged NH groups of OZO. On the other hand, the Raman band of the CF3 symmetric stretching at 1245 cm-1 shows no obvious changes with addition of OZO in Figure 8b, consistent with the previous report.67 This further proves that OZO mainly interacts with the TFSI- anion via the SO2 group in the LiTFSI-OZO complex. The spectral changes of the LiTFSI-OZO complex system indicate clearly that the Li+ ions coordinate with the CdO group of OZO while the NH group of OZO interacts with the SO2 group in TFSI- anion, consistent with the spectral variation of the room-temperature complex system based on LiTFSI and organic compound with acylamino group.48 The breaking of the hydrogen bonding among the OZO molecules and the dissociation of LiTFSI result in the formation of a complex system. Such interactions lead to the transition of the resonance form of OZO and are reflected in the IR and Raman spectra. To confirm the above spectral analysis, quantum chemistry calculations are performed in the following by optimizing the geometries of the organic molecule and coordinating ion and by calculating the energies. Figure 10 shows optimized molecular structure and partial bond lengths of OZO and OZO(Li+) at the BLYP/DNP level. The calculation results of the Mulliken charge distributions and bond lengths of the OZO molecule before and after coordination with the Li+ ion and the binding energies for the coordination are listed in Table 2. After coordination with the Li+ ion, the Mulliken charge of the carbonyl oxygen becomes more negative while that of the nitrogen atom and the adjacent oxygen atom are less negative. This explains the equilibrium of resonance form of OZO organic compounds. The variation of the bond length in the system exhibits that the coordination elongates the CdO bond but shortens the C-N and the C-O bond length. As a result, the C-N and C-O bonds are more characteristic of a double bond in the complex, consistent with the IR and Raman spectroscopic results.



TABLE 2: Calculated Mulliken Charges, Bond Lengths, and Binding Energy (BE) of OZO and Its Coordination Ion with Li+, and Total Energy, Frontier Molecular Orbital Energy, and Dipole Moment of OZO Mulliken charge bond

Oa

C1

O

N

C2

C3

OZO OZO(Li+)

-0.419 -0.562

0.600 0.757

-0.460 -0.441

-0.390 -0.353

-0.038 -0.039

0.111 0.083

bond length (nm) atom

r(C1dO)

r(C1-N)

r(C1-O)

r(C2-N)

r(C3-O)

r(C2-C3)

OZO OZO(Li+)

12.15 12.52

13.93 13.49

13.94 13.51

14.68 14.76

14.63 14.90

15.44 15.49

OZO a

interaction

r(Li-O) (nm)

BEb (kJ mol-1)

Li+ + OZO f OZO(Li+)

17.79

4.0804

ET (Ha)

EHOMO

frontier molecular orbital energy (Ha) ELUMO

∆Egc

dipole moment (Debye)

-322.5933143

0.2329

-0.0039

0.2290

5.3694

Oxygen atom in acylamino group. b BE ) -{E(OZO(Li+)) - E(OZO) - E(Li+)}. c ∆Eg ) ELUMO - EHOMO.

On the basis of the molecular orbital theory, the ability to gain and lose electrons is judged by the energy level of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The total energy, the frontier molecular orbital energy, the energy gaps between the HOMO and LUMO, and the dipole moment of the OZO molecule are also shown in Table 2. It can be seen in Figure 11 that the HOMO is mainly located around the carbonyl oxygen of the OZO molecule. This indicates that the Li-O coordination is easy to occur in the interaction between the lithium salt and OZO. On the basis of the hybrid orbital theory, the outer-shell electron of Li atom is 2s1 and the Li+ ion comes from Li atom losing one electron, creating four 2s2p vacant orbitals. Then sp3 hybridization occurs between one 2s orbital and three 2p

Figure 11. Frontier molecular orbital (HOMO (a), LUMO (b)) of OZO at the BLYP/DNP level.

orbitals, leading to the formation of four sp3 vacancy orbitals. These four sp3 vacancy orbitals combine with lone-pair electrons of four ligating atoms to form a coordination bond. To determine the coordination number of the Li+ ion, nonlocal DFT calculations were performed on the interaction between Li+ ion and OZO. The optimized structures of the supermolecules Li+(OZO)n (n ) 1-5) are shown in Figure 12. Calculations show that the Gibbs free energies of the supermolecules Li+(OZO)n (n ) 1-4) are negative while that for the supermolecules Li+(OZO)5 is positive. Therefore, it is suggested that the coordination number of Li+ ion with OZO is 4 at low salt concentration. This is consistent with previous reports.68-71 3.2.3. Ionic Species. There are various ionic species in the complex system, such as the “free” anions, contact ion pairs, and aggregates, due to the strong interaction between LiTFSI and OZO in the complex system and the differential concentration in the various molar ratios. The configuration of ions (free ion or ion pairs) and the interaction between them have an important influence on the electrochemical performance of the LiTFSI-OZO complex system, especially their ionic conductivities. The possible structures of different ionic species that were fully optimized at the BLYP/DNP level using the Dmol3 program package are shown in Figure 13, corresponding to aggregates (Figure 13a), contact ion pairs (Figure 13b,c) and the free anions (Figure 13d,e). Moreover, the behavior of the specific Raman spectra is more sensitive to the different kinds of ionic species. The spectral evolution of the 740 cm-1 band of LiTFSI has been studied extensively.72-76 Bakker et al.77 reported that its position is determined by the coordinate and type of cations in the system. Edman78 systematically studied this band in the poly(ethylene oxide)n-LiTFSI system. They found that this

Figure 12. Optimized structures for the Li+(OZO)n (n ) 1∼5) at the BLYP/DNP level.


Chen et al.

Figure 13. Possible optimized structures of different ionic species in LiTFSI-OZO RTMS calculated at the BLYP/DNP.

Figure 14. Raman spectra of LiTFSI-OZO complex between 765 and 730 cm-1 at various molar ratios.

band blue-shifts and becomes broadened upon the increase of the lithium salt concentration and attributed these results to the formation of ion pairs in the system. In the present LiTFSIOZO system, the position of this band shifts to the highfrequency side with increasing content of LiTFSI in the system from 741 cm-1 at LiTFSI-OZO ) 1:6.5 to 747 cm-1 at LiTFSI-OZO ) 1:1.5 (Figure 14). The shifting is steady and well reproducible despite that it is not remarkable. Consequently, the slight position variation of the 745 cm-1 band is still the evidence for the formation of ion pairs in the complex system. It is understandable that the number of ion pairs increases with increasing salt content in the complex system. On the basis of the above discussion, it is clear that those hydrogen bonds among the OZO molecules and dissociation of the lithium salt play an important role in the phase transformation process although these two factors are contradictory to each other. At high salt concentration, although the hydrogen bonds are abruptly broken due to the Li ions coordination with the O atom of CdO group, the lithium salt is not fully dissociated and existed as aggregates or higher aggregates (Figure 13a) in the complex. At low salt concentration, the lithium salt is fully dissociated whereas hydrogen bonds

among uncoordinated OZO molecules are extensively present in the complex. These samples are just the opaque mixture of OZO and the eutectic complex. Accordingly, the LiTFSI-OZO complex system can become homogeneous liquid in the specific molar ratio range. Furthermore, the appropriate proportions of the LiTFSI-OZO complex must be chosen to gain the high ionic conductivity accord with a great amount of the free ions in the system. 3.3. Electrochemical Evaluation. The ionic conductivity of the LiTFSI-OZO electrolyte as a function of temperature is shown in Figure 15. The nonlinear (curved) profile indicates that their conductivity-temperature relationships do not follow the Arrhenius equation in Figure 15a. It is well known that the empirical Vogel-Tammann-Fulcher (VTF) equation is valid for polymer and glassy electrolytes and concentrated electrolyte solutions.79,80 Therefore, the VTF equation can be applied to describe the temperature-dependent conductivity of the complex system (Figure 15b)

σ(T) ) AT-1/2 exp[-Ea/R(T - T0)] where A is the pre-exponential factor proportional to T-1/2 and T0 is the temperature at which the transport function ceases to exist or the solvent structural relaxation becomes zero and may be regarded as the glass transition temperature. Adam et al.81 found that T0 in the VTF equation is about 50 K lower than the glass transition temperature. Therefore, the corresponding Tg value is used as the T0 on drawing Figure 15b in the following. Ea is the activation energy for ionic conduction. Figure 15b indicates clearly that the conductivity-temperature relationship of the complex system is linear. It is well known that when the ion transport is dominated by the mobility of the solvent molecule, the conductivity can be shown to depend on the free volume of the solvent and may be correlated by the VTF equation. Consequently, the excellent agreement of the conductivity-temperature behavior with the VTF equation is consistent with a solvent-assisted ionic conduction mechanism. We attribute this to the strong interaction between LiTFSI and OZO.



Figure 16. The relationship between conductivity and concentration of LiTFSI in LiTFSI-OZO complex at various temperatures.

Figure 15. (a). The Arrhenius plots of conductivity for LiTFSI-OZO complex at various molar ratios. (b) The Vogel-Tammann-Fulcher (VTF) plots of conductivity for LiTFSI-OZO complex at molar ratio 1:4.0. T0 in the VTF equation is ca. 50 K lower than the glass transition temperature.

TABLE 3: Ionic Conductivities for LiTFSI-OZO Complex with Various Molar Ratios at Different Temperatures (10-3 S cm-1) molar ratio (LiTFSI-OZO)

molar ratio (LiTFSI)

1:2.5 1:3.5 1:4.0 1:4.5 1:5.5 1:6.5

0.29 0.22 0.20 0.18 0.15 0.13

conductivity (σ/10-3 S cm-1) 0 °C 25 °C 60 °C 80 °C 0.03 0.08 0.11 0.14 0.07 0.01

0.25 0.52 0.65 0.75 0.67 0.41

1.63 2.80 3.30 3.50 4.50 4.64

3.37 5.15 5.74 6.19 7.92 8.10

Of all the LiTFSI-OZO samples, the electrolyte at molar ratio of 1:4.5 shows the better ionic conductivity (i.e., 0.75 × 10-3 S/cm at 25 °C and 3.5 × 10-3 S/cm at 60 °C) as summarized in Table 3. As can be seen, ionic conductivity increases with increasing temperature over the entire salt concentration studied. It should be emphasized that the growth rate of the ionic conductivity of the samples at low salt content are remarkably higher than that of other samples at high salt content. The electrolyte at molar ratio of 1:6.5 owns the highest ionic conductivity above 50 °C, 4.64 × 10-3 S/cm at 60 °C, and 8.10 × 10-3 S/cm at 80 °C, respectively. Figure 16 shows that when the molar ratio of [OZO]/[LiTFSI], n, decreases from 6.5 to 2.5, the conductivity of the LiTFSI-OZO complex changes at various temperatures. It is clearly shows that there are critical points of the ionic conductivity in the different temperature. This behavior is mainly attributed to the formation of effective dissociation of LiTFSI at low salt contents and the dramatic ion association at high salt contents. The formation

Figure 17. CVs of LiTFSI-OZO at the molar ratio of 1:4.5 with a microelectrode electrochemical cell, platinum wire (Φ ) 0.1 mm) as the working electrode and Li foil (99.9%) as the reference and counter electrodes (scan rate ) 2 mV/s; T ) 25 °C).

of large amounts of contact ion pairs do not contribute to the ionic conductivity in the complex system, supported with the Raman spectroscopic investigation. Another important reason for the quick decrease of conductivity is the increasing viscosity of the complex system with increasing salt content. The electrochemical stability window of the LiTFSI-OZO complex is determined by CV at 25 °C. To improve the measurement accuracy, microelectrode cell is assembled with platinum wire (Φ ) 0.1 mm) as the working electrode and Li foil (99.9%) as the reference and counter electrodes. Figure 17 shows that an irreversible oxidation peak starts at 4.8 V and a reduction peak is observed at 1.8 V for the LiTFSI-OZO complex with a molar ratio of 1:4.5. Therefore, the electrochemical stability window of the molten salt is about 3.0 V. 3.4. Application Characteristic of an EDLC. The RTMS electrolytes composed of imidazolium-type or N-diethyl-Nmethyl-N-(2-methoxyethyl)ammonium cations and various anions were mostly chose in the studies of RTMS electrolytes for EDLCs at present.82-86 The results indicated some capacitors had a higher capacity and a better charge/discharge cycling durability than those with nonaqueous electrolyte at high temperature above 40 °C. However, the low cathodic stability and the high viscosity hinder their practical applications. Previously, we chose the LiTFSI-acetamide complex as the electrolyte applied in the EDLC with carbon nanotubes and activated carbon as the electrode respectively, due to its lower glass transition temperature, higher room-temperature ionic


Chen et al.

Figure 19. Charge-discharge profile of a capacitor with LiTFSIOZO at a molar ratio of 1:4.5 as the electrolyte at room temperature.

Figure 18. (a) CV of an EDLC with LiTFSI-OZO at a molar ratio of 1:4.5 as electrolyte (scan rate ) 1 mV/s; T ) 25 °C) cycled in different potential ranges; (b) CV of EDLC in a complex electrolyte at different scan rates (T ) 25 °C).

conductivity, and broad electrochemical stability as shown in the following.87,88 As a result, the EDLCs with this electrolyte can be charged up to 2.5 V with a good rate performance and high capacitance retention. To evaluate the performance of the LiTFSI-OZO complex system in practical electrochemical devices, an EDLC based on the activated carbon is assembled with the complex sample at the molar ratio of 1:4.5 as the electrolyte. Traditional EDLCs use an aqueous solution as an electrolyte and cannot be charged to a high voltage because the solvent will decompose at high potentials. With the LiTFSI-OZO complex as the electrolyte, the capacitor can be charged to a higher voltage and therefore, a higher capacity can be obtained because the complex system has a very wide electrochemical stability window. Figure 18a shows the CV of the capacitor with the complex electrolyte between 0 and 3.0 V at 1 mV/s scan rate. The welldefined rectangular voltammogram indicates a pure electrostatic attraction (i.e., a typical capacitive behavior). The curve in the potential range between 0 and 2.0 V is also boxlike, a characteristics of EDLC. Rate capability is an important feature of the EDLCs. The rectangular CVs of the EDLC over a wide range of scan rates (Figure 18b) reflect the ability of the complexes-based EDLCs to cycle at high current densities. Until the scan rate increases to 50 mV/s, the voltammetric curve still remains approximately rectangular. Galvanostatic charge/discharge test between 2.5 and 0.0 V was carried out to evaluate the electrochemical performances of the complex electrolyte. This cutoff charge voltage is rather high for capacitors but much lower than its electrochemical window. The linear voltage-time dependence demonstrates the typical capacitive behavior of the cell (Figure 19). The specific capacitance reaches 32.9 F/g based on the active materials. The

Figure 20. Cycle test of an EDLC using LiTFSI-OZO complex system as an electrolyte.

Figure 21. Charge-discharge curves of EDLC with LiTFSI-OZO at a molar ratio of 1:4.5 as electrolyte at different environmental temperatures.

capacity retention of this cell is not as good as that of the EDLCs with organic solvent electrolytes (Figure 20), but much better than those with aqueous or viscous ionic liquid electrolyte. The above capacitance fading is attributed to the reactions between the active surface functional groups on the porous carbon electrodes and the active hydrogen on the N-H groups in the complex system at high voltages. The most attractive characteristics of the LiTFSI-OZO complex electrolyte are its outstanding thermal performances such as high thermal stability, nonflammability, and low vapor pressure. The galvanostatic charge/discharge voltage profiles at 20, 40, 60, and 80 °C at the same current density (Figure 21) all retain an isoceles triangle shape, indicating typical capacitive

RT Electrolyte Based on LiTFSI and 2-Oxazolidinone behavior. The specific capacitance increases from 18.2 F/g at 20 °C to 38.7 F/g at 40 °C, 69.9 F/g at 60 °C, and even 92.9 F/g at 80 °C. It is well known that the resistance of the electrolyte, which is sensitive to temperature, strongly affects the capacitor performance. Therefore, the increasing capacitance with the increasing temperature is mainly attributed to the decreasing resistance of the LiTFSI-OZO complex electrolyte. Considering that the complex system as electrolyte possess a number of excellent properties such as negligible vapor pressure and high thermal stability, the above preliminary results demonstrate that the LiTFSI-OZO complex system is a promising electrolyte candidate for the EDLCs and probably other electrochemical devices with high safety and stability. One has to be aware, however, that electrochemical application results present here are preliminary. Evaluations on the compatibility of the LiTFSI-OZO complex electrolyte with the electrode materials and the separator are underway. In addition, the match has to be optimized between the pore size of the carbon material and ion size in the complex system to meet the practical demands to the EDLC. IV. Conclusions The novel room-temperature complex system based on LiTFSI and OZO has been prepared and characterized. The LiTFSI-OZO complex system appears as a liquid at room temperature between the molar ratios of 1:1.5 and 1:6.5. OZO can coordinate with both the lithium cations and TFSI- anions because of its two polar groups in the LiTFSI-OZO complex system. Vibration spectroscopic and quantum chemistry calculations studies show that Li+ ions coordinate with the CdO group of OZO whereas the SO2 group of TFSI- anions interacts with the NH group of OZO via hydrogen bonding. Such interactions lead to the breakage of the hydrogen bonds between OZO molecules and the weakening of the Coulombic interaction between the anions and the cations of LiTFSI, resulting in the formation of this homogeneous, stable, and highly ionic conductive room-temperature complex system. The ionic conduction behavior of the complex system agrees well with the VTF equation. The ion transport in the complex system is dominated by the mobility of the solvent molecule and the conductivity is correlated with the free volume model. Of the complex system, the LiTFSI-OZO complex at a molar ratio 1:4.5 has the highest ionic conductivity, 0.75 × 10-3 S/cm at room temperature. In addition, CV analysis shows that the electrochemical windows of these samples are ca. 3V. Finally, EDLC with the LiTFSI-OZO electrolyte shows a stable capacitor performance and rather good thermal performance, indicating that the novel binary room-temperature complex system based on LiTFSI and OZO is a promising candidate of electrolyte for the electrochemical devices. Further works concerning the match between carbon electrode and the LiTFSI-OZO electrolyte and development of ion conductive polymers based on this complex system are in progress. Acknowledgment. This work was supported by the National 973 Program (Contract No. 2002CB211800) and the National Key Program for Basic Research of China (Contract No. 2001CCA05000). The authors thank Professor C. M. Hong (College of Chemistry and Molecular Engineering, Peking University) for the critical reading of this manuscript. The authors are grateful to 3M Company for providing the LiTFSI sample. References and Notes (1) Wikes, J. S.; Zaworotko, M. J. Chem. Commun. 1992, 9, 965. (2) Welton, T. Chem. ReV. 1999, 99, 2071.

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Novel Binary Room-Temperature Complex System Based on LiTFSI

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