the structure–activity relationship studies of binary room temperature complex electrolytes based...

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Vibrational Spectroscopy 44 (2007) 297–307

The structure–activity relationship studies of binary room temperature

complex electrolytes based on LiTFSI and organic compounds

with acylamino group

Renjie Chen a,c, Feng Wu b,c,*, Li Li b,c, Xinping Qiu a, Liquan Chen a, Shi Chen b,c

a Department of Chemistry, Tsinghua University, Beijing 100084, Chinab School of Chemical Engineering and the Environment, Beijing Institute of Technology, Beijing 100081, China

c National Development Center of High Technology Green Materials, Beijing 100081, China

Received 30 May 2006; received in revised form 29 December 2006; accepted 30 January 2007

Available online 9 February 2007

Abstract

Binary room temperature complex electrolytes based on lithium bis(trifluoromethane sulfone) imide (LiN(SO2CF3)2, LiTFSI) and organic

molecules with acylamino (amide) groups, such as ethyleneurea, acetamide, etc., have been synthesized and evaluated with differential scanning

calorimetry (DSC) and ac impedance spectroscopy. Most of the complex systems with proper molar ratio have excellent thermal stability and

electrochemical performance. Infrared (IR) and Raman spectroscopic studies have been carried out to understand the formation these electrolytes.

It is shown that the organic compounds with amide group can coordinate with the Li+ cation and the TFSI� anion via their polar groups (the C O

and NH groups). Such strong interactions lead to the dissociation of LiTFSI and the breaking of the hydrogen bonds among the organic molecules,

resulting in the formation of the complex systems. In order to have a comprehensive understanding of the above interactions and the structure–

activity relationship of these complex systems, the Mulliken charges on the O and N atoms, the equilibrium configuration and the bonding energy

of the systems have been determined by quantum chemistry calculations with non-local density function theory (DFT). The calculations indicate

that the structure and the substitution group of organic molecules influence the charge density and coordination strength of the carbonyl oxygen of

these molecules. In addition, the strength of hydrogen bonding between the organic molecules influences the physico-chemical properties of the

complex electrolyte.

# 2007 Elsevier B.V. All rights reserved.

Keywords: Room temperature complex electrolyte; LiTFSI; Acylamino group; Structure–activity relationship

1. Introduction

Ionic compounds are usually solid with 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 due to the steric hindrance of these

anions and/or cations [1,2]. Room temperature molten salts

(RTMS) have long been the subject of fundamental research in

many fields [3–14], owing to their unusual properties, such as a

* Corresponding author at: School of Chemical Engineering and the Envir-

onment, Beijing Institute of Technology, Beijing 100081, China.

Tel.: +86 10 68912657; fax: +86 10 68451429.

E-mail address: [email protected] (F. Wu).

0924-2031/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.vibspec.2007.01.003

wide liquid-phase range, high thermal stability, low vapor

pressure, high ionic conductivity, and so forth. Previously we

reported several novel complex systems based on solid LiTFSI

and solid organic compounds with acylamino group, urea, for

example [15–19]. It is noticeable that these complex systems

possess similar characteristics as RTMS, especial low vapor

pressure and nonhygroscopic. Furthermore, these systems are

easy to prepare, and show high thermal stability and good

electrochemical properties. Electric double layer capacitors

based on the active carbon or the carbon nanotube with the

LiTFSI/acetamide complex system as electrolyte showed a

high specific capacity and rather good cycling performance,

indicating that the complex system is a promising candidate of

electrolyte for supercapacitor and other electrochemical

devices [19,20]. Preliminary analysis indicates that the

structure (chain or loop type) and the substitute in the organic

mailto:[email protected]

http://dx.doi.org/10.1016/j.vibspec.2007.01.003

Scheme 1. Structure of organic compounds with acylamino group.

R. Chen et al. / Vibrational Spectroscopy 44 (2007) 297–307298

molecules and the activities of their hydrogen bonds strongly

influence the physico-chemical performances of these complex

electrolytes.

Clearly it is necessary to make a comprehensive study on

the interaction between lithium salt and the organic

compound, the relations between the microstructure in the

complex systems and their thermal and electrochemical

performances, especially how and why the structure of the

organic compound will influence the interactions and the

formation of the microstructures in the complex systems. It is

a pity, however, that most authors stopped at characterizing

the properties of the complex systems. Only very few of them

tried to understand the intrinsic reasons for the experimental

observations, and even fewer scientists take efforts to find out

these important questions in the scale of microstructures and

in the aspects of theoretical calculations. Maginn and co-

workers [21–23] used Monte Carlo method to study

extensively the kinetics and the related properties of

imidazolium-based ionic liquids. Liang et al. [15] thought

that the strong interactions, mainly the Li–O coordination,

between the two components, lead to the formation of the

LiTFSI/urea composite. Hu et al. [24,25] studied the

vibration spectra of the LiCF3SO3/acetamide complex and

observed obvious band shifting for the C O stretching of

acetamide.

In this paper, binary complex electrolytes based on LiTFSI

and organic compounds with amide groups are synthesized.

The thermal properties and ionic conductivities of these

complex electrolytes are characterized by DSC and ac

impedance, respectively. Most of these organic compounds,

such as acetamide and urea, are common candidates used to

lower the melting point of a composite and form a eutectic

system with different organic or inorganic compounds owing

to their ‘water-like’ physical properties (e.g., high dielectric

constant and dissociation constant) [26,27]. In order to clarify

the influence of the molecular structure on the thermal and

electrochemical properties of the binary complex electro-

lytes, ethyleneurea is chosen because it possesses a five-

membered ring. Moreover, it is interesting to find out how the

properties of the complex systems will be changed when the

NH2 groups in an organic compound are replaced with other

functional groups. Methylurea (NMU) and 1,3-dimethylurea

(DMU) are introduced to examine the effects of the

substitution of methyl for the hydrogen atom on the NH

group in urea. As the purpose of this work is to

comprehensively understand the interactions and microstruc-

tures of the complex electrolyte, vibration spectroscopy as a

powerful tool is applied. Furthermore, quantum chemistry

calculations should be a simply but appropriate method to

characterize the interactions and the structure–activity

relationship of the LiTFSI-based composites. The Cerius2

program is matured commercial software, and its DMol3

module has been popularly used for quantum chemistry

calculations. This module will be applied upon the geometry

optimization of each organic molecule before and after

coordinated with Li+ and the calculations about the total

energy, frontier energy orbital, etc.

2. Experimental

2.1. Sample preparation

LiTFSI (3 M Inc., 99%) was dried at 140 8C for 12 h in

vacuum. Acetamide (Acros Inc., AP) and ethyleneurea (Tokyo

Kasei Kogyo Co. Inc., W > 97%) were recrystallized with

chloroform and urea (Beijing Chemical Reagents Inc., 99%),

NMU (Acros Inc., 97%) and DMU (Acros Inc., 98%) were

recrystallized with anhydrous ethanol. These materials were

then dried at 55 8C for 10 h in vacuum. All the complex systems

were prepared by simply mixing LiTFSI and the organic

compound with an acylamino group at various molar ratios in

an argon-filled MBraun LabMaster 130 glovebox

(H2O < 5 ppm). Homogeneous and stable liquids for partial

composites with the proper molar ratio were obtained directly

after the mixtures were mechanically stirred at room

temperature. Moreover, the complex systems with other recipes

were prepared by slightly heating the mixture and then cooled

down to room temperature (see Scheme 1). The water content in

the complex electrolyte was determined to be less than 39 ppm

by Karl–Fischer titration method on DL37KF Coulometer,

Mettler Toledo.

2.2. Thermal and electrochemical measurement

The melting points of the complex systems were determined

on a DSC 2010 differential scanning calorimeter (TA Inc.) by

R. Chen et al. / Vibrational Spectroscopy 44 (2007) 297–307 299

sealing ca. 10 mg of the composite in an aluminum pan. The

pan and the electrolyte were first cooled to about�100 8C with

liquid nitrogen and then heated to 100 8C at a rate of 5 8C/min.

Special attention was paid to avoid exposing the hygroscopic

samples to moisture by continuous nitrogen flowing around the

sample during measurement. Ionic conductivity measurements

were carried out with an electrochemical cell with Pt electrode.

The cell constant was determined with standard KCl solution

(0.01 M) at 25 8C. The ac impedance of the samples was

measured on a CHI660a electrochemical workstation (1–

100 kHz, 0–80 8C).

2.3. IR and Raman spectra

The IR spectra of the samples were recorded on a Nicolet

Magna 750 FTIR spectrometer between 4000 and 400 cm�1

with the resolution set at 4 cm�1. The solid sample was mixed

with dry KBr and pressed into pellet while a droplet of the

liquid sample was spread on a dry KBr pellet for the IR

spectroscopic measurements. The Raman spectra of the

electrolyte sealed in a test tube were recorded on a Nicolet

950 FT-Raman spectrometer between 3700 and 100 cm�1

with the resolution set at 4 cm�1. The Raman and IR spectra

shown here were the average results of 400 and 50 scans,

respectively.

2.4. Computational methods

The configurations of the organic molecules coordinated

with the Li+ ions were optimized with the BLYP function of

non-local DFT with DNP (double numerical with polariza-

tion) basis set using the DMol3 module of the Cerius2

program. The Mulliken charges before and after the

coordination between the organic molecule and the Li+

ion, the binding energies for the interactions between the

organic molecule and the Li+ ion, and the total energy and

Frontier molecular orbital energy of each organic molecule

are calculated with this program. The sizes of the DNP basis

sets are comparable to the Gaussian 6-31G** basis sets, giving

the p polarization functions on hydrogen apart form 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 [28].

Table 1

Thermal properties and ionic conductivities of some complex systems

Composition Glass temperature at various molar ratios, Tg (8C)

1:2.0 1:3.0 1:3.3 1:4.0 1:4.5

LiTFSI/acetamide – �57.1 �60.4 �60.9 �62.2

LiTFSI/ethyleneurea �29.7 – 5.4 9.5 �35.1

LiTFSI/urea22 – �31.9 �31.0 – –

LiTFSI/NMU �34.8 �38.0 – �40.0 –

LiTFSI/DMU �28.0 – – – –

a At room temperature.b Molar ratios are the data in parenthesis.c Supercooled liquid.

3. Results and discussion

3.1. Physical chemical properties analysis

It is known that the melting points of LiTFSI and organic

compounds as studied are above 273 8C and 81 8C, respectively

(see Table 1). It is interesting that two solids can form directly a

homogeneous liquid at room temperature when they are mixed

with the proper molar ratio. For the complex systems with

LiTFSI/acetamide molar ratios between 1:2.5 and 1:7.5, the salt

and the organic molecule become wet immediately after

contact with each other and liquid drops can be observed on the

wall of the container. DSC analyses indicate the eutectic

temperatures of these complex electrolytes are as low as

�60 8C. The composites of LiTFSI and ethyleneurea with a

five-membered ring with molar ratios between 1:2.5 and 1:4.0

need heating until they become a homogeneous, and remain

liquid after cooled down to room temperature. They are

supercooled and have to be stored in a sealed container. We

previously reported that mechanically mixing LiTFSI and urea

with molar ratios between 1:3.0 and 1:3.8 led to the formation

of homogeneous liquids at room temperature [15]. In

comparison, the eutectic temperature of the LiTFSI/NMU

complex system is lower and their range of liquid phase is wider

than that of the LiTFSI/urea system. The formation of the

LiTFSI/DMU complex system is very slow at room tempera-

ture. However, slight heating leads to the quick formation of a

homogeneous liquid. The lowest eutectic temperature (�28 8C)

is reached at LiTFSI:DMU = 1:2.0.

Experimental results for the glass transition temperature,

melting point and range of liquid phase are summarized in

Table 1. In comparison, the LiTFSI/ethyleneurea composite has

higher eutectic temperature than the other complex systems.

The phenomena observed during sample preparation and the

DSC measurement results indicate that a eutectic system can be

formed by mixing different organic compounds with LiTFSI.

The acylamino group takes an important role on the interaction

between the organic compound and LiTFSI and the formation

of a complex electrolyte. In the present case, it is reasonable to

assume that acylamino groups work as a complexing agent for

both the cations and the anions due to their polarity (C O and

NH2) capable of coordinating with cations and anions,

respectively. The weakening and even breaking down the

Melting point of organic

compounds, Tm (8C)

Range of liquid

phasea

Ionic conductivity

(�10�3 S cm�1, 25 8C)

81 1:2.0–1:6.0 1.21 (1:6.0)b

132 1:2.5–1:4.0c 0.26 (1:4.0)

132.7 1:3.0–1:3.8 0.17 (1:3.3)

102 1:2.0–1:4.0 0.18 (1:4.0)

105 1:2.0–1:3.0 0.01 (1:2.0)

Fig. 2. The glass transition temperature (A) and the ionic conductivities (B)

variations as a function of salt concentration in the various complex systems.

Inset: the correlation of the glass transition temperatures Tg with the log(s)

values for the binary complex samples at a molar ratio 1:4.0.


bonding between the Li+ cation and TFSI� anion results in the

formation of the complex electrolyte.

In order to further find out the influence of the molecular

structure on the electrochemical performance of the molten

salts, their conductivities are examined. Of all the complex

electrolytes, the LiTFSI/acetamide system shows the highest

ionic conductivity at the same temperatures and molar ratios.

The data can be one order higher than that of other recipes as

summarized in Table 1. Fig. 1 shows the Arrhenius plots of

different binary complex systems. The temperature-dependent

conductivity plots of these complex systems are protruding

upward, indicating that their conductivity–temperature rela-

tionships do not follow the Arrhenius equation. It is well known

that the Vogel–Tammann–Fulcher (VTF) equation is valid for

polymer and glassy electrolytes and concentrated electrolyte

solutions [29–31]. The inset of Fig. 1 indicates clearly that the

conductivity–temperature relationship of the composite is

linear. As the VTF equation is closely correlated to the free-

volume model, the excellent agreement of the conductivity

versus temperature behavior with the VTF equation implies a

solvent-assisted ionic conduction mechanism for these complex

systems.

Fig. 2 clearly show that the glass transition temperature and

the ionic conductivities variations as a function of lithium salt

concentration in the various complex systems. It is noticeable to

find that there is the relativity between the thermal and the

electrochemical properties of the complex systems. The ionic

conductivities of the complex sample with the lower glass

transition temperature are higher than that of those with the

higher glass transition temperature. Accordingly, the correla-

tion of the glass transition temperatures Tg with the log(s)

values for the binary complex samples at a molar ratio 1:4.0 is

list in inset of Fig. 2. This behavior is attributed to the

differences of the structure–activity relationship of these

complex systems based on LiTFSI and the various organic

compounds as will be shown in the subsequent spectroscopic

study and quantum chemistry calculations.

Fig. 1. The Arrhenius plots of different LiTFSI-based binary complex systems

at various molar ratios. Inset: the Vogel–Tammann–Fulcher (VTF) plots of

some typical binary complex systems based on LiTFSI. T0 in the VTF equation

is ca. 50 8C lower than the glass transition temperature.

3.2. Spectroscopic characterization and quantum

chemistry calculations

In recent years, LiTFSI have 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 conflict

with each other in the literature due to its complicated structure

and vibration spectrum [32–39]. It seems that the assignments

of the IR and Raman bands by Rey et al. [40] are more

reasonable than others’ attribution and will be adopted in this

paper. The recognition to the vibration spectra of several

organic compounds in this work, such as ethyleneurea with

five-membered ring, is based on the reports of molecules with

similar structures [41–43]. The vibration spectra of acetamide

[44–46] and urea [47,48] have been extensively studied in

literature. The assignments of Ganceshsrinivas et al. [49] for

acetamide and of Keuleers et al. [50] for urea will be borrowed

hereafter.

The molecules of solid ethyleneurea are all associated with

hydrogen bonding due to the coordination between the O atom

on the C O group and the H atom on the NH group (N–H� � �O).

Obvious spectral changes are also observed throughout the

spectra when solid ethyleneurea is mixed with solid LiTFSI and

a homogeneous liquid is obtained. The spectrum varies for two

reasons when the physical state of a substance changes from

solid to liquid: damage to its crystal field and the breaking of the

hydrogen bonds. The former will lead to the breakdown of the

structural symmetry of a crystal and changes the distribution of

its vibration modes. That means some new modes may appear

and some old modes may disappear. Although the hydrogen

bonding may not change the symmetry of a molecule, its

breaking will definitely result in variation of interactions

between molecules and therefore, their spectral changes. Fig. 3

shows the IR spectra of the symmetric (3282 cm�1) and

asymmetric (3406 cm�1) stretching of NH of ethyleneurea.

Both bands blue-shift gradually with increasing LiTFSI content

and reach 3423 and 3583 cm�1, respectively at LiTFSI/

ethyleneurea = 1:3.0. This demonstrates the breaking of the

hydrogen bonds among the ethyleneurea molecules and the

Fig. 3. FT-IR spectra of the NH stretching of ethyleneurea and LiTFSI/

ethyleneurea composite at various molar ratios.Fig. 4. Comparison of the FT-IR spectra of the n(C O) mode of ethyleneurea

at various molar ratios of LiTFSI/ethyleneurea.


coexistence of the associated and non-associated (free) NH

groups in the complex system. It is known that the NH

symmetric stretching band is much stronger than the

asymmetric one in the presence of strong hydrogen bonding

whereas weak hydrogen bonding results in more free NH

groups corresponding to stronger NH asymmetric stretching

band [51]. Accordingly, the NH asymmetric stretching band is

broadened and its intensity increases with increasing LiTFSI

concentration. These results suggest that the hydrogen bonding

in ethyleneurea is weakened and even broken down due to the

competitive Li+–oxygen interaction (see the following) in

concentrated complex.

As a lactam, ethyleneurea does not have NH bending mode.

The bands at 1643 and 1626 cm�1 in the Raman spectrum are

attributed to the NH bending (Table 2) of chain-structured

acetamide and urea, respectively. The NH bending band, be

different from the NH stretching band, red-shifts due to the

breaking of the hydrogen bonding among the acetamide (or

urea) molecules. All the bands shift to lower frequencies with

Table 2

Frequencies and vibration assignments of the Raman spectra of some complex sam

Assignmenta LiTFSI LiTFSI/acetamide Acet

1:3.0 1:3.3 1:4.5

nasNH – 3366 3365 3360 3320

nsNH – 3230 3226 3211 3163

nCO – 1666 1668 1676 1682

dNH – 1607 1611 1618 1643

nCN – 1406 1406 1405 1407

nSO2 1128 1138 1140 1141 –

vSO2c 388 407 408 411 –

a The symbols in the above table have their usual meanings: n, stretching, d, beb Classification of intensity: m, middle and s, strong.c one of the Raman spectra of out-of-plane wagging of the SO2 group.

the addition of lithium salt into the organic compounds. The

red-shifting of the NH bending band is also originated from the

destruction of the hydrogen bonding induced by the addition of

LiTFSI.

The strong band at 1689 cm�1 in the IR spectrum of solid

ethyleneurea is attributed to its C O stretching. The frequency

of this band is higher than that of the chain-structured

acetamide and urea due to the tension effect of the five-

membered rings of solid ethyleneurea [52] as shown in Table 2.

Moreover, the position of the C O stretching mode is mainly

affected with the neighboring nitrogen atom and the NH

radical. The nitrogen atom with free electrons pairs is very

prone to be polarized and form p-p conjugation with the

carbonyl group due to its (sp2 + p) conformation in the solid

state. Verbist et al. [53] studied the crystalline structure of the

LiI�2OC(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

ples and pure LiTFSI, acetamide and urea

amide LiTFSI/urea Urea Intensityb

1:2.0 1:3.0 1:4.0

3412 3408 3400 3354 m

3240 3229 3222 3240 m

1638 1640 1641 1649 s

1600 1606 1610 1626 s

1472 1470 1468 1472 s

1133 1134 1134 – s

406 407 409 – s

nding, v, wagging, as, asymmetry, and s, symmetry.

Scheme 2. The resonance form of ethyleneurea.

Fig. 6. Raman spectra of the out-of-plane (SO2) wagging of pure LiTFSI and

LiTFSI/ethyleneurea composite at various molar ratios.


strongest electronegativity [54]. These studies clearly indicate

that the Li+ ions tend to coordinate with the carbonyl oxygen

rather than the nitrogen of the acylamino group of the organic

molecule. Fig. 4 compares the IR spectra of the C O stretching

mode of the LiTFSI/ethyleneurea complex with different

molar ratios. It shows that the position of this band shifts

toward low frequency with increasing LiTFSI content in the

composite.

Considering the resonance forms of ethyleneurea (Scheme

2), the position of the C–N stretching band at 1277 cm�1 in pure

ethyleneurea shifts downward slightly in frequency (Fig. 5)

owing to the influences of the Li+–oxygen coordination and the

breaking of the hydrogen bonds. With increasing LiTFSI

content in ethyleneurea, the interaction between the Li ions and

the O atom on the C O group becomes more obvious and the

C–N stretching band keeps blue-shifting. This will lead to the

increase of the amount of ethyleneurea with resonance forms

(A) or (C). As a result, the C–N bonding is more like a double

bond in the liquid composite than in solid ethyleneurea. The

spectral variation of the C–N band is in good agreement with

that of the C O band.

The Raman band at 387 cm�1 is assigned to the out-of-plane

SO2 wagging of solid LiTFSI in Fig. 6. It shifts to higher

frequencies with the decrease of LiTFSI concentration.

Similarly, the band at 1128 cm�1 for the SO2 stretching mode

Fig. 5. Evolution of the FT-IR spectra of the n(CN) mode of ethyleneurea at

various molar ratios of LiTFSI/ethyleneurea.

of solid LiTFSI red-shifts to 1134 cm�1 upon the introduction

of LiTFSI. 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 [40].

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 ethyleneurea. However, the position

variation of these bands is less obvious with the salt

concentration, consistent with the previous observations [15].

This might be due to the narrower range of salt concentration

change in this work. Similar phenomena have also been

observed for the LiTFSI/acetamide and LiTFSI/urea complex

systems in Fig. 7, and relative data are listed in Table 2. It is

understandable that part of the O atoms on the SO2 group

possess negative charges when the Li ions coordinate strongly

with the C O group of ethyleneurea. Considering the

resonance forms of ethyleneurea, these O atoms tend to

interact with the partially positive-charged NH groups of

ethyleneurea. On the other hand, the Raman band of the CF3

symmetric stretching at 1245 cm�1 shows no obvious changes

with addition of ethyleneurea, consistent with the previous

report [55]. This further proves that ethyleneurea mainly

interacts with the TFSI� anion via the SO2 group in the LiTFSI/

ethyleneurea composite.

There are various ionic species in the complex system, such

as the ‘‘free’’ anions and contact ion pairs, due to the strong

interaction between LiTFSI and ethyleneurea 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 complex system,

especially their ionic conductivities. The possible structures of

different ionic species in the LiTFSI–ethyleneurea complex

system which were fully optimized at the BLYP/DNP level

using the Dmol3 program package are shown in Fig. 8,

Fig. 7. The observed band frequency variations of the Raman spectra as a function of lithium salt concentration for several typical binary complex systems based on

LiTFSI.

Fig. 8. Possible optimized structures of different ionic species in LiTFSI–ethyleneurea complex system calculated at the BLYP/DNP.

Fig. 9. Raman spectra of LiTFSI/ethyleneurea composite at various molar

ratios between 770 and 670 cm�1.


corresponding to contact ion pairs (Fig. 8a and b) and the

‘‘free’’ anions (Fig. 8c and d). 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 [56–60]. Bakker et al. [61] reported

that its position is determine by the coordinate and type of

cations in the system. Edman [62] systematically studied this

band in the (PEO)n–LiTFSI system. They found that this 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. The band at 708 cm�1 in

Fig. 9 is assigned to the out-of-plane wagging of the NH group

of ethyleneurea. It is partially superposed with the ca. 745 cm�1

band of LiTFSI when the composites are formed. OriginPro

software is used to fit the band and the results are shown in

Fig. 9. The three fitted Guassian components are located at 746,

740 and 708 cm�1, corresponding to the contact ion pairs, the

‘‘free’’ anions and the out-of-plane wagging of the NH group,

respectively.

The NH wagging band shifts to higher frequencies with the

increase of LiTFSI concentration but its relative intensity

becomes weak, clearly demonstrating that the interaction

between the NH group and TFSI- anion is enhanced in the

Table 3

Calculated Mulliken charges for various organic compounds with acylamino group and each with Li+ coordinationa

Atom Mulliken charge

O Cb Nc Li

Ethyleneurea (Li+) �0.458 (�0.573) 0.518 (0.670) �0.390 (�0.367) 0.694

CH3CO(Li+)NH2 �0.435 (�0.535) 0.357 (0.452) �0.352 (�0.276) 0.732

NH2CO(Li+)NH2 �0.479 (�0.578) 0.486 (0.595) �0.388 (�0.335) 0.713

NH2CO(Li+)NHCH3 �0.493 (�0.588) 0.535 (0.644) �0.416 (�0.380)d 0.697

CH3NHCO(Li+)NHCH3 �0.506 (�0.594) 0.588 (0.696) �0.422 (�0.389) 0.679

a Mulliken charges of each organic compounds with Li+ coordination are the data in parenthesis (excluding the H atom).b Carbon atom in acylamino group.c Nitrogen atom in acylamino group, the Mulliken charges on the two nitrogen atoms are the same for ethyleneurea, urea and 1,3-dimethylurea.d Nitrogen atom adjacent with methyl.


samples with high LiTFSI concentrations. The position of the

multicomponent band shifts to the high frequency side with

increasing content of LiTFSI in the system, from 741 cm�1 at

LiTFSI/ethyleneurea = 1:4.5 to 745 cm�1 at LiTFSI/ethyle-

neurea = 1:2. This slight position variation evidences that the

number of ion pairs increases with increasing salt content in the

complex system.

The above results indicate clearly that the Li+ ions

coordinate with the C O group of ethyleneurea while the

NH group of ethyleneurea interacts with the SO2 group in

TFSI� anion, consistent with the spectral variation of the

LiTFSI/acetamide and the LiTFSI/urea system (Table 2). The

breaking of the hydrogen bonding among the ethyleneurea

molecules and the dissociation of LiTFSI result in the

formation of a eutectic system. Such interactions lead to the

transition of resonance form of ethyleneurea and are reflected in

the IR and Raman spectra. In order to confirm the above

discussions, Quantum chemistry calculations are performed in

the following by optimizing the geometries of the organic

molecules and coordinating ions and by calculating the

energies.

The difference in structure and substituting group for various

organic molecules has an important influence on the

physicochemical properties of the related composite. Acet-

amide can be regarded as urea with one of its NH2 groups

substituted with a methyl group. This substitution changes the

charge density on the carbonyl oxygen and determines the

strength of coordination of Li+ ion with the carbonyl oxygen.

Quantum chemistry calculations will give the further insight

into these discussions.

Table 4

Selected bond lengths, bond angle and binding energy (BE) of various organic co

Atom Bond length (�10�1 nm)

r(C O) r(C–N)

Ethyleneurea (Li+) 1.229 (1.267) 1.405 (1.363)

CH3CO(Li+)NH2 1.234 (1.268) 1.380 (1.342)

NH2CO(Li+)NH2 1.235 (1.272) 1.387 (1.356)

NH2CO(Li+)NHCH3 1.237 (1.274) 1.385 (1.355)c

CH3NHCO(Li+)NHCH3 1.239 (1.275) 1.388 (1.360)

a The data in parenthesis represent the bond length of each organic compound cb BE = �{E(RCO(Li+)NH2) � E(RCONH2) � E(Li+)}.c Nitrogen atom adjacent with methyl.

The results of the charge distributions are shown in Table 3.

It is seen that the negative charge on the carbonyl oxygen is

larger than on the nitrogen, agreeing with the fact that the Li+

ion is prone to coordinate with the carbonyl oxygen. After

coordination with Li+ ion, the Mulliken charge of the carbonyl

oxygen becomes more negative while that of the nitrogen atom

is less negative. This explains the equilibrium of resonance

form of organic compounds, ethyleneurea, for example.

The calculated bond lengths, bond angle of various organic

compounds before and after coordination with the Li+ ion as

well as the binding energies for these coordinations are listed in

Table 4. The variation of the bond length in all the systems

exhibits the same trend, i.e. the coordination elongates the C O

bond but shortens the CN bond length. As a result, the CN bond

is more characteristic of a double bond in the complex,

consistent with the IR and Raman spectroscopic results. More

information focus on the structural parameters and geometries

optimized for ethyleneurea, acetamide and each with Li+

coordination from BLYP/DNP are shown in Fig. 10.

Based upon 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 unoccu-

pied molecular orbital (LUMO). The total energy, the Frontier

molecular orbital energy, the energy gaps between the HOMO

and LUMO, and the dipole moment of various organic

molecules are listed in Table 5. The LUMO energy of

ethyleneurea is higher than that of other molecules. So it is

difficult for the ethyleneurea to accept electrons. On the

contrary, the LUMO energy of the acetamide is low. Therefore,

it can easily accept electrons and bears a high reaction activity.

mpounds with acylamino group and each with Li+ coordinationa

Bond angle of

Li–O–C (8)BEb (kJ/mol)

r(Li–O)

1.745 180.0 223.581

1.758 170.6 203.615

1.746 180.0 216.225

1.740 178.0 224.115

1.736 180.0 231.280

oordinated with Li+.

Fig. 10. Structural parameters and geometries optimized for ethyleneurea, acetamide and each with Li+ coordination from BLYP/DNP.

Table 5

Total energy, Frontier molecular orbital energy and dipole moment of various organic molecules

Organic molecule ET (Ha) Frontier molecular orbital energy (Ha) Dipole moment (Debye)

EHOMO ELUMO DEga

Ethyleneurea �302.7100 �0.2140 0.0086 0.2226 4.4165

CH3CONH2 �209.2479 �0.2106 �0.0171 0.1935 3.8280

NH2CONH2 �225.3175 �0.2160 �0.0085 0.2075 4.2799

NH2CONHCH3 �264.6124 �0.2115 �0.0052 0.2063 4.1850

CH3NHCONHCH3 �303.9070 �0.2007 �0.0035 0.1927 4.0804

a DEg = ELUMO � EHOMO.


The HOMO energies of urea and its ramifications are in the

subsequence of 1,3-dimethylurea > methylurea > urea. The

ability of 1,3-dimethylurea to lose electrons is stronger than

other molecules due to the electron donation of the methyl. It

can be seen in Fig. 11 that the HOMO is mainly located around

the carbonyl oxygen. This indicates that the Li–O coordination

is easy to occur in the interaction between the lithium salt and

organic compounds.

Both ethyleneurea and acetamide have two N–H bonds.

However, as a cyclic organic molecule with acylamino group,

ethyleneurea has stronger intermolecular tension and larger

steric hindrance than acetamide does. As a result, the LiTFSI/

ethyleneurea complex owns the highest glass transition

temperature. Most of the complexes are in their supercooled

state partially due to the big Li–O bond length and low binding

energy between the Li ion and the ethyleneurea molecules.

Moreover, the low ionic conductivity of LiTFSI/ethyleneurea is

attributed to the high viscosity and low ionic migration rate of

this composite. Table 4 also shows that the LiTFSI/acetamide

Fig. 11. Frontier molecular orbital

system has the biggest Li–O bond length and lowest binding

energy in all the complex electrolytes. Compared with urea, the

acetamide molecule has only one NH2 radical, thus the lithium

salt is apt to be dissociate in it by weak hydrogen bonding and

forms a complex system with low melting point and plastic

viscosity. Consequently, the thermal and electrochemical

properties of the LiTFSI/acetamide complex are superior to

that of LiTFSI/urea. Comparing the molecular structures of

urea, methylurea and 1,3-dimethylurea, it seems that, as an

electron donator, the methyl group can enhance the electro-

negativity of the carbonyl oxygen. On the other hand, the

influence of the methyl on the steric hindrance of organic

molecule is negative to the coordination with the Li+ ion. The

LiTFSI/methylurea system prefers to form a liquid at room

temperature while the LiTFSI/urea system has a high ionic

conductivity due to the easy migration of the ions in them. A

balance has to be built up between the association and

dissociation of the salt in order to reach high ionic conductivity

and low eutectic temperature.

of ethyleneurea and acetamide.


Clearly the thermal and electrochemical properties of the

complex systems are influenced by the structure of the organic

molecules, especially their configurations and the properties of

the substituent. On one hand, forcing the coordination between

the lithium salt and organic molecules will promote the

dissociation of lithium salt and decrease the eutectic

temperature of the composite. On the other land, it is difficult

for the Li ions with very strong coordination to the carbonyl

oxygen to migrate in the complex in the viewpoint of

electrochemical applications. Consequently, the compromise

has to be made between the salt dissociation and the Li ion

migration in order to reach optimized general properties for the

complex electrolyte.

4. Conclusion

The complex systems based on the LiTFSI and the above-

mentioned organic compound with acylamino group appear as a

liquid at room temperature in the relevant range of the molar

ratios. The formation mechanism of the complex systems are due

to the strong interaction between the O atom of the C O group in

the organic compounds and the Li cations in addition to the

interaction of the TFSI� anions with the NH2 group in the organic

compounds via hydrogen bonding. Large coordinated cations are

formed and their positive charges are shielded within the organic

molecules. The charge of the TFSI� anion is partially

delocalized, leading to charge shielding in the whole molecule

due to the impact of the CF3 end-group. The coulombic forces

between the cations and anions are very weak. In this way, a

homogeneous, stable and highly ionic conductive room

temperature complex electrolyte can be obtained from two

solid components. Moreover, the calculations indicate that the

molecular structure and the substituting group affect the charge

density and coordination strength of the carbonyl oxygen in an

organic molecule and that the hydrogen bonding interaction

between the organic molecules determines the properties of the

composite, its thermal stability, for example. In addition, a

compromise has to be made between the salt dissociation and ion

migration in the complex system in order to synthesize complex

electrolytes with excellent properties.

Acknowledgements

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 Prof. C.M. Hong (College of Chemistry and

Molecular Engineering, Peking University) for a critical

reading of this manuscript, and Dr. J. Weng (NeoTrident

Technology Ltd.) for quantum chemistry calculation assistance.

The authors are grateful to 3M Company for providing the

LiTFSI sample.

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