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Electrochemistry Communications 8 (2006) 460–464

Ionic liquids containing an ester group as potential electrolytes

J.S. Lee a,b, N.D. Quan b, J.M. Hwang a, J.Y. Bae b, H. Kim b, B.W. Cho b,H.S. Kim a,*, H. Lee b,*

a Department of Chemistry, Kyung Hee University, 1 Hoegi-dong, Dongdaemoon-gu, Seoul 130-701, Republic of Koreab Division of Environment and Process Technology, Korea Institute of Science and Technology, Sungbuk-gu, Seoul 136-791, Republic of Korea

Received 15 November 2005; received in revised form 23 December 2005; accepted 11 January 2006Available online 15 February 2006

Abstract

Various types of ionic liquids containing a methyl acetate group were synthesized and their physical and electrochemical propertieswere investigated. These ionic liquids exhibited wide electrochemical windows of at least 5.0 V and relatively high conductivities. Unlikethe case of dialkyl-substituted ionic liquids, the conductivities of ionic liquids with methyl acetate group on the cations were reduced to amuch smaller extent only by 30% upon mixed with a lithium salt. In addition, a severe deterioration of C-rate performance was notobserved when an ionic liquid with a methyl acetate group was added to an electrolyte solution consisting of EC, DMC and LiPF6. Suchfavorable electrochemical properties of these ionic liquids are likely to be attributed to the interaction between methyl acetate group andlithium ions. FT-IR study clearly demonstrates that the methyl acetate group of the ionic liquids interacts with Li+ ions through carbonylgroup. Upon interaction with LiPF6, the carbonyl stretching frequency shifted to a lower frequency, whereas the peak associated withC–O single bond shifted to a higher frequency.� 2006 Elsevier B.V. All rights reserved.

Keywords: Ionic liquids; Electrolyte; Ionic conductivity; Lithium secondary battery

1. Introduction

The current lithium battery system has an inherentsafety problem arising from the use of volatile and flamma-ble organic carbonate compounds as electrolyte solvents.Besides the safety problem, the organic carbonate-basedelectrolyte systems for lithium ion batteries have manyother drawbacks in terms of charge–discharge capacity,operating temperature limit, and ionic conductivity, andthus there is a room for possible improvement. Recently,imidazolium-based room temperature ionic liquids(RTILs) have received considerable interest as potentialelectrolytes for lithium batteries, due to their favorableproperties, such as wide electrochemical window, high

1388-2481/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.elecom.2006.01.009

* Corresponding authors. Tel.: +82 2 961 0432; fax: +82 2 966 3701.E-mail addresses: khs2004@khu.ac.kr (H.S. Kim), hjlee@kist.re.kr

(H. Lee).

ionic conductivity, non-flammability, and wide operatingtemperature range [1–5].

However, the application of imidazolium-based ionicliquids as electrolytes of lithium ion battery has beenrestricted due to their low cathodic stability toward lithium[6,7]. For this reason, many attempts were made to developalternative ionic liquids with better cathodic stability. Ionicliquids based on 2-substituted imidazolium [8], tetraalky-lammonium [9,10], pyrrolidinium [11], and piperidinium[12] cations were found to exhibit high cathodic stabilitytoward lithium.

Recently, Egashira et al. [13] showed that the intro-duction of cyano group onto the quaternary ammoniumcation significantly improved the cycle performance andthe conductivity of the corresponding quaternary ammo-nium-based ionic liquids. Even though this result clearlydemonstrates that the electrochemical property of anionic liquid can be tuned by the substitution of anappropriate functional group on the cation, detailed

J.S. Lee et al. / Electrochemistry Communications 8 (2006) 460–464 461

investigation on the interaction between a functionalgroup and a lithium salt has rarely been attempted.

In this regard, we have prepared several ionic liquidscontaining a methyl ester group on the cations with a hopethat the favorable interaction between the ester group anda lithium salt could improve various electrochemical prop-erties of the resulting electrolytes. We report here the prep-aration and physical and electrochemical properties of aseries of ionic liquids containing a methyl ester group. Inaddition, the interaction of lithium salt with the ester groupis investigated by means of a FT-IR technique.

2. Experimental

All the chemicals were obtained from commercial sup-pliers and used without further purification. Solvents werefreshly distilled prior to use according to the literatureprocedure.

2.1. Synthesis of methyl 1-methylimidazolium-3-acetate

trifluoromethanesulfonimide ([MMEIm][TFSI])

1-Methylimidazole (16.4 g, 0.2 mol) was dissolved inCH3CN (100 mL) in a 250 mL round bottomed flask.Methyl chloroacetate (29.3 g, 0.27 mol) was added slowlyto the above solution with a vigorous stirring under a nitro-gen atmosphere. After the 5 h reaction at 60 �C, white pre-cipitates were collected, washed with CH3CN (30 mL), anddried under vacuum to give [MMEIm][Cl] (35.4 g, 93%).[MMEIm][Cl] (19.0 g, 0.1 mol) was dissolved in CH2Cl2(100 mL) in a 250 mL round bottomed flask and thenreacted with lithium trifluoromethanesulfonimide (LiTFSI)(28.7 g, 0.1 mol) for 5 h under a nitrogen atmosphere. Afterfiltration of precipitated LiCl, the filtrate was washed withwater to remove dissolved LiCl and treated with Na2SO4.The resulting solution was subjected to filtration and thendried under vacuum to produce [MMEIm][TFSI] (45.4 g,95%). Elemental analysis calcd (%) for C8H9F6N3O6S2:C, 22.8; H, 2.2; N, 9.9; O, 22.8. Found: C, 23.0; H, 1.9;N, 10.1; O, 23.1. 1H NMR (300 MHz, D2O, 25 �C): d(ppm) = 9.00, 7.73 (s, d, 3H, CH-Im), 5.30 (s, 2H, CH2),4.09 (s, 3H, CH3N), 3.78 (s, 3H, CH3O).

[MMEPyr][TFSI], [MMEMor][TFSI], and [EME-Pip][TFSI] were prepared in a manner analogous to thatused for preparing [MMEIm][TFSI].

Thermogravimetric analysis (TGA) was performedusing a Metter TGA 50 in a nitrogen atmosphere between50 and 900 �C at a heating rate of 10 �C/min. The meltingtemperatures of ionic liquids were determined using a TAInstruments Q10 differential scanning calorimeter (DSC)in the range from �90 to 100 �C at a scan rate of 10 �C/min under a nitrogen atmosphere. FT-IR spectra wererecorded on a Mattson Infinity spectrophotometerequipped with a MCTA detector. To avoid contact withwater and air, IR measurements were performed using aspecially designed IR cell equipped with two CaF2 windows[16].

The ionic conductivities were measured by a Solartron1260A frequency response analyzer (FRA). The electro-chemical stabilities of the ionic liquids were also analyzedby using a cyclic voltammetry (CH Instruments Electro-chemical Work Station) at an ambient temperature. Aglassy carbon working electrode of 3 mm diameter wasused with a platinum wire as the counter electrode and asilver wire as the reference electrode. The electrochemicalwindows were calibrated using a redox potential of ferro-cene/ferricenium (Fc/Fc+) couple measured in each ionicliquid.

The galvanostatic charge–discharge test was performedin one-stack laminated cells consisting of a LiCoO2 cath-ode, a lithium foil anode, and 1 M LiPF6 in EC:DEC(1:1) electrolytic solution containing a 10 wt% ionic liquid.All the cells were assembled in a glove box under argonatmosphere and the charge–discharge test was carried outwith a battery cycler (Won A Tech WBC 3000) at a rateof 1 C in the potential range of 3.0–4.3 V.

3. Results and discussion

Imidazolium, pyrrolidinium, morpholinium, and pipe-ridinium-based ionic liquids were prepared in two stepsby reacting methyl chloroacetate with 1-methylimidazole,N-methylpyrrolidine, N-methylmorpholine, or N-methylpi-peridine followed by anion exchange with LiN(SO2CF3)2(LiTFSI). A typical example is depicted in Scheme 1.

The thermal stabilities of these ionic liquids were testedin a nitrogen atmosphere between 50 and 900 �C at a heat-ing rate of 10 �C/min. As shown in Fig. 1 and Table 1, allthe ionic liquids tested exhibited high thermal stabilitiesand similar TGA tendencies, showing two distinct decom-position temperatures between 290 and 440 �C. Thedecompositions at lower temperatures are likely to beattributed to the decomposition of the methyl acetategroup. The melting temperatures of these ionic liquids werealso determined. As listed in Table 1, the ionic liquids con-taining an ester group melt at around �42 �C, demonstrat-ing wide liquid temperature ranges for these ionic liquids.Glass transition temperatures of these ionic liquids werenot observed from the DSC curves obtained under theexperimental condition (from �90 to 100 �C).

The ionic conductivities are presented in Figs. 2 and 3 asa function of temperature. The conductivities of these ionicliquids increased with increasing temperature, possibly dueto the decrease in viscosity at higher temperatures. It isworth to note that the conductivity values of pyrrolidiniumand morpholinium ionic liquids containing a methyl estergroup are higher than those of the corresponding alkylatedsalts, [P13][TFSI] (P13: N-methyl-N-propylpyrrolidinium)[12] and [Mor1,4][TFSI] (Mor1,4: N-butyl-N-methylmorph-olinium) [14].

It is well known that the conductivity of ionic liquiddecreases significantly upon mixing with a lithium salt.For example, the conductivity of 1-ethyl-3-methylimidazo-lium trifluoromethanesulfonimide ([EMIm][TFSI]) was

NNH3CCl

O

OCH3

N

N

H3C

OO

CH3

LiN(SO2CF3)2

Cl-

N

N

H3C

OO

CH3

-N(SO2CF3)2

++

Scheme 1.

2.9 3.0 3.1 3.2 3.3

-4.0

-3.5

-3.0

-2.5

-2.0

log

(σ

/ Scm

-1)

1000 T-1 / K-1

[MMEIm][TFSI] [MMEPyr][TFSI] [MMEMor][TFSI] [EMEPip][TFSI] [P13][TFSI] [Mor

1,4][TFSI]

Fig. 2. The ionic conductivities of ionic liquids as a function oftemperature.

2.9 3.0 3.1 3.2 3.3-3.5

-3.0

-2.5

-2.0

-1.5

1000 T-1 / K-1

[EMIm][TFSI] [EMIm][TFSI]+LiTFSI [MMEIm][TFSI] [MMEIm][TFSI]+LiTFSI [MMEPyr][TFSI] [MMEPyr][TFSI]+LiTFSI

log

(σ /

Scm

-1)

Fig. 3. The ionic conductivities of ionic liquids with 1 M LiTFSI as afunction of temperature.

0 200 400 600 800 1000

100

80

60

40

20

0

Mas

s lo

ss (

%)

Temperature (oC)

[MMEIm][TFSI] [MMEPyr][TFSI] [MMEMor][TFSI] [EMEPip][TFSI]

Fig. 1. Thermogravimetric analysis (TGA) of various types of ionicliquids with a methyl ester group on the cations.

Table 1Thermal properties of ionic liquids

Tm/�Ca Tdecomp1/�Cb Tdecomp2/�Cb

[MEMIm][TFSI] �42.5 340 440[MEMPyr][TFSI] �42.1 290 400[MEMMol][TFSI] �42.0 310 400[EMEPip][TFSI] �42.2 335 410

MMEIm, methyl 1-methylimidazolium-3-acetate; MMEPyr, methyl N-methylpyrrolidinium-N-acetate; MMEMor, methyl N-methylmorpholin-ium-N-acetate; EMEPip, methyl N-ethyl-piperidinium-N-acetate; TFSI,trifluoromethanesulfonimide.a Tm, melting temperature.b Tdecomp1, Tdecomp2, decomposing temperatures.

462 J.S. Lee et al. / Electrochemistry Communications 8 (2006) 460–464

reduced by about 70% with the addition of LiPF6 [15]. Thedrop in the C-rate performance of lithium ion cells was alsoobserved when an imidazolium-based ionic liquid, 1-butyl-2,3-dimethylimidazolium hexafluorophosphate ðBDMIþ

PF�6 Þ, was added to a conventional electrolyte consisting

of LiPF6 and propylene carbonate. Such a performancedrop was explained in terms of the difference in diffusioncoefficients of two cations (BDMI+ and Li+). Lithium ionsare more easily solvated by organic carbonate moleculesand consequently the diffusion coefficient or mobility ofsolvated lithium ions becomes smaller compared with thatof BDMI+. Therefore, during the charging process,BDMI+ ions with larger diffusion coefficient are able to

reach the anode more rapidly than solvated lithium ionsand form a BDMI+ barrier around the anode. As a result,the approach of lithium ions to the anode becomes limited,causing a significant drop in C-rate performances of lith-ium ion batteries [17]. Interestingly, however, such adverseeffects of ionic liquids were greatly diminished when analkyl group of ionic liquids was replaced by methyl acetate.As shown in Fig. 3, the degree of conductivity decrease by

1800 1600 1400 1200

(e)

(d)

(c)

(b)

(a)

1209

1190

1209

1203

17831753

17931760

1747

1751

1186

1760

Tra

nsm

ittan

ce

Wavenumber (cm-1)

Fig. 5. FT-IR spectra showing the interaction of [MMEIm][TFSI] withLiPF6 at 25 �C under Ar(g). (a) [MMEIm][TFSI], (b) [MMEIm][TFSI] +LiPF6 (1 equiv.), (c) [MMEIm][TFSI] + LiPF6 (2 equiv.), (d)[MMEIm][TFSI] in propylene carbonate, and (e) [MMEIm][TFSI] +LiPF6 (2 equiv.) in propylene carbonate.

[EMEPip][TFSI]

[MMEMor][TFSI]

[MMEPyr][TFSI]

[MMEIm][TFSI]

J.S. Lee et al. / Electrochemistry Communications 8 (2006) 460–464 463

the addition of LiTFSI was much smaller for the ionicliquid containing a methyl acetate group comparing withthat for the dialkyl-substituted ionic liquid. Furthermore,no significant deterioration of C-rate performances wasobserved when pyrrolidinium and morpholinium-basedionic liquids containing a methyl acetate group were mixedwith LiPF6 (Fig. 4). These results strongly imply that theester group on the ionic liquid is playing a favorable rolein transporting lithium ions through an interaction with alithium salt.

The interaction of the methyl acetate group with LiTFSIis clearly demonstrated by a FT-IR experiment. As shownin Fig. 5, the peak associated with the carbonyl group ofmethyl 1-methylimidazolium-3-acetate trifluoromethane-sulfonimide ([MMEIm][TFSI]) shifted to a lower frequencyfrom 1760 to 1747 cm�1 when two equivalents of LiPF6

was added. Whereas, the stretching frequency assigned tothe C–O single bond moved to a higher frequency to1209 from 1184 cm�1, possibly due to a resonance betweenthe carbonyl group and the C–O single bond [16]. The IRspectra in Fig. 5 clearly indicate that the methyl acetategroup of the ionic liquid, [MMEIm][TFSI] is also capableof interacting with lithium ions even in the presence of pro-pylene carbonate (see Fig. 5d and e). From these results, itis conceivable that the diffusion of lithium ions in organicsolution will be facilitated by the interaction with the addedionic liquid. The positive effect of ionic liquids containing amethyl acetate group is supported by the improved C-rateperformances in comparison with those of electrolytes con-taining ionic liquids without an ester group.

The electrochemical stabilities of the ionic liquids werealso analyzed by using a cyclic voltammetry at an ambienttemperature. Fig. 6 displays the cyclic voltammograms forfour different types of ionic liquids containing a methylester group. All the ionic liquids exhibited wide electro-chemical windows of at least 5.0 V, indicating the contribu-

0 20 40 60 80 100 120 140 1603.0

3.2

3.4

3.6

3.8

4.0

4.2

Specific Capacity / mAh g-1

Cel

l Vol

tage

/ V

Control[MMEPyr][TFSI][P12][TFSI][MMEMor][TFSI][Mor

1,4][TFSI]

Fig. 4. Charge–discharge curves of 1 M LiPF6 in EC:DEC (1:1) (control)and mixed ionic liquid (10 wt%) electrolytes at initial cycle at roomtemperature (3.0–4.3 V, constant current at 1 C mA).

-4 -3 -2 -1 0 1 2 3 4

Potential (Vvs. Fc/Fc+)

Fig. 6. Cyclic voltammograms of ionic liquids at 25 �C. Scan rate,10 mV s�1; working electrode, glassy carbon; counter electrode, Pt;reference electrode, Ag. Potential was calibrated using the redox potentialof ferrocene/ferricenium (Fc/Fc+) redox couple measured in each ionicliquid.

tion of the ester group to the electrochemical stability ofthese salts.

4. Conclusions

The presence of an ester group on the cations of ionicliquids was found to improve thermal and electrochemicalproperties of room temperature ionic liquids, such as ionicconductivities and C-rate performances. It is likely that theimproved electrochemical properties seem to be originated

464 J.S. Lee et al. / Electrochemistry Communications 8 (2006) 460–464

from the interaction between ester group and lithium ions.The interaction of methyl acetate group of ionic liquidswith lithium ions was clearly demonstrated by FT-IRexperiments. Due to their excellent thermal and electro-chemical stability, it is expected that these ionic liquidsmay find some application as additives or alternatives tothe conventional carbonate-based electrolytes used in thesecondary batteries.

Acknowledgment

This work was supported by the KIST institutionalfund.

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