Electrochemistry Communications 10 (2008) 1761–1764

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Electrochemistry Communications

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Effect of ether group on the electrochemical stability of zwitterionicimidazolium compounds

H. Kim a, D.Q. Nguyen b, H.W. Bae a, J.S. Lee a, B.W. Cho b, H.S. Kim a,*, M. Cheong a,*, H. Lee b,*

a Department of Chemistry, Kyung Hee University, Seoul 130-701, Republic of Koreab Energy and Environment Division, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea

a r t i c l e i n f o

Article history:Received 12 July 2008Received in revised form 26 August 2008Accepted 5 September 2008Available online 12 September 2008

Keywords:Ionic liquidsZwitterionLithium batteryElectrolytesCycle performance

1388-2481/$ - see front matter � 2008 Elsevier B.V. Adoi:10.1016/j.elecom.2008.09.006

* Corresponding authors. Tel.: +82 2 961 0432; faxtel.: +82 2 961 0239 (M. Cheong); tel.: +82 2 958 586

E-mail addresses: khs2004@khu.ac.kr (H.S.(M. Cheong), hjlee@kist.re.kr (H. Lee).

a b s t r a c t

The cathodic stability of the zwitterionic imidazolium compounds was significantly enhanced by theintroduction of an ether group at 1 or 2-position on the imidazolium ring. The cycle performance testsshowed that the initial cell capacity was maintained almost unchanged up to 100 cycles at 0.5 and 1 Cwhen 2.5 wt.% of 2-butoxymethyl-1-methylimidazolium-3-propylsulfonate or 2-butoxymethyl-1-buty-limidazolium-3-propylsulfonate was added to the model electrolyte (1 M LiPF6 in ethylene carbonate,dimethyl carbonate and ethylmethyl carbonate (1/1/1 v/v/v)).

Structures of zwitterionic compounds and their interactions with lithium ions were theoreticallyinvestigated.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction

Current lithium ion batteries suffer from an inherent safetyproblem arising from the use of volatile and flammable organiccarbonates as components of electrolytes [1–3].

Recently, room temperature ionic liquids (RTILs) have attractedgrowing interest as promising alternatives electrolytes to replacepartly or completely the traditional organic carbonate-based elec-trolytes for the lithium ion batteries, due to their nonflammability,thermal stability and nonvolatility [4,5]. Among the various seriesof RTILs, ILs consisting of 5- and 6-membered cycloaliphatic ammo-nium cations and imide anions were found to exhibit relatively highelectrochemical stability. However, there still exist some limitationsfor the application of these ILs as electrolytes or electrolyte addi-tives, due to their high viscosity and low ionic conductivity [6–11].

As a means of increasing ionic conductivity, imidazolium-basedILs have been extensively studied because of their favorable elec-trochemical properties such as wide electrochemical window andhigh conductivity [12–14]. However, the attempts to use theseILs as electrolytes for lithium ion batteries were not very successfuldue to their low cathodic stability caused by the presence of acidicC-2 hydrogen atom as well as to the migration of imidazolium cat-ions along with lithium ions [15–19]. Some improvement on the

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electrochemical stability was achieved by substituting C-2 hydro-gen with an alkyl group, but the cathodic stability influencingthe cycle performance of the battery was hardly improved [20–23].

Herein, we report on the synthesis and characterization ofhighly cathodic stable zwitterionic imidazolium compounds bear-ing an ether group at C-2 position and a sulfopropyl group at 3-po-sition on the imidazolium ring. The interactions of lithium ionswith sulfopropyl group and/or ether group are also discussed basedon the optimal structures obtained using density functionalcalculations.

2. Experimental

2.1. Materials

All the chemicals were purchased from Aldrich Co. and used asreceived. A model electrolyte containing 1 M LiPF6 solution in eth-ylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methylcarbonate (EMC) (1/1/1 v/v/v) was donated from Cheil Industries,Inc. 1-Methyl-2-(hydroxymethyl)imidazole, 1-butyl-2-(hydroxy-ethyl)imidazole, 1-methylimidazolium-3-propylsulfonate, 6 and1,2-dimethylimidazolium-3-propylsulfonate, 7 were prepared bythe literature procedures [6,7,24].

2.2. Synthesis of 1-(2-methoxyethyl)imidazolium-3-propylsulfonate, 1

A solution of imidazole (6.8 g, 100 mmol) in 100 mL DMF wasadded dropwise to NaH (2.6 g, 110 mmol) in 50 mL of anhydrous

Fig. 1. Ionic conductivities of model electrolyte itself (N) and model electrolytescontaining 2.5 wt.% of zwitterionic imidazolium compound 1 (j), 2 (h), 3 (d), 4(s), 5 (D), 6 (�) and 7 (e).

1762 H. Kim et al. / Electrochemistry Communications 10 (2008) 1761–1764

DMF at room temperature. The mixture was reacted at 60 �C for1 h. After the reaction, excess NaH was filtered off. To this solu-tion, CH3OC2H5Cl (9.4 g, 100 mmol) was added slowly at roomtemperature. After the removal of NaCl, the resulting solutionwas reacted with 1,3-propanesultone (12.8 g, 105 mmol) at 80 �Covernight to give 1 (Yield: 82%). Elemental analysis calcd (%) forC9H16N2O4S: C, 43.5; H, 6.5; N, 11.3. Found: C, 43.2; H, 6.3; N,10.9. 1H NMR (300 MHz, D2O, 25 �C): d(ppm) = 8.67 (s, 1H,NCHN), 7.57 (d, 2H, NCHCHN), 4.42 (t, 2H, NCH2), 4.38 (t, 2H,OCH2), 3.85 (t, 2H, NCH2), 3.39 (s, 3H, OCH3), 2.93 (t, 2H, SCH2),2.23 (m, 2H, CH2).

1-(2-methoxyethyl)-2-methylimidazolium-3-propylsulfonate,2, was similarly prepared from 2-methylimidazole to 1.

2.3. Synthesis of 2-butoxymethyl-1-methylimidazolium-3-propylsulfonate, 3

2-(Hydroxymethyl)-1-methylimidazole (5.6 g, 50 mmol) andNaOH (20 g, 500 mmol) in distilled water (30 mL) was heated at80 �C for 2 h until the mixture became transparent. (C4H9)4NBr(1.6 g, 5 mmol) and C4H9Cl (5.1 g, 55 mmol) were added to thesolution and stirred at 80 �C overnight. The upper organic layerwas further reacted with 1,3-propanesultone (7.3 g, 60 mmol) inacetone at reflux for 6 h to produce white precipitates.

2-Butoxymethyl-1-butylimidazolium-3-propylsulfonate, 4 and2-butoxyethyl-1-methylimidazolium-3-propylsulfonate, 5 weresimilarly prepared from 1-butyl-2-(hydroxymethyl)imidazole and1-methyl-2-(hydroxyethyl)imidazole, respectively.

2.4. Instruments

The ionic conductivities were measured on a Solatron 1260Afrequency response analyzer. The galvanostatic charge–dischargetests were conducted in one-stack laminated cells consisting of aLiCoO2 cathode, a graphite anode, model electrolyte and azwitterionic compound. Charge–discharge tests were conductedusing a battery cycler (Won A Tech WBC 3000) at a constantrate of 0.2 C for the first cycle and 0.5 or 1 C for the rest ofcycles.

Scheme

3. Results and discussion

The syntheses of the zwitterionic imidazolium compoundsbearing an ether group were depicted in Scheme 1.

The effect of added ether-containing zwitterionic compound onthe electrochemical properties of a model electrolyte containing1 M LiPF6 in a mixture of EC, DMC and EMC (1/1/1 v/v/v) was inves-tigated (Fig. 1). The electrolytes containing a zwitterionic com-pound with an ether group at C-2 position (compounds 3 and 4)exhibited higher conductivities than that containing 1, 2, or 6,implying that C-2 substituted ether group is more effective forfacilitating the dissociation of LiPF6. It is worthwhile to notice thatthe electrolytes containing a zwitterionic compound bearing theether group at C-2 position (2.5 wt.%) showed higher conductivitiesthan the model electrolyte. In general, the conductivity of an elec-trolyte consisting of organic carbonates and a lithium salt de-creases significantly by the addition of an IL, due to the increase

1.

Fig. 2. Cycling performances for LiCoO2 cathode and graphite anode using anelectrolyte consisting of model electrolyte and 2.5 wt.% zwitterionic compound(top: 0.5 C-rate, bottom: 1 C-rate, 3.0–4.2 V): model (N), 1 (j), 2 (h), 3 (d), 4 (s), 6(�) and 7 (e).

Fig. 3. Optimized structures of (a) 1, (b) 1 + LiPF6 (1 equiv.), (c) 2, (d) 2 + LiPF6 (1 equiv.10�10 m).

H. Kim et al. / Electrochemistry Communications 10 (2008) 1761–1764 1763

of the viscosity of the resulting electrolyte [25]. Such an adverse ef-fect of the IL can be suppressed to a certain extent by introducing afunctional group or groups on the cation of the IL [18,25]. It is alsoreported that the conductivity of an IL decreases significantly uponmixing with a lithium salt [19]. In this context, the conductivitybehaviors of the ether-containing zwitterionic imidazolium com-pounds, 3, 4 and 5, are quite unusual.

The cycle performances were also evaluated with the electro-lytes consisting of the model electrolyte and a zwitterionic com-pound. As shown in Fig. 2, the initial discharge capacities of thebatteries containing a zwitterionic imidazolium compound withan ether group at 0.5 C were maintained without appreciable fad-ing up to 100 cycles, whereas the cell capacity reduced rapidlywith the cycles when a zwitterionic imidazolium compound with-out an ether group, 6 or 7, was added to the model electrolyte. Theether group on the imidazolium ring seems to play a role in stabi-lizing C-2 methyl group during the charging/discharging process.Surprisingly, the cell capacity of the battery containing 3 or 4was higher than that of the battery containing the model electro-lyte only. The effect of the substitution at C-2 by an ether groupwas more pronounced at higher C-rate of 1 C. The battery contain-ing 3 or 4 exhibited better cycle performance at 1 C than that con-taining the model electrolyte, strongly implying that thesubstitution at C-2 position by an ether group on the imidazoliumring is highly effective in improving the cathodic stability of theimidazolium compounds. On the contrary, the cell capacity of thebattery containing 1 or 2 as an additive decreased continuouslywith the cycle at 1 C. The lower cathodic stabilities of 1 and 2 athigher C-rate can be attributed in part to the acidic characters ofC-2 hydrogen and methyl hydrogen, respectively [15–17,19].

To have a better understanding of the role of the ether group inenhancing the electrochemical stability of imidazolium com-pounds, density functional calculations were made at the B3LYP(6–31 + G* for C, O, H and S) level of the theory using Gaussian03 program [26]. The optimized structures of1, 2, 3 and 5 and theircomplexes with Li+ are shown in Fig. 3. Both ether and sulfonategroups in compound 1, 2, 3 and 5 are positioned on the oppositeside of the imidazolium ring. The C-2 hydrogen atom of 1 wasfound to interact strongly with two oxygen atoms of the sulfonate

), (e) 3, (f) 3 + LiPF6 (1 equiv.), (g) 5 and (h) 5 + LiPF6 (1 equiv.) (bond lengths are in

1764 H. Kim et al. / Electrochemistry Communications 10 (2008) 1761–1764

group. Similar interaction was also observed in 2, 3 and 5 betweenthe oxygen atom of the sulfonate group and the methyl or methy-lene group on the C-2 carbon. The optimized molecular structuresof 1 � Li+, 2 � Li+ and 3 � Li+ complexes clearly show the stronginteraction between the sulfonate group and Li+. The lithium ionsbound to sulfonate groups are found to intra-molecularly interactwith the oxygen atoms of the ether groups for the complexes1 � Li+ and 2 � Li+. With such additional interactions, the dissocia-tion of a lithium salt seems to be facilitated, thereby resulting inthe increase of the conductivity. Interestingly, as in 1 and 2, thesulfonate groups in 1 � Li+ and 2 � Li+ are found to interact withC-2 hydrogen and C-2-methyl group, respectively, even after thebonding with Li+. These interactions may contribute to the stabil-ization of the imidazolium compounds with hydrogen or methylgroup at C-2 position during the charging/discharging process. Un-like 1 � Li+ and 2 � Li+, additional intra-molecular interaction isnot observed in 3-Li+. The absence of intra-molecular interactioncan be easily conceivable from the structures of 3, where the pro-pylsulfonate and the ether groups are located on the opposite sideof the imidazolium ring.

In contrast to 3-Li+, intra-molecular interaction can be found for5-Li+ even though the two functional groups of 5 are also locatedon the opposite side of the imidazolium ring with each other.The increase of carbon number between C-2 and ether oxygenatom from –CH2O– to –CH2CH2O– seems to provide a space forthe favorable intra-molecular interaction of the lithium ion bondedto the propylsulfonate group with the ether oxygen atom.

From the experimental and computational results, it is likelythat the improved electrochemical stability and high conductivityof the zwitterionic compounds with an ether group at 1 or C-2 po-sition are originated from to the multi-interactions among lithiumion, propylsulfonate and ether groups. As for 1 and 2, the interac-tion between sulfonate group and C-2 hydrogen or C-2 methylgroup can also be considered as an important factor in improvingtheir electrochemical properties.

4. Conclusions

The electrochemical stability and the conductivity of a zwitter-ionic imidazolium compound can be improved significantly byintroducing an ether group at 1 or C-2 position, especially at C-2position. Computational study suggests that the improvement in

electrochemical properties of these compounds could be attributedto the multi-interactions among lithium ion, propylsulfonate andether groups.

Acknowledgment

This work was supported by an Institutional fund from KoreaInstitute of Science and Technology.

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