Effects of Alkyl Chain in Imidazolium-Type Room-Temperature Ionic Liquids as Lithium Secondary...

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Effects of Alkyl Chain in Imidazolium-Type Room-TemperatureIonic Liquids as Lithium Secondary Battery Electrolytes

Shiro Seki,a,z

Yuichi Mita,a,* Hiroyuki Tokuda,

bYasutaka Ohno,

a

Yo Kobayashi,a,* Akira Usami,

aMasayoshi Watanabe,

b,* Nobuyuki Terada,a

and

Hajime Miyashiro

a

a Materials Science Research Laboratory, Central Research Institute of Electric Power Industry,

2-11-1, Iwado-kita, Komae, Tokyo 201-8511, Japanb Department of Chemistry and Biotechnology, Yokohama National University, 79-5 Tokiwadai,

Hodogaya-ku, Yokohama, Kanagawa 240-8501, Japan

Lithium secondary batteries that use a room-temperature ionic liquid as an electrolyte were investigated for the purpose of realizing high-safe batteries. For the improvement of stability under charge/discharge operation with electrodes, we focusedattention on a series of 1-alkyl-3-methyl-imidazolium bistrifluoromethane sulfonylimide. The temperature dependence of ionicconductivity and battery charge-discharge performance were examined by changing the alkyl chain lengths: -methyl/-ethyl/-butyl/-hexyl/-octyl. According to the results, the effects of extending the alkyl chain were confirmed in, for example, the increase incarrier ion number, and the improvement of battery charge-discharge performance characteristics.© 2007 The Electrochemical Society. DOI: 10.1149/1.2768168 All rights reserved.

Manuscript submitted May 8, 2007; revised manuscript received July 6, 2007. Available electronically August 3, 2007.

With the establishment of new ways of using electric power inJapan, such as all-electric homes, heat pump water heaters commer-cial name: Eco Cute, large-scale energy storage devices for storingelectric power at the household level electric-load leveling systemsare expected to be realized.1 Not only can savings in electrical billsbe achieved by the use of cheaper late-evening electric power duringthe daytime, but also positive effects for useful applications such asemergency power supplies and uninterruptible power sources can beexpected in a time of disaster. Lithium secondary batteries have ahigher energy density per unit volume and weight compared withother battery systems,2 and the wide range of application fields isstrongly desired, from consumer equipment applications such as cel-lular phones and notebook-type personal computers to high-powerpower supplies such as those in electric vehicles.3,4 For this reason,we have recently been promoting the research and development of “very safe” lithium secondary batteries that use room-temperature

ionic liquids5 or solid polymers6-8 as electrolyte materials instead of conventional flammable organic electrolyte solutions. In particular,room-temperature ionic liquids have received considerable attentionbecause the newly developed third liquid differs from “water” and“organic solvent,”9,10 and they are considered to be one candidatefor new battery electrolytes of large-scale and safe electric storagedevices. Since the room-temperature ionic liquids are organic saltsthat consist only of cations positive ions and anions negativeions, they show relatively low volatility, low combustibility, highthermal stability, and relatively high ionic conductivity, and are veryattractive materials for consideration as new electrolytes. For ex-ample, as for room-temperature ionic liquids geared toward “energyconversion and storage,” their applications for use as safe electro-lytes in electric double-layer capacitors,11,12 fuel cells,13 dye-synthesized solar cells,14,15 lithium secondary batteries,16,17 andactuators,18 among others, have been examined. Also, the optimiza-

tion of cell preparation conditions long charge/discharge cycle lifeusing a quaternary-ammonium cation based room-temperature ionicliquid,19 the protection of the cathode/electrolyte interface highvoltage, high capacity by using a stable ZrO2 layer at the LiCoO2

cathode surface,20 and the development of high-stability room-temperature ionic liquids prepared by substituting the chemicallystable protective group at the second position of the imidazoliumcation,21 among others,22 have so far been reported as methods of improving battery characteristics. However, simple 1,3-alkyl imida-zolium cation-based room-temperature ionic liquids are most com-

monly used because of their relatively easy synthesis. In this report,to improve the performance of lithium secondary batteries that use1,3-alkyl imidazolium cation-based room-temperature ionicliquids,23 we examined the molecular design of various room-temperature ionic liquids. In particular, to promote charge delocal-ization in the imidazolium cation ring and to control the three-dimensional attack at the second position of the imidazolium cationring, room-temperature ionic liquids that lengthened the alkyl chainsin the first position third position were examined.

Experimental

Five types of room-temperature ionic liquids with variouslengths of alkyl chains in the first position third position of theimidazolium cation ring, mmim CF3SO22N methyl, emim

CF3SO22N ethyl, bmim CF3SO22N butyl, hmim

CF3SO22N hexyl, and omim CF3SO22N octyl shown inFig. 1 were synthesized, respectively, according to previousreports.22 First, all room-temperature ionic liquids were dried in avacuum chamber at 323 K for more than 24 h, and stored in a dry-argon-filled glove box O2 0.4 ppm, H2O 0.1 ppm, MiwaMFG Co., Ltd.. Next, the room-temperature ionic liquid-lithiumsalt LiNCF3SO22, Kishida Chemical battery grade, dried in avacuum chamber at room temperature and stored in a dry-argon-filled glove box mixed electrolytes were prepared by dissolving a

* Electrochemical Society Active Member.z E-mail: [email protected]

Figure 1. Chemical structures of mmim CF3SO22N, emim CF3SO22N, bmim CF3SO22N, hmim CF3SO22N, and omim CF3SO22N.

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given amount concentration: 0.32 mol kg−1 of LiNCF3SO22 intothe room-temperature ionic liquids, and stored at room temperaturein the glove box.

The battery performance was investigated using LiCoO2

cathoderoom-temperature ionic liquid-LiNCF3SO22 mixedelectrolytelithium metal anode cells. The cathode sheets were pre-pared from LiCoO2 85 wt % as a cathode active material, acety-

lene black 9 wt %, Denka as an eletrically conductive additive,and polyvinylidene fluoride PVdF, 6 wt %, Kureha Chemical,

N -methylpyrrolidone solution as a binder polymer. These constitu-

ent materials were thoroughly agitated together in a homogenizer for20 min. The obtained paste was applied to an aluminum currentcollector using an automatic applicator at 353 K for more than 12 h.After drying the coated paste, the cathode sheet was compressed toincrease packing density for the improvement of the electrical con-ductivity. The dried cathode sheet, a separator, the room-temperatureionic liquid-LiNCF3SO22 mixed electrolyte and the lithium metalanode were encapsulated into 2032-type coin cells. For favorablepenetration of the electrolyte into the pressed cathode sheet, thermalaging of the prepared battery was performed at 333 K for more than12 h.

The temperature dependences of the ionic conductivities of

various room-temperature ionic liquid-LiNCF3SO22 mixed elec-trolytes were measured in SUS stainless steelelectrolyteSUScells and determined by the complex impedance method, using an acimpedance analyzer Princeton Applied Research, PARSTAT-2263,

200 kHz to 50 mHz; impressed voltage: 10 mV between 353 and233 K at 10 K intervals with cooling.Charge-discharge cycle tests of the batteries were performed at

3.0–4.2 V vs Li/Li+ V at a current density of 0.05 mA cm−2 1/8C at 303 K constant current charge–constant current discharge,Hokuto Denko HJ-1010mSM8A.

Results and Discussion

Figure 2 shows the temperature dependence Arrhenius plots of ionic conductivity for the mmim CF3SO22N, emim

CF3SO22N, bmim CF3SO22N, hmim CF3SO22N, and

omim CF3SO22N–LiNCF3SO22 mixed electrolytes used in

this study. The Arrehenius-type temperature dependences of for allelectrolytes in this study exhibited convex curved profiles, following

the Williams–Landel–Ferry or Vogel–Tamman–Fulcher VTF

behavior. Although mmim CF3SO22N–LiNCF3SO22 andemim CF3SO22N–LiNCF3SO22 mixed electrolytes exhibited

close values throughout the entire temperature range in this study, largely decareases monotonically with increasing alkyl length of theimidazolium cation. These trends are similar to the case of the neatroom-temperature ionic liquid systems.23 They correlate to the orderof the viscosity values and the self-diffusion coefficients on the basisof pulsed-gradient spin-echo nuclear magnetic resonance in the caseof bulk room-temperature ionic liquid systems, and the microscopicionic mobility room-temperature ionic liquid cations, NCF3SO22

anions, and lithium cations might be similarly reflected even if LiNCF3SO22 was added into room-temperature ionic liquids.

In this research, the temperature dependences of the ionic con-ductivities of all electrolyte systems showed convex curved profiles,and they are analyzed using the VTF equation24,25

= A T

−1/2

exp− B / T − T 0 1where A parameter is a pre-exponential constant proportional to thenumber of carrier ions in this study, room-temperature ionic liquidcations, TFSI anions, and lithium cations in the case of extremelyhigh temperatures, B parameter is the pseudo-activation energy forionic conduction expressed in temperature units, and T 0 is the idealglass transition temperature. Figure 3 shows the relationships be-tween best-fit VTF equation parameters obtained from ionic conduc-tivity data with error bars and the alkyl carbon numbers of the fivetypes of room-temperature ionic liquid–LiNSO2C F32 mixture

electrolytes a: A parameter, b: B parameter, c: T 0. A param-eters and B parameters monotonically increased, whereas T 0monotonically decreased, with increasing length of the alkyl chain.In a previous report, the density values of imidazolium cation-based

Figure 2. Arrhenius plots of the ionic conductivities of the five types of room-temperature ionic liquid-LiNSO2CF32 mixture electrolytes.

Figure 3. VTF equation = AT −1/2 exp− B / T − T 0 parameters forionic conductivity data for the five types of room-temperature ionic liquid-LiNSO2CF32 mixture electrolytes a: A parameter, b: B parameter, c:T 0.

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room-temperature ionic liquids follow the order mmim CF3SO22N emim CF3SO22N bmim CF3SO22N hmim CF3SO22N omim CF3SO22N in the tempera-

ture range of 288–313 K. Although an increase of density indicatesthe increased affinity of the molecules, A parameters can increase asthe alkyl chain extends as a result of the formation of space for theadded lithium cations and for the entry of CF3SO22N anions intothe “vacant” space. In other words, an increase of ion content pereach unit volume has been suggested. The increase in B parameteraccompanying LiNCF3SO22 concentration can explain the de-

creasing tendency of the ionic conductivity in the low-temperatureregion. On the other hand, T 0 showed a different tendency from thatof neat room-temperature ionic liquid systems.23 In this study, theconcentration of the dissolved LiNCF3SO22 is constant at

0.32 mol kg−1, and the molar ratio of LiNCF3SO22 to room-temperature ionic liquid increases with alkyl chain length. More-over, in particular, LiNCF3SO22 might not influence the alkyl part

in the imidazolium cation ring. Therefore, it is thought that the in-teraction between imidazolium cations and lithium cations becamesmall with increasing length of the alkyl chain. Of course, lithiumcations most likely have a very weak interaction with the imidazo-lium cations no matter what the alkyl chain length, and the lithiumcations will be strongly interacting with the anions for all of theelectrolytes.

Because the ionic conductivity of all electrolytes was higher than10−3 S cm−1 303 K, the characteristics of the electrolytes for actual

lithium secondary batteries were examined. Figure 4 shows thecycle number dependences of the cathode-limited discharge capaci-ties a and the Coulombic efficiencies b of the LiCoO2

cathoderoom-temperature ionic liquid—LiNCF3SO22 mixedelectrolytelithium metal anode cells with various room-temperatureionic liquids. The initial discharge capacities of all the electrolytesystems except the mmim cation were approximately135 mAh g−1 which is close to the theoretical capacity of Li x CoO2

0.5 x 1; between 3.0 and 4.2 V. The cycle performance of the battery was improved with the extension of the alkyl chainlength of the imidazolium cation ring, in particular when the alkylchain is longer than butyl contains more than 4 carbon atoms. Forexample, hmim and omim cation room-temperature ionic liquidelectrolyte systems maintained a high capacity retention over

110 mAh g−1 after 100 charge/discharge cycles. The Coulombicefficiency, calculated from the charge capacity and discharge capac-ity, was also improved with the extension of the alkyl chain lengthin the imidazolium cation ring, as well as in case of using secondposition substituted imidazolium cation.21 In other words, these re-sults suggest that side reactions at electrode / electrolyte interfaces

probably, reductive decomposition at lithium metal anode de-creased with the extension of the alkyl chain length due to formationof stable interfaces, for example, solid electrolyte interface SEI.For example, in order to overcome the problem of insufficient sta-bility with lithium metal electrode, proposal of the quaternary pip-eridinium cation-based room-temperature ionic liquids26-28 or appli-cations of the stable SEI forming room-temperature ionic liquid29

and additive30 have been reported. Of course, the possibility of theSEI formation by all ionic species cations and anions should not beneglected31-33 at the present stage. On the other hand, in neat room-temperature ionic liquid systems, with increasing number of carbonatoms in the alkyl chain, the van der Waals inductive forces amongalkyl chains of imidazolium cations were increased.23 For example,recently, Lopes and Pádua have reported the formation of mi-crophase intramolecular separation as the length of the alkyl chainincreases using computer simulations.34 In particular, they reported

the aggregation formation of the alkyl chains in nonpolar domaincharge delocalization in the imidazolium cation ring, in the case of the alkyl chain is longer than butyl. That is, the separation of hydro-philic part and hydrophobic part might be promoted i.e., intramo-lecular phase dissociation might promote hydrophobic nature. Notonly a single-molecular order charge delocalization of the imidazo-lium ring and control of the attack at the second position by thesteric barrier, but also a multimolecular order for example, a smallamount of moisture control due to a phase separation of the room-temperature ionic liquids structure might be affected by the alkylchain length of the imidazolium cation ring.

Conclusion

Rechargeable lithium batteries were fabricated using the fivekinds of 1,3-alkyl imidazolium cation alkyl: methyl—octyl basedroom-temperature ionic liquids, and their battery properties wereinvestigated. The results are summarized as follows:

1. Ionic conductivity of room-temperature ionic liquid–lithiumsalt mixed electrolytes lithium salt concentration: 0.32 mol kg−1was higher than 10−3 S cm−1 at 303 K in all room-temperature ionicliquid systems. Ionic conductivity decreases monotonically with in-creasing alkyl length carbon atom numbers of the imidazoliumcation of the room-temperature ionic liquids.

2. Charge/discharge cycle performances of the battery have beenimproved with the extension of the alkyl chain length of the imida-zolium cation ring, in particular when the alkyl chain is more thanbutyl. For example, high capacity retention over 110 mAh g−1 af-ter 100 cycles were realized when the alkyl chain is longer thanhexyl contains more than 6 carbon atoms.

Central Research Institute of Electric Power Industry assisted in meeting

the publication costs of this article.

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Effects of Alkyl Chain in Imidazolium-Type Room-Temperature Ionic Liquids as Lithium Secondary...

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