Journal of Power Sources 194 (2009) 1068–1074
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Journal of Power Sources
journal homepage: www.elsevier.com/locate/jpowsour
Electrochemical stability of bis(trifluoromethanesulfonyl)imide-based ionic
liquids at elevated temperature as a solvent for a titanium oxide bronze electrode
Junyoung Mun, Yoon Seok Jung
1
, Taeeun Yim, Hyun Yeong Lee, Hyo-Jin Kim,
Young Gyu Kim, Seung M. Oh
∗
Department of Chemical and Biological Engineering, Research Center for Energy Conversion & Storage, Seoul National University, 56-1,
Shillim-dong, Gwanak-ku, Seoul 151-744, Republic of Korea
article info
Article history:
Received 22 October 2008
Received in revised form 13 April 2009
Accepted 25 May 2009
Available online 9 June 2009
Keywords:
Lithium-ion battery
Room-temperature ionic liquids
Titanium oxide bronze
Thermal stability
Cathodic stability
Electrolyte decomposition
abstract
Four different electrolytes are prepared by dissolving a Li salt in three different room-temperature ionic
liquids (RTILs) and also in a conventional organic solvent. The cathodic (electrochemical reduction) sta-
bility of these electrolytes is compared at both ambient and elevated temperature by potential cycling on
aTiO
2
-B electrode. At room temperature, the stability of pyrrolidinium- and piperidinium-based RTILs
is comparable with that of the carbonate-based organic solvent, which is in contrast to the severely
decomposed imidazolium-based RTIL. At elevated temperature (120
◦
C), the imidazolium-based RTIL
undergoes even more significant cathodic decomposition that results in the deposition of a resistive
surface film and leads to eventual cell degradation. By contrast, the cathodic decomposition and con-
comitant film deposition are not serious with pyrrolidinium- and piperidinium-based RTILs even at this
high-temperature, so that the TiO
2
-B/Li cell operates with reasonably good cycle performance. The latter
two RTILs appear to be promising solvents for lithium-ion batteries that are durable against occasional
exposure to high-temperature.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
During the past decade, lithium-ion batteries (LIBs) have been a
major power source for portable electronic devices such as mobile
phones and notebook computers. Recently, the issues of oil price
and global warming have brought an increasing awareness to the
need for more fuel-efficient and less-polluting hybrid electric vehi-
cles (HEVs), for which LIBs are being considered as replacements
for the present nickel-metal hydride cells [1–3]. The performance
of LIBs, however, still falls short of the requirements for many
transportation applications with respect to specific energy, specific
power, cell life, and safety characteristics. The safety characteris-
tics of LIBs are often deeply associated with the high-temperature
stability of the cell constituents. For instance, thermal runaway is
often triggered by internal/external electric shorts or over-charging
and is accompanied by decomposition of the electrode/electrolyte
constituents and the eventual ignition of combustible ingredients.
Thermally stable and non-flammable electrodes/electrolytes may
thus reduce the danger of thermal runaway. Cell life is also affected
∗
Corresponding author. Tel.: +82 2 880 7074; fax: +82 2 872 5755.
E-mail address: seungoh@snu.ac.kr (S.M. Oh).
1
Present address: Department of Mechanical Engineering, University of Texas at
Austin, Austin, TX 78712, USA.
by the thermal stability of electrodes/electrolytes. That is, if these
materials are vulnerable to decomposition under occasional high-
temperature exposure, the cells deteriorate and fail prematurely
[4–6].
Room-temperature ionic liquids (RTILs) have been proposed
as solvents for long-lived and safer LIBs, since they are generally
non-flammable and have low volatility as well as superior thermal
stability [7,8]. Unfortunately, however, the cathodic (electrochemi-
cal reduction) stability of RTILs is poorer than that of conventional
organic solvents [9–12]. For instance, it has been reported that
imidazolium-based RTILs are cathodically decomposed at <1.0 V
(vs. Li/Li
+
) on a glassy carbon electrode at room temperature [10].
By contrast, pyrrolidinium- and piperidinium-based RTILs exhibit
better cathodic stability on the same inert electrode and aredecom-
posed at <0.4–0.5 V [11,12]. Up to now, however,therehave been few
detailed reports of the cathodic stability of these RTILs at elevated
temperatures.
The prime aim of this work is to examine whether RTILs can
be used as solvents for long-lived LIBs. To this end, the high-
temperature electrochemical stability of RTILs and the elevated
temperature cell degradation mechanism have been examined.
Three different electrolytes are prepared by dissolving a Li salt in
three different RTIL solvents based on imidazolium, pyrrolidinium
and piperidinium, and their high-temperature (120
◦
C) cathodic
stability is examined by cyclic voltammetry and charge–discharge
0378-7753/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jpowsour.2009.05.048