COMMUNICATION
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Exothermic and thermal runaway behaviour of some ionic liquids
at elevated temperaturesw
a
b
a
a
b
R. Vijayaraghavan, M. Surianarayanan, V. Armel, D. R. MacFarlane* and V. P. Sridhar
Received (in Cambridge, UK) 12th June 2009, Accepted 14th August 2009
First published as an Advance Article on the web 7th September 2009
DOI: 10.1039/b911568d
The exothermic behaviour and intrinsic safety of a number of
ionic liquids being considered for battery and solar cell
applications have been investigated at elevated temperatures
by analysing data from accelerated rate calorimetric (ARC)
studies.
bis(trifluoromethanesulfonyl)amide, BF
sulfonyl)amide and tricyanomethanide.
4 4
, B(CN) , bis(fluoro-
The accelerated rate calorimetry (ARC) technique is a
relatively simple and direct way to study the thermal
behaviour of a compound or a mixture of compounds. It
has been used to identify potentially dangerous exothermic
7
,8,12–14
Ionic liquids have been the subject of intense study in recent
years because of their attractive properties for a variety of
applications. To a large extent it is the thermal, chemical and
electrochemical stability of selected ILs that are the features of
interest. Where this is the case they offer the potential of very
behaviour
and also to show that certain ionic liquids
can be used as moderators to control exothermic behaviour in
1
5
polymerisation reactions. The process involves heating the
sample in an adiabatic calorimeter in small temperature steps
and evaluating, at each step, the tendency of the sample to
attain isothermal conditions, or, if an exothermic process is
present, to self-heat. Knowledge of the thermal inertia (f) of
the calorimeter bomb and the sample itself allows calculation
of the enthalpy of reaction. Further details can be found in the
ESIw.
1
,2
stable, non-flammable and non-volatile reaction solvents,
3
4
electro-deposition solvents, extractants and battery or solar
5
cell electrolytes. However, though more detailed analyses of
6
breakdown behaviour have begun to appear in recent years, a
full understanding of the high temperature behaviour of most
ionic liquids is not yet available. In any application where
even a moderate quantity of ionic liquid may become
exposed to elevated temperatures, this information is vital
from a safety standpoint. For example, the use of ionic liquids
in large battery packs for electric vehicles, where elevated
temperatures may occur under extreme service or collision/
accident conditions, requires an understanding of their
The ionic liquids tested in this study were as follows: 1-ethyl-
3
-methylimidazolium tricyanomethanide ([C mim][TCM])
2
1
was synthesised as per the literature procedure, while
6
1
-ethyl-3-methylimidazolium
[C and
mim)][FSI]z)
bis(fluorosulfonyl)amide ([C
from Dai-chi Kogyo Seikayu. 1-ethyl-3-methylimidazolium-
tetracyanoborate ([C mim][B(CN) )]) and 1-ethyl-3-methyl
imidazolium tetrafluoroborate ([C mim][BF )]) were purchased
bis(fluorosulfonyl)amide
(
2
N-ethyl-N-methylpyrrolidinium
3
mpyr][FSI]) were purchased
7
,8
behaviour under these conditions. As we will show, a wide
range of behaviours is exhibited by the ionic liquids that are
being considered for such applications; some are encouragingly
stable, while others are not.
2
4
2
4
from MERCK. The ionic liquids were intentionally used as
received, in order to test them under practical conditions. The
synthesis of 1-ethyl-3-methyl imidazolium bis(trifluorometha-
nesulfonyl)amide ([C mim][NTf ]), N-ethyl-N-methylpyrroli-
Explosions due to thermal runaway are also one of the
9
major safety issues faced by chemical industries. In an
2
2
exothermic reaction, if the heat generation in the system
exceeds the heat removal capacity, thermal runaway can
occur, leading to secondary reactions and/or potentially to
dinium bis(trifluoromethanesulfonyl)amide ([C
2
mpyr][NTf
mpyr][I])
followed literature procedures. The preparation of diethyl-
2
])
and N-butyl-N-methylpyrrolidinium iodide ([C
4
1
7
1
0
1
8
an explosion. Knowledge of the thermokinetics of exothermic
reactions assists in mitigating such disastrous events and
phosphate ionic liquids followed the literature methods. The
synthesis of 1-ethyl-3-methylimidazolium diethylphosphate
1
1
provides a means for designing inbuilt safety systems.
(
2
[C mim][DEP]) involved dropwise addition of triethyl-
Hence the main objective of the present study was to
investigate the safety of a number of commonly used ionic
liquids and generate thermo-kinetic data for them. The output
of this study will be of direct relevance to the use of these ionic
liquids in any application where the onset temperature for
exothermic processes is even a remote possibility. The ionic
liquids chosen for this study are a series of methylimidazolium
and methylpyrrolidinium salts of anions including
phosphate (46 g, 0.252 mol) to a flask containing 21 g of
N-methylimidazole at 140 1C under nitrogen and stirring. The
liquid was stirred overnight at the same temperature, then
cooled and dried in vacuo for 24 h. The product was liquid at
room temperature. (Crude yield 90%) MS (ESI): ES+ m/z:
+
ꢀ
1
11.09 (C
similar manner N-ethyl N-methylpyrrolidinium diethyl-
phosphate ([C mpyr][DEP]) was prepared by a slow addition
6
H
11
N
2
), ESꢀ m/z: 153.03 ((C
2
H
5
O)PO
2
). In a
2
of triethylphosphate (17 g, 0.094 mol) into a flask containing
N-methylpyrrolidine (8 g, 0.094 mol) at 160 1C under nitrogen
and stirring. The liquid was stirred overnight at the same
temperature, then cooled and dried under vacuum overnight.
A liquid was formed at room temperature. (Crude yield 92%)
a
School of Chemistry, Monash University, Clayton, VIC 3800,
Australia. E-mail: d.macfarlane@sci.monash.edu.au
Chemical Engineering Department, CLRI, Chennai 600 020, India
b
w Electronic supplementary information (ESI) available: The instru-
mental techniques, adiabatic thermo-kinetics and the time-to-temperature
plots for each IL. See DOI: 10.1039/b911568d
+
ꢀ
ES+ m/z: 114.13 (C H N ), ESꢀ m/z: 153.03 ((C H O)PO ).
7
16
2
5
2
This journal is ꢁc The Royal Society of Chemistry 2009
Chem. Commun., 2009, 6297–6299 | 6297