Melting/freezing points of high concentrations of AlCl3 in a saturated chloroaluminate ionic liquid

Abstract

Melting/freezing points of AlCl3 in saturated chloroaluminate ionic liquids [molar ratio 2:1 AlCl3:1-ethyl-3-methyl imidazolium chloride (EMIC)] are measured by differential scanning calorimetry (DSC). A critical range of temperature data (50-130 °C) for AlCl3 dissolution and precipitation from saturated chloroaluminate ionic liquids is obtained. This range of temperature data is of significance to control phase transition of AlCl3 in saturated chloroaluminate ionic liquids. By applying the chloroaluminate ionic liquids to electrolytes for energy storage usage, solid AlCl3 can be partially dissolved and precipitated out during the charging/discharging cycle of energy storage equipment. Therefore, the energy density of the electrolytes is expected to be greatly improved.

Full text

Melting/freezing points of high concentrations of AlCl
3
in a saturated chloroaluminate ionic liquid
Mengqi Zhang
1
•
Richard Groves
2
•
Robert M. Counce
1
•
Jack S. Watson
1
•
Thomas A. Zawodzinski
1,3
Received: 10 April 2015 / Accepted: 6 November 2015
 Akade
´
miai Kiado
´
, Budapest, Hungary 2015
Abstract Melting/freezing points of AlCl
3
in saturated
chloroaluminate ionic liquids [molar ratio 2:1 AlCl
3
:
1-ethyl-3-methyl imidazolium chloride (EMIC)] are mea-
sured by differential scanning calorimetry (DSC). A critical
range of temperature data (50–130 C) for AlCl
3
dissolu-
tion and precipitation from saturated chloroaluminate ionic
liquids is obtained. This range of temperature data is of
significance to control phase transition of AlCl
3
in satu-
rated chloroaluminate ionic liquids. By applying the
chloroaluminate ionic liquids to electrolytes for energy
storage usage, solid AlCl
3
can be partially dissolved and
precipitated out during the charging/discharging cycle of
energy storage equipment. Therefore, the energy density of
the electrolytes is expected to be greatly improved.
Keywords Ionic liquids  DSC  AlCl
3
 Energy storage
Introduction
It has been widely known that AlCl
3
can form eutectic
melts at room temperature, i.e., ionic liquids, with different
imidazolium chloride compounds [1–4]. The most typical
imidazolium chloride compounds are 1-ethyl-3-methyl
imidazolium chloride (EMIC). The AlCl
3
/EMIC ionic
liquids have low viscosity, high conductivity and wide
electrochemical window [2, 4, 5]. The excellent physi-
cal/chemical properties of AlCl
3
/EMIC ionic liquid indi-
cate that it has potential for energy storage applications
[6–10]. However, in the industrial practice of the AlCl
3
/EMIC
ionic liquids, the water content should be limited to less
than 0.1 ppm to avoid the hygroscopic reaction. The anions
composition in AlCl
3
/EMIC ionic liquid varies with the
molar ratio of AlCl
3
/EMIC (MR). Typically, the chemistry
of AlCl
3
/EMIC ionic liquid can be described in the fol-
lowing chemical reactions [11].
MR AlCl
3
þ EMIC ! MR AlCl

4
þ 1  MR
ðÞ
Cl

þ EMI
þ
; when MR  1
MR AlCl
3
þ EMIC ! MR  1ðÞAl
2
Cl

7
þ 2  MRðÞAlCl

4
þ EMI
þ
; when MR [ 1
Therefore, at room temperature the maximum molar
ratio of AlCl
3
:EMIC is 2:1 for the Al
2
Cl
7
-
to be the main
part of total anions in the electrolyte. This characteristic
affects the energy density of AlCl
3
/EMIC ionic liquid for
energy storage applications.
Differential scanning calorimetry (DSC) is a funda-
mental technique in the thermal analysis and records the
heat flow curve as measured, while the temperature varies
continuously [12]. In this paper we use DSC to measure
melting/freezing point of excess AlCl
3
in the saturated
AlCl
3
/EMIC ionic liquid. The melting/freezing point data
indicate when the AlCl
3
starts to dissolve/precipitate in the
saturated AlCl
3
/EMIC ionic liquid. As the concern of
thermal stability of EMIC, the experimental temperature is
controlled under 200 C since the onset decomposition
temperature of EMIC is 285 C[13–15]. Additionally, the
different shapes of heat flow curves from different mole
& Mengqi Zhang
seventh_zmq@hotmail.com
1
Department of Chemical and Biomolecular Engineering,
University of Tennessee, Knoxville, TN 37996, USA
2
Clean Energy Events, Wilmington, NC 28409, USA
3
Materials Science and Technology Division, Oak Ridge
National Laboratory, Oak Ridge, TN 37831, USA
123
J Therm Anal Calorim
DOI 10.1007/s10973-015-5154-3

fractions of AlCl
3
in the ionic liquids are discussed based
on their phase transition mechanism.
Experimental
The saturated AlCl
3
/EMIC ionic liquid was prepared in
glovebox using AlCl
3
and 1-ethyl-3-methyl imidazolium
chloride (EMIC) in a 2:1 molar ratio. Molar ratio 1.5:1 and
1:1 ionic liquids were also prepared to test against the data
from literature [5]. The transparent AlCl
3
crystal was
purified from yellowish AlCl
3
powder (Fluka, [99 %
anhydrous) based on the reported method [1, 16]. EMIC is
synthesized under the help from Trulove [16].
1-Methylimidazole is further purified by reflux and distil-
lation before used. EMIC is dissolved and recrystallized in
acetonitrile (Sigma-Aldrich, 99.8 %) and ethyl acetate
(Sigma-Aldrich, 99.8 %) for 3 times before it is used to
prepare the ionic liquid. Table 1 shows the purity and
purification methods used for chemicals synthesis and
preparation.
Thermal analyses were performed on the samples using
differential scanning calorimetry with a TA Instruments
DSC Q2000, and data were evaluated using TA Instru-
ments Universal Analysis 2000. The instrument was cali-
brated using a standard material, i.e., indium. The
calibration process involved the rising temperature exper-
iment with the temperature range from -90 to 300 Cata
heating rate 10–20 C min
-1
and at a nitrogen flow rate
50 mL min
-1
[17]. The average mass of samples is 7.0 mg.
The heating rate for experimental samples is 10 Cmin
-1
under the nitrogen flow rate 50 mL min
-1
. The maximum
temperature is limited to 195 C to avoid the high vapor
pressure and thermal decomposition of ionic liquids at high
temperature. Samples for DSC measurements were pre-
pared inside the glovebox using a Tzero press, Tzero
standard aluminum pans, and Tzero hermetic lids. The
samples were prepared by applying 1, 3, or 5 lL of ionic
liquid inside the pans and then adding excess AlCl
3
. The
ratios were determined by subtracting the prerecorded
mass of each pan and average mass of at least 3 samples
each of 1, 3, and 5 lL ionic liquid from the total sealed
sample mass.
Results and discussions
Samples with different mole fractions of AlCl
3
are mea-
sured by DSC. Generally, during the heating/cooling pro-
cess the peaks on the heat curve represent the phase
transitions, such as melting/freezing, at the certain tem-
perature range. Melting/freezing points of the samples
marked by the software’s calculation can be easily
obtained. However, to interpret the different shapes of heat
curve peaks is difficult. Here, we show two representative
heat curves in Figs. 1 and 2 to discuss the reasons of dif-
ferent shapes of peaks.
AlCl
3
/EMIC ionic liquid is the combination of different
anions, i.e., Al
2
Cl
7
-
, AlCl
4
-
and Cl
-
, and EMI
?
cation. So
this ionic liquid belongs to the mixture compound and the
melting curves are concave in shape. Their peak maxima
are usually considered as the melting point. In Fig. 1 no
obvious endothermic peak is observed during the heating
process indicating that the melting phase transition does
not occur during this temperature range. However, a wide
exothermic peak on the heating curve marked by software
indicates that a possible supercooling crystallization pro-
cess occurs after the temperature of ionic liquid decreases
below -85 C. This characteristic peak is unique in the
sample of the molar ratio 2:1 AlCl
3
/EMIC ionic liquid.
Reference literature reported that the ionic liquid at molar
ratio 2:1 undergoes a glassy transition process at the low
temperature and no obvious melting/freezing phase tran-
sition occurs.
In Fig. 2 the DSC heat curve shows the heat flow
changes in heating/cooling process for the sample of molar
ratio 2.7:1 AlCl
3
/EMIC ionic liquid. At room temperature
the maximum molar ratio for the chloroaluminate ionic
liquid is only 2:1, as the AlCl
3
reacts with EMIC to form
the ionic liquid Al
2
Cl
7
-
and EMI
?
. The excess AlCl
3
will
stay as the solid in the ionic liquid at molar ratio 2:1.
Although the existence form of AlCl
3
in ionic liquid is
Table 1 Purity and purification of chemical compounds used in ionic liquid preparation
Compound Source Purity/mass fraction Purification method
Aluminum chloride Fluka 0.99 Sublimation
1-Methylimidazole Aldrich 0.99 Distillation and reflux
Chloroethane Aldrich 0.997 –
Acetonitrile Sigma-Aldrich 0.998 –
Ethyl acetate Sigma-Aldrich 0.998 –
M. Zhang et al.
123

uncertain, by elevating the temperature to over 50 C,
AlCl
3
is possible to be further dissolved into the ionic
liquid. The heat from the phase transition (or maybe
reaction transition) is recorded in Fig. 2. Both endothermic
and exothermic peaks are recorded during the melting and
freezing phase transitions of the excess AlCl
3
in the satu-
rated ionic liquid. The peaks, especially the endothermic
peaks, are wider than the normal peaks obtained from the
samples of pure EMIC, pure AlCl
3
and ionic liquids with
molar ratio less than 2:1. This indicates that the slow dif-
fusion and nucleation process exists during melting and
freezing phase transitions of AlCl
3
in the saturated AlCl
3
/
EMIC ionic liquid. Similar characteristics occur in both
samples with 0.73 and 0.75 mole fractions of AlCl
3
, while
in the samples with higher mole fractions of AlCl
3
, such as
0.82, 0.85 and 0.89, this characteristic is not obvious and
shape of peaks are more similar to those from the pure
AlCl
3
.
The melting/freezing points of AlCl
3
/EMIC ionic liq-
uids at different mole fractions of AlCl
3
are listed in
Table 2. The freezing point of the sample with mole
fraction of AlCl
3
0.6 is not available because the DSC heat
curve shows a characteristic of glassy transition during
cooling process and no obvious freezing phase transition is
obtained. These data together with related data from ref-
erence literature are plotted in Fig. 3 as the complete phase
diagram of AlCl
3
/EMIC ionic liquids.
As seen from the phase diagram of AlCl
3
/EMIC ionic
liquid in Fig. 3, the melting/freezing curves are steeper
between the mole fraction of AlCl
3
0.68 and 0.8. Once the
mole fraction of AlCl
3
is over 0.8, the melting/freezing
point data slowly approaches the value of pure AlCl
3
. The
difference between melting point and freezing point is
larger when the mole fraction of AlCl
3
is over 0.68. This
may result from the slow dissolving and crystallization
processes of AlCl
3
in the saturated AlCl
3
/EMIC ionic liq-
uid. From this phase diagram a range of temperature suit-
able to control AlCl
3
dissolving and precipitating from the
saturated ionic liquid is established. It is best to adjust
–0.3
–0.2
–0.1
0
0.1
0.2
0.3
–100 –50 0 50 100 150
Temperature/°C
Heat flow/W g
–1
Endo ↔ Exo
Cooling
Heating
–61.23 °C
0.215 J g
–1
–56.77 °C
7.155 J g
–1
–20.15 °C
Heating rate: 10 °C min
–1
Fig. 1 DSC heat curve of molar ratio 2:1 AlCl
3
:EMIC
–50 0 50 100 150 200 250
–0.4
–0.2
0
0.2
0.4
0.6
Temperature/°C
Heat flow/W g
–1
Endo ↔ Exo
74.29 °C
Cooling
86.29 °C
33.12 J g
–1
70.50 °C
28.52 J g
–1
Heating
131.38 °C
Heating rate: 10 °C min
–1
Fig. 2 DSC heat curve of molar ratio 2.7:1 AlCl
3
:EMIC (Mole
fraction of AlCl
3
is 0.73)
Table 2 Melting/freezing point of AlCl
3
/EMIC ionic liquids at dif-
ferent mole fractions of AlCl
3
Mole fraction of AlCl
3
Melting point/C Freezing point/C
0 88.7 ± 0.2 14.8 ± 0.1
0.5 8.5 ± 0.1 1.1 ± 0.1
0.6 -31.8 ± 0.35 N/A
0.724 119.4 64
0.729 131.4 86.3
0.743 140.2 108.6
0.76 151.8 120.8
0.82 185 162
0.85 187.5 156
0.89 189.1 159
1 192 161 ± 8
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Mole fraction of AlCl
3
in Ionic liquid
–50
0
50
100
150
200
250
Temperature/°C
Melting point from literature
Melting point
Freezing point
Critical region for AlCl
3
dissolving and precipitation by
temperature control
Excess AlCl
3
in saturated
AlCl
3
/EMIC ionic liquids
Fig. 3 The phase diagram of AlCl
3
/EMIC ionic liquids including
data from reference literature [5]
Melting/freezing points of high concentrations of AlCl
3
in a saturated chloroaluminate ionic…
123

temperature in the range from 50 to 130 C to dissolve and
precipitate the excess amount of AlCl
3
.
Conclusions
DSC heat curves are measured from samples of different
AlCl
3
mole fractions of AlCl
3
/EMIC ionic liquids. Melt-
ing/freezing points of ionic liquids with different AlCl
3
mole factions are obtained from these DSC heat curves.
Phase diagram of AlCl
3
in ionic liquid, especially the
region where excess AlCl
3
exists in the saturated ionic
liquid, is plotted. A suitable range of temperature
(50–130 C) is chosen based on the phase diagram to
control the excess amount of AlCl
3
dissolving and pre-
cipitating from the saturated ionic liquid.
Acknowledgements We thank the TN-SCORE program, NSF EPS-
1004083, for funding this work.
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