New inorganic ionic liquids possessing low melting temperatures and wide electrochemical windows: Ternary mixtures of alkali bis(fluorosulfonyl)amides

Article abstract of DOI:10.1016/j.electacta.2012.01.097

Four kinds of ternary phase diagrams of alkali bis(fluorosulfonyl)amides, LiFSA-NaFSA-KFSA, LiFSA-NaFSA-CsFSA, LiFSA-KFSA-CsFSA and NaFSA-KFSA-CsFSA, have been constructed. All the ternary systems possess the only one eutectic point around equimolar compo

Full text of DOI:10.1016/j.electacta.2012.01.097

Electrochimica Acta 66 (2012) 320–324
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New inorganic ionic liquids possessing low melting temperatures and wide
electrochemical windows: Ternary mixtures of alkali bis(fluorosulfonyl)amides
∗
Keigo Kubota, Toshiyuki Nohira, Rika Hagiwara
Graduate School of Energy Science, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 March 2011
Received in revised form 25 January 2012
Accepted 28 January 2012
Available online 4 February 2012
Four kinds of ternary phase diagrams of alkali bis(fluorosulfonyl)amides, LiFSA–NaFSA–KFSA,
LiFSA–NaFSA–CsFSA, LiFSA–KFSA–CsFSA and NaFSA–KFSA–CsFSA, have been constructed. All the ternary
systems possess the only one eutectic point around equimolar composition with the temperature of
3
0
09–318 K. The electrochemical window of the eutectic LiFSA–KFSA–CsFSA ((xLi, xK, xCs) = (0.30, 0.35,
.35)) is as wide as 5.4 V at 323 K with the cathode limit being lithium metal deposition. The electro-
chemical window of the eutectic NaFSA–KFSA–CsFSA ((xNa, xK, xCs) = (0.40, 0.25, 0.35)) is 4.9 V at 323 K
with sodium metal deposition as the cathode limit reaction.
Keywords:
Bis(fluorosulfonyl)amide
Molten salt
©
2012 Elsevier Ltd. All rights reserved.
Ionic liquid
Phase diagram
Electrochemical window
1
. Introduction
temperatures are around 373 K, much lower than those for MTFSA
single salts. However, the decomposition temperatures are also
low (typically, 413–443 K). We investigated ten kinds of MFSA
binary mixtures as shown in Table 2 [14]. All the binary systems
exhibit a simple eutectic type. Their eutectic temperatures range
from 325 K to 360 K, which is lower than the melting tempera-
tures of constituent single salts. According to the electrochemical
measurements, it was revealed that the eutectic LiFSA–KFSA and
NaFSA–KFSA melts possess wide electrochemical windows of over
5 V with the cathode limits of lithium metal deposition and
sodium metal deposition, respectively [14]. These ionic liquids
are unique in the point that they contain only inorganic com-
pounds. Since they contain only alkali metal cations, they are
essentially free from electrochemical instability originating from
organic cations. Moreover, the absence of organic solvent/ions
offers very interesting opportunities in investigating the effects of a
solid-electrolyte interface and/or stability of the eutectic elec-
trolyte in contact with active electrodes in battery applications.
Since the eutectic temperatures of ternary systems are expected
to be lower than those of binary systems, thermal proper-
ties of four kinds of MFSA ternary systems, LiFSA–NaFSA–KFSA,
LiFSA–NaFSA–CsFSA, LiFSA–KFSA–CsFSA and NaFSA–KFSA–CsFSA,
were investigated in this study. Furthermore, the electrochemi-
cal windows and ionic conductivities were measured for selected
eutectic melts, LiFSA–KFSA–CsFSA and NaFSA–KFSA–CsFSA.
We have been investigating new class of ionic liquids or molten
salts which possess high thermal and electrochemical stability and
are capable of electrodepositing alkali metals reversibly. Firstly,
we focused on binary [1,2] and ternary [3] mixtures of alkali
bis(trifluoromethylsulfonyl)amides, MTFSA (M = Li, Na, K, Rb, Cs).
We found that some selected binary and ternary mixtures of
MTFSAs give reasonably lower melting temperatures (typically
3
90–423 K) than constituent single salts (typically over 473 K).
In addition, it was revealed that some selected mixed melts
possess more desirable properties as electrolytes than the sin-
gle salts [2]. As one of the promising fields of application, the
LiTFSA–KTFSA–CsTFSA ternary melt ((xLiTFSA, xKTFSA, xCsTFSA) = (0.20,
0.10, 0.70 in mole fraction)) was tested as an electrolyte for a
rechargeable lithium metal battery, in which a Li/LiFePO4 cell
exhibited a good cycle performance at 423 K [4].
Secondly, in order to lower melting temperatures, we aimed
at alkali bis(fluorosulfonyl)amides, MFSAs. For some MFSAs, the
route of synthesis and the crystal structures have been stud-
ied [5–8]. Especially, LiFSA is expected to be used as electrolyte
salts for lithium batteries and has been useful as an electrolyte
salt for lithium batteries with PEO-based gel [9,10], EC/DMC [11]
and ionic liquid [12] electrolytes for lithium ion batteries with
LiFePO4 cathodes. Table 1 summarizes the melting and decom-
position temperatures of MFSA single salts [13]. The melting
2. Experimental
∗ Corresponding author. Tel.: +81 75 753 5822; fax: +81 75 753 5906.
E-mail address: hagiwara@energy.kyoto-u.ac.jp (R. Hagiwara).
MFSA salts were prepared in the same manner as given
in the previous report [6]. Several important points and new
0
013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2012.01.097

K. Kubota et al. / Electrochimica Acta 66 (2012) 320–324
321
Table 1
The melting and decomposition temperatures of MFSA single salts [13].
LiFSA
00
Tm (K)
Td (K)
4
LiFSA
NaFSA
KFSA
RbFSA
CsFSA
403a
379
375
368
386
343
413
423
435
443
390
380
3
70
a
The decomposition rate of LiFSA was slow enough to measure its melting point
−1
by DSC at 10 K min [13].
3
60
350
EB
EB
procedures are described here. KFSA (Dai-ichi Kogyo Seiyaku
Co., Ltd., purity > 99%, Li < 0.01 ppm, Na < 6 ppm) was used after
drying in a vacuum line at 353 K. LiFSA and NaFSA were syn-
thesized by the reactions of KFSA with LiClO4 (Wako Pure
Chemical Industries, purity > 0.99 in weight) and NaClO4 (Aldrich,
purity > 0.98 in weight) in acetonitrile (Wako Pure Chemical Indus-
tries, purity > 99.5%), respectively. CsFSA was synthesized by the
reaction of HFSA with CsCl (Wako Pure Chemical Industries,
purity > 0.99 in weight) in nitromethane (Wako Pure Chemi-
cal Industries, purity > 99%), where HFSA was prepared by the
reaction of KFSA and HClO4 beforehand. In order to remove
water, LiFSA and NaFSA were added thionyl chloride (Wako Pure
Chemical Industries, purity > 99%) and stirred for 1 h under N2
atmosphere. The samples were filtered off under vacuum and
washed by dichloromethane (Wako Pure Chemical Industries,
purity > 99%) three times. After that, the residual dichloromethane
were dried under vacuum at 323 K. CsFSA was recrystallized twice
in tetorahydrofuran (Wako Pure Chemical Industries, purity > 99%).
The products, LiFSA, NaFSA and CsFSA were identified by Raman
spectroscopic analysis and elemental analysis. Ternary mixtures of
340
350
3
50
ET
360
3
70
3
60
KFSA
EB
NaFSA
Fig. 1. Phase diagram of LiFSA–NaFSA–KFSA system.
argon-filled glove box. A copper disk (6 mm in diameter, 0.1 mm in
thickness) or a glassy carbon rod (3 mm in diameter) was used for
the working electrode according to the scanning potential range.
Lithium foil was used for the counter and reference electrodes
in LiFSA–KFSA–CsFSA melts. In the cases of NaFSA–KFSA–CsFSA,
sodium metal lump was pressed to make a foil for the counter and
reference electrodes. The scan rate for the cyclic voltammetry was
−
1
1
0 mV s
Ionic conductivities of the eutectic LiFSA–KFSA–CsFSA melts
.
(
Li, Na, K), (Li, Na, Cs), (Li, K, Cs) and (Na, K, Cs) were prepared by
were measured with an AC impedance method using an electro-
chemical interface and impedance analyzer (IVIUMSTAT, Ivium
technologies). The measurement device was composed of Pyrex
glass container, platinum disk electrodes connected with copper
bar and a thermocouple covered with a Pyrex glass tube. The
cell constant was determined by the measurement of a KCl stan-
thoroughly mixing the constituent single salts using a mortar and
a pestle.
Melting and thermal decomposition temperatures of the ternary
mixtures were measured by means of a differential scanning
calorimeter (DSC-60 (Shimadzu Co., Ltd.)) and a thermogravime-
ter (DTG-60/60H (Shimadzu Co., Ltd.)), respectively. Observations
of the phase transitions were conducted for mixtures at intervals of
dard solution at 291 K and LiNO –KNO3 eutectic molten salt at
3
4
13–573 K.
0.10 in the mole fraction range of 0.10–0.90. Samples were placed
in aluminum sealed pans. The DSC measurements were performed
−1
3. Results and discussion
at rate of 2 K min under N2 atmosphere. The transition temper-
atures were determined in the heating process in order to avoid
uncertainty by supercooling. The phase diagrams of the ternary
MFSA systems were constructed by plotting the temperatures of
endothermic peaks found in the DSC curves against the composi-
tions of the salts.
3.1. Phase diagrams of MFSA ternary systems
The ternary phase diagrams constructed for LiFSA–NaFSA–KFSA,
LiFSA–NaFSA–CsFSA, LiFSA–KFSA–CsFSA and NaFSA–KFSA–CsFSA
systems are shown in Figs. 1–4. In the diagrams, liquidus surfaces
are shown as a contour map. Temperatures are given in K and
the following abbreviations are used: EB, binary eutectic point; ET,
ternary eutectic point.
Fig. 1 shows the ternary phase diagram of LiFSA–NaFSA–KFSA
system. This system is classified into a simple eutectic type as
expected by the results of the binary mixtures. The ternary eutectic
Electrochemical windows of the eutectic LiFSA–KFSA–CsFSA
and NaFSA–KFSA–CsFSA melts were measured by means of cyclic
voltammetry. The limiting potentials are defined as the potentials
−2
at a current density of 0.1 mA cm . The electrochemical mea-
surements were performed by the three-electrode method in an
Table 2
point is found at (x , xNa, xK) = (0.30, 0.40, 0.30) with the tempera-
Li
The eutectic temperatures and compositions of MFSA binary systems [13].
ture of 318 K. The eutectic composition was not determined more
accurately because similar DSC traces were observed at the com-
System
Composition
T (K)
positions around (x , xNa, xK) = (0.30, 0.40, 0.30). Figs. 2–4 show the
LiFSA–NaFSA
LiFSA–KFSA
LiFSA–RbFSA
LiFSA–CsFSA
NaFSA–KFSA
NaFSA–RbFSA
NaFSA–CsFSA
KFSA–RbFSA
KFSA–CsFSA
RbFSA–CsFSA
xLi = 0.40, xNa = 0.60
xLi = 0.41, xK = 0.59
xLi = 0.38, xRb = 0.62
xLi = 0.47, xCs = 0.53
xNa = 0.56, xK = 0.44
xNa = 0.50, xRb = 0.50
xNa = 0.47, xCs = 0.53
xK = 0.31, xRb = 0.69
xK = 0.54, xCs = 0.46
xRb = 0.65, xCs = 0.35
349
341
337
335
334
328
325
354
336
360
Li
ternary phase diagrams of LiFSA–NaFSA–CsFSA, LiFSA–KFSA–CsFSA
and NaFSA–KFSA–CsFSA systems, respectively. These three systems
are also classified into a simple eutectic type as expected. Table 3
summarizes the compositions and temperatures for eutectic points
of these systems. It is found that the ternary MFSA systems possess
the only one eutectic point at around equimolar composition with
the temperatures of 309–318 K, which are lower than those of con-
stituent binary subsystems. In this study, we constructed the basic

3
22
K. Kubota et al. / Electrochimica Acta 66 (2012) 320–324
Table 3
400
The eutectic temperatures and compositions of MFSA ternary systems.
System
Composition
T (K)
3
90
LiFSA–NaFSA–KFSA
LiFSA–NaFSA–CsFSA
LiFSA–KFSA–CsFSA
NaFSA–KFSA–CsFSA
E: xLi = 0.30, xNa = 0.40, xK = 0.30
E: xLi = 0.30, xNa = 0.40, xCs = 0.30
E: xLi = 0.30, xK = 0.35, xCs = 0.35
E: xNa = 0.40, xK = 0.25, xCs = 0.35
318
311
312
309
3
80
370
360
EB
50
350
E
phase diagrams in order to investigate physicochemical properties
of these alkali metal salts at the lowest possible melting tempera-
tures. Construction of detailed phase diagrams is interesting future
task.
The thermal decomposition temperatures of all the eutectic
mixtures were measured by thermogravimetric analysis. It was
confirmed that the decomposition temperature of each ternary sys-
tem is determined by the lowest decomposition temperature of the
constituent single salt. Thus, there is no effect of the mixing of MFSA
salts on the decomposition temperatures.
B
3
350
3
60
E
T
360
3
30
3
70
3
70
340
3
80
3
80
CsFSA
EB
NaFSA
Fig. 2. Phase diagram of LiFSA–NaFSA–CsFSA system.
According to our previous study [13], the thermal decomposi-
tion temperature of LiFSA is as low as 343 K. Concerning the four
binary eutectic mixtures containing LiFSA, the eutectic tempera-
tures range from 335 to 349 K, as shown in Table 2. Thus, the binary
systems have no or very narrow temperature ranges in which the
melts are stable as liquid. For the ternary systems, however, they
possess the eutectic temperatures of 311–318 K, indicating that the
temperature ranges of stable liquid are extended. Moreover, all the
ternary MFSA eutectic melts are unique in the point that they con-
sist of entirely inorganic ions. To the best of our knowledge, the
eutectic temperatures for ternary MFSA melts are lower than melt-
ing temperatures for any other inorganic ionic liquids or molten
salts containing alkali metal cations.
LiFSA
4
00
3
90
3
80
70
60
3
3
3
50
EB
E
B
3
30
E
T
3
60
3.2. Electrochemical widows of selected eutectic melts
3
40
3
70
350
360
70
In our previous study, the order of deposition potential was
determined to be Na > Li > (K, Rb, Cs) for binary MTFSA melts [2].
Concerning the binary MFSA melts, it was assumed that the depo-
sition potential of sodium was also more positive than that of
lithium. Thus, in order to deposit lithium from the ternary MFSA
melts, NaFSA should not be included as a constituent salt based
on this assumption. Therefore, the LiFSA–KFSA–CsFSA eutectic
melt was selected for electrochemical measurement. On the other
hand, it is expected that the deposition of sodium always occurs
at the most positive potential in the melts containing sodium
ion. Considering the relatively poor thermal stability of LiFSA, the
NaFSA–KFSA–CsFSA eutectic melt was selected as the electrolyte
for sodium deposition avoiding lithium salt in the system.
3
50
3
80
3
CsFSA
KFSA
EB
Fig. 3. Phase diagram of LiFSA–KFSA–CsFSA system.
NaFSA
3
70
Fig. 5 shows cyclic voltammograms in the LiFSA–KFSA–CsFSA
eutectic melt at 323 K. A pair of cathodic and anodic currents is
observed at 0 V vs. Li /Li on a copper electrode. These cathodic and
3
60
+
3
50
3
anodic currents are interpreted as the deposition of lithium metal
and its dissolution, respectively. On the other hand, an anodic cur-
40
EB
EB
3
30
+
rent is observed at 5.4 V vs. Li /Li on a glassy carbon electrode. This
ET
340
50
anodic current is considered to correspond to the irreversible oxi-
dation of FSA anion, though more studies are required to elucidate
the detail of this reaction. The electrochemical window is deter-
mined as 5.4 V at 323 K, when the limiting potentials are defined
3
3
60
−
2
as the potential at a current density of 0.1 mA cm . This new inor-
ganic ionic liquid is highly promising as an electrolyte for lithium
secondary batteries due to its high content of lithium ion, reversible
electrodeposition of lithium metal and wide electrochemical win-
dow.
3
70
3
80
370
EB
CsFSA
KFSA
360
350
Fig. 6 shows cyclic voltammograms in the NaFSA–KFSA–CsFSA
eutectic melt at 323 K. A pair of cathodic and anodic currents is
Fig. 4. Phase diagram of NaFSA–KFSA–CsFSA system.

K. Kubota et al. / Electrochimica Acta 66 (2012) 320–324
323
2
.5
1
1.4
1
0
1
.2
1
.5
0
0
.8
-
-
0.5
0.6
-
1
0
0
.4
.2
0
1.5
-
2
-
2
-1
0
1
2
3
4
5
6
7
8
+
Potential vs. (Li/Li )/V
310
320
330
340
Temperature / K
Fig. 5. A combined cyclic voltammogram at a Cu electrode (negative potential
region, −1.0 to 2.7 V) and a glassy carbon electrode (positive potential region,
−
1
Fig. 7. Temperature dependence of ionic conductivity in LiFSA–KFSA–CsFSA eutec-
2
.8–7.5 V) in LiFSA–KFSA–CsFSA eutectic melt at 323 K (scan rate: 10 mV s ).
tic melt at 313–338 K.
+
observed at 0 V vs. Na /Na on a copper electrode. These cathodic and
anodic currents are interpreted as the deposition of sodium metal
solid-electrolyte interface and/or stability of the eutectic elec-
trolyte in contact with active electrodes in battery applications.
and its dissolution, respectively. An anodic current is observed at
.9 V vs. Na /Na on a glassy carbon electrode. This anodic current
is considered to correspond to the oxidation of FSA anion as in
the case of LiFSA–KFSA–CsFSA eutectic melt. The electrochemical
window in the same definition as above is determined as 4.9 V at
+
4
4. Conclusions
The ternary phase diagrams were constructed for
LiFSA–NaFSA–KFSA, LiFSA–NaFSA–CsFSA, LiFSA–KFSA–CsFSA
and NaFSA–KFSA–CsFSA systems and the ternary eutectic tem-
peratures and compositions were determined. All the ternary
systems possess the only one eutectic point around equimolar
composition with the temperatures of 309–318 K. These tempera-
tures are lower than melting temperatures for any other inorganic
ionic liquids or molten salts containing alkali metal cations. The
electrochemical window of the eutectic LiFSA–KFSA–CsFSA ((xLi,
xK, xCs) = (0.30, 0.35, 0.35)) was determined to be 5.4 V at 323 K
with the cathode limit identified as lithium metal deposition. The
electrochemical window of the eutectic NaFSA–KFSA–CsFSA ((xNa,
xK, xCs) = (0.40, 0.25, 0.35)) was 4.9 V at 323 K with sodium metal
deposition as the cathode limit reaction. The results of this study
evidenced the possibility to prepare new ionic liquids starting
from alkali salts that exhibit much higher melting points. The
new inorganic ionic liquids of ternary MTFSA are expected to be
used for electrochemical applications such as lithium and sodium
secondary batteries.
3
23 K. The NaFSA–KFSA–CsFSA eutectic melt is full of promise as
an electrolyte for sodium secondary batteries for its high content
of sodium ion, reversible electrodeposition of sodium metal and
wide electrochemical window. This melt has another advantage
of higher thermal stability compared with the LiFSA–KFSA–CsFSA
eutectic melt.
Fig. 7 shows ionic conductivity of LiFSA–KFSA–CsFSA eutec-
tic melt at 313–338 K. At 323 K, the conductivity of this melt is
−1
0.41 mS cm , which is much lower than that of the salt mixture
containing an organic cation-based ionic liquids such as PY14FSA-
LiFSA [12]. However, the absence of organic solvent/ions will offer
very interesting opportunities in investigating the effects of a
2
1
0
.5
1
Acknowledgement
.5
0
This work was financially supported by a Grant in Aid for Sci-
entific Research for Priority Area “Science of Ionic Liquids” from
Japanese Ministry of Education, Culture, Sports, Science and Tech-
nology.
-
-
0.5
-1
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[
[
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