Ionic liquids based on (fluorosulfonyl)(pentafluoroethanesulfonyl)imide with various oniums

Article abstract of DOI:10.1016/j.electacta.2010.06.085

New hydrophobic ionic liquids based on (fluorosulfonyl) (pentafluoroethanesulfonyl)imide ([(FSO₂)(C₂F ₅SO₂)N]^-, FPFSI^-) anion with various oniums, including imidazolium, tetraalkyl ammonium, pyrrolidinium, and piperidinium, were prepared and characterized. Their physicochemical and electrochemical properties, including phase transitions, thermal stability, viscosity, density, specific conductivity and electrochemical windows, were extensively characterized, and were comparatively studied with the corresponding ionic liquids containing the isomeric but symmetric TFSI^- ([(CF ₃SO₂)₂N]^-) anion. These new FPFSI^--based ionic liquids display low melting points, low viscosities, good thermal stability, and wide electrochemical windows allowing Li deposition/dissolution. All these desired properties suggest they are potential electrolyte materials for Li (or Li-ion) batteries.

Full text of DOI:10.1016/j.electacta.2010.06.085

Electrochimica Acta 55 (2010) 7145–7151
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Ionic liquids based on (fluorosulfonyl)(pentafluoroethanesulfonyl)imide
with various oniums
a
a
a
b
a,∗
a
b
Kai Liu , Yi-Xuan Zhou , Hong-Bo Han , Si-Si Zhou , Wen-Fang Feng , Jin Nie , Hong Li ,
b
c,∗
a,∗
Xue-Jie Huang , Michel Armand , Zhi-Bin Zhou
School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan 430074, China
Institute of Physics, Chinese Academy of Sciences, 3rd South Street, Zhongguancun, Beijing 100080, China
a
b
c
Laboratoire de Réactivité et de Chimie des Solides (LRCS), CNRS 6007, UPJV, 33 rue St. Leu, 80039 Amiens, France
a r t i c l e i n f o
a b s t r a c t
Article history:
New hydrophobic ionic liquids based on (fluorosulfonyl)(pentafluoroethanesulfonyl)imide
(
Received 13 April 2010
Received in revised form 29 June 2010
Accepted 30 June 2010
− −
[(FSO2)(C2F5SO2)N] , FPFSI ) anion with various oniums, including imidazolium, tetraalkyl ammo-
nium, pyrrolidinium, and piperidinium, were prepared and characterized. Their physicochemical and
electrochemical properties, including phase transitions, thermal stability, viscosity, density, specific con-
ductivity and electrochemical windows, were extensively characterized, and were comparatively studied
Available online 6 July 2010
−
−
with the corresponding ionic liquids containing the isomeric but symmetric TFSI ([(CF3SO2)2N] ) anion.
Keywords:
Ionic liquids
−
These new FPFSI -based ionic liquids display low melting points, low viscosities, good thermal stability,
and wide electrochemical windows allowing Li deposition/dissolution. All these desired properties
suggest they are potential electrolyte materials for Li (or Li-ion) batteries.
© 2010 Elsevier Ltd. All rights reserved.
(
Fluorosulfonyl)(pentafluoroethanesulfonyl)imide
Oniums
Lithium batteries
1
. Introduction
In recent years, ionic liquids (ILs) have been intensively stud-
not only with Li metal electrode [19] but also with graphitized
carbon electrode for Li-ion batteries [20], which was not previ-
ously seen with ILs with other anions. Additionally, better rate
−
ied as potentially safe solvents for Li (or Li-ion) batteries and
dye-sensitized solar cells, and as solvent-free electrolytes for elec-
trochemical capacitors, because of their desirable properties, such
as low-volatility and non-flammability, and high thermal stability
capabilities were obtained with the low-viscosity, FSI -based ILs
as solvents [19]. Although the mechanism underlying the interfa-
−
cial formation between the FSI -based ILs and carbon (or Li metal)
anode is yet to be understood [21–24], it seems that the fluorosul-
−
[
1–4].
Previous work on fundamental understanding and applications
fonyl (FSO -)group in the FSI anion plays a crucial role in creating
2
this unique interfacial compatibility. For further understanding the
interactions at the ILs/carbon (or Li) anode interface and ultimately
realizing ILs as solvents for Li (or Li-ion) batteries, it is important to
design ILs based on new fluorinated sulfonimide anions containing
of ILs as possible safe solvents or co-solvents for Li (or Li-ion)
batteries mainly focused on the combination of bis(trifluoro-
methanesulfonyl)imide ([N(SO CF ) ] , TFSI ) anion with various
−
−
2
3 2
cations [5–18]. Recently, more effort has been devoted to the ILs
and lithium salts based on fluorosulfonylimide anions as elec-
trolytes for Li (or Li-ion) batteries [19–32], since the discovery
the FSO -group.
2
Thus far, a large number of ILs based on symmetric perfluori-
nated sulfonimide anion, such as TFSI [5–18] and FSI [19–29],
have been reported. However, little attention has been paid
−
−
of lithium bis(fluorosulfonyl)imide (Li[N(SO F) ], LiFSI), being a
2
2
potential conducting salt for Li-ion batteries [33,34]. It has been rec-
ognized that pure ionic liquid electrolytes based on the FSI anion,
to the ILs based on the asymmetric (fluorosulfonyl)(perfluoro-
−
−
alkanesulfonyl)imide [(FSO )(RFSO )N]
(RF = CmF2m+1
,
m > 1)
2
2
without any additives other than a lithium salt, were compatible
anions, except for the (fluorosulfonyl)(trifluoromethanesulfonyl)
−
−
imide [(FSO )(CF SO )N]
(FTFSI ) reported by Matsumoto
2
3
2
[
30,31]. This is mainly caused by lack of effective preparation
−
methods for asymmetric [(FSO )(RFSO )N] anions. Very recently,
2
2
∗ Corresponding authors.
E-mail addresses: fengwenfang@mail.hust.edu.cn (W.-F. Feng),
michel.armand@sc.u-picardie.fr (M. Armand), zb-zhou@mail.hust.edu.cn
we have developed a convenient method to prepare (fluoro-
sulfonyl)(pentafluoroethanesulfonyl)imide ((FSO )(C F5SO )NH,
2
2
2
(
Z.-B. Zhou).
H[FPFSI]) and its alkali salts [32]. All these alkaline salts show low
0
013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2010.06.085

7
146
K. Liu et al. / Electrochimica Acta 55 (2010) 7145–7151
◦
from room temperature to 600 C under a flow of helium. The
loss of 5% weight was defined as the decomposition temperature
(
T ).
d
The water content in ionic liquids was detected by a coulometric
Karl Fischer titrator (Metrohm KF 831).
The viscosity (ꢀ) of ionic liquids was measured on a pro-
◦
grammable viscometer (Brookfield, DV-III+) at 25 C, and the
◦
temperature was accurately controlled to ± 0.1 C with a Brookfield
TC-502 oil bath.
The density (ꢁ) of ionic liquids was the mean of three measure-
ment values, which were determined by measuring the weight of
three 1.0 mL samples at 25 C.
Fig. 1. Structure of the cation and anion used for ionic liquids.
◦
The specific conductivity (ꢂ) of ionic liquids was measured
in a sealed platinum black disk conductivity cell (cell constant:
melting points and interesting plastic crystal properties in ambient
temperature region [32], making them attractive for electrolyte
materials.
−
1
9
.96 cm ) by using ac impedance spectroscopic technique. The
ac impedance spectra was recorded with an Autolab PGSTAT302N
impedance analyzer (Eco Chemie, Netherland) in the frequency
In the present study, we wish to introduce a new family of
range from 10 to 106 Hz. The temperature of the cell was accu-
−
2
−
ILs based on the novel asymmetric anion [(FSO )(C F5SO )N]
2
2
2
◦
rately controlled to ± 0.1 C with a JULABO F12 oil bath. The cell
−
(
1
FPFSI ). Four representative ammonium cations, including
was calibrated with 0.1 M KCl aqueous solution before and after
each sample measurement.
+
-ethyl-3-methylimidazolium
(EMI ),
tetraethylammonium
(
N2222+), N-methyl-N-propylpyrrolidinium (PY13+), and N-methyl-
The linear sweep voltammetry of ionic liquids was measured
on an Autolab PGSTAT302N workstation (Eco Chemie, Nether-
N-propylpiperidinium (PI13+), were chosen to be combined with
−
FPFSI to form the ILs. Their physicochemical and electrochemical
land) in an argon-filled glove box (O2 and H O < 1 ppm) by
2
properties were measured, and comparatively studied with those
using a 5 mL beaker-type three-electrode cell equipped with a
−
of the ILs containing the isomeric but symmetric TFSI anion. The
−
3
2
glassy carbon electrode (surface area: 7.85 × 10 cm ), a Pt wire
structures of the cation and anion for the ILs in the present study
are shown in Fig. 1.
−
−
counter electrode, and an I /I reference electrode consisting
3
−
3
−3
of Pt wire/0.015 mol dm
I2 + 0.060 mol dm [(n-C H7) N]I in
3
4
EMI[TFSI] contained in a side glass-frit tube. The potential was
referred to ferrocene (Fc)/ferricinium (Fc ) redox couple for each
2
. Experimental
+
salt. The data for each salt were collected in the first cathodic and
anodic scan. For Li deposition/stripping, Ni discs (0.06 cm ) served
2.1. General remarks
2
as working electrode, and Li foil used as both counter and quasi-
reference electrodes.
Commercially available reagents were obtained from
Sinopharm Chemical Reagent Co. (China), Shanghai Chemical
Reagent Co. (China), and Sigma–Aldrich, and were analytical
grade and used as received, unless otherwise specified. Lithium
bis(trifluoromethanesulfonyl)imide (LiTFSI) was purchased from
Rhodia. All the solvents were dried and purified using standard
procedures before use. All the procedures with moisture sensitive
reagents were performed under a dry argon atmosphere or in a
2.3. Preparation
General procedures for preparation of ionic liquids. The
ionic liquids based on (fluorosulfonyl)(pentafluoroethanesulfo-
−
−
nyl)imide ([(FSO )(C F5SO )N] , FPFSI ) and bis(trifluoro-
2
2
2
glove box (Mikrouna, H O and O < 1 ppm).
− −
methanesulfonyl)imide ([(CF SO ) N] , TFSI ) were prepared
3 2 2
2
2
Potassium (fluorosulfonyl)(pentafluoroethanesulfonyl)imide
KFPFSI) was prepared according to recent literature [32].
by a metathesis reaction between the corresponding bromide
salt with the desired cation and a slight excess of KFPFSI or
LiTFSI (1.05 equiv.). In a typical reaction, to a stirred solution of
1-ethyl-3-methylimidazolium bromide (EMI-Br) (5.7 g, 30 mmol)
in water (12 mL) was added KFPFSI (10.1 g, 31 mmol) at room
temperature. The mixture was stirred for an additional 30 min.
The bottom layer was separated, dissolved in CH2Cl2 (15 mL), and
washed with doubly-distilled deionized water (5 mL) until no
more bromide anion was detected by AgNO3 test. After removal of
CH2Cl2 at reduced pressure, the liquid obtained was dried under
(
2
.2. Instrumentation
1H NMR (399.65 MHz), and 19F NMR (376.05 MHz) spectra were
recorded on a Bruker AV400 spectrometer. Chemical shift values in
1
19
ppm for H NMR spectra were referenced to TMS, and those for
F
NMR spectra were referenced to external CCl F. Element analyses
3
(
C, H, and N) were performed on an Elementar Vario MicroCube
elemental analyzer.
◦
vacuum (70 C, 1–2 mm Hg) for 24 h to afford EMI[FPFSI] (10.2 g,
The phase transitions for ionic liquids were measured on a
Netsch 200F3 DSC instrument. Samples of average weight of ca.
yield 86%) as a colorless liquid (water content 30 ppm). The others
were similarly prepared in a 30-mmol scale, and the water content
in the ionic liquids was less than 30–40 ppm after drying under
vacuum.
5
mg were hermetically sealed in an aluminum pan in a glove box,
◦
−1
◦
and then heated and cooled at a rate of 10 C min from −150 C
to the temperature just past the melting point under nitrogen flow.
The glass transition temperature (Tg, onset of the heat capacity
change), crystallization temperature (Tc, onset of the exothermic
peak), solid–solid transition (Ts–s, onset of the endothermic peak),
and melting point (Tm, onset of the endothermic peak) were taken
during the heating scans.
2.3.1. 1-Ethyl-3-methylimidazolium
(
fluorosulfonyl)(pentafluoroethanesulfonyl)-imide (EMI[FPFSI])
1
H NMR (399.76 MHz; acetone-d ; TMS): 1.58 (t, J = 7.2 Hz, 3H),
6
4
1
.06 (s, 3H), 4.41 (q, J = 7.2 Hz, 2H), 7.70 (s, 1H), 7.77 (s, 1H), 9.0 (s,
19
H). F NMR (376.05 MHz; acetone-d ; CCl F): 56.4 (s, 1F), −80.1
Thermal gravimetric analysis (TGA) was performed on
a
6
3
(s, 3F), −117.9 ppm (s, 2F). Anal. Found: C, 24.39; H, 2.72; N, 10.65.
PerkinElmer Pyris 1 TGA system. Samples of average weight of ca.
◦
−1
Calc. for C H F N O S : C, 24.55; H, 2.83; N, 10.74.
5
mg were placed in a platinum pan and heated at 10 C min
8
11
6
3
4 2

K. Liu et al. / Electrochimica Acta 55 (2010) 7145–7151
7147
2.3.2. Tetraethylammonium
2.3.7. N-Methyl-N-propylpyrrolidinium
(
fluorosulfonyl)(pentafluoroethanesulfonyl)imide (N2222[FPFSI])
bis(trifluoromethanesulfonyl)imide (PY13[TFSI])
Colorless liquid, yield 88%. 1H NMR (399.76 MHz; acetone-d ;
Colorless liquid, yield 89%. 1H NMR (399.76 MHz; acetone-d6;
TMS): ı = 1.02 (t, J = 7.2 Hz, 3H), 1.96 (m, 2H), 2.33 (m, 4H), 3.25 (s,
3H), 3.50 (m, 2H), 3.72 (m, 4H). 19F NMR (376.05 MHz; acetone-d6;
6
9
1
TMS): ı = 1.37 (t, J = 7.2 Hz, 4 × 3H), 3.46 (q, J = 7.2 Hz, 4 × 2H).
F
NMR (376.05 MHz; acetone-d ; CCl F): 56.4 (s, 1F), −80.1 (s, 3F),
6
3
−
117.9 (s, 2F). Anal. Found: C, 29.62; H, 4.86; N, 6.93. Calc. for
CCl F): ı = −79.9 (s, 3F). Anal. Found: C, 29.54; H, 4.28; N, 6.65. Calc.
3
C
10H20F N O S : C, 29.27; H, 4.91; N, 6.83.
for C H F N O S : C, 29.41; H, 4.44; N, 6.86.
10 18 6 2 4 2
6
2
4
2
2.3.8. N-Methyl-N-propylpiperidinium
2
.3.3. N-Methyl-N-
bis(trifluoromethanesulfonyl)imide (PI13[TFSI])
propylpyrrolidinium(fluorosulfonyl)(pentafluoroethanesulfonyl)imide
Colorless liquid, yield 92%. 1H NMR (399.76 MHz; acetone-d6;
TMS): ı = 1.03 (t, J = 7.2 Hz, 3H), 1.75 (m, 2H), 1.93 (m, 2H), 2.07 (m,
(
PY13[FPFSI])
Colorless liquid, yield 87%. 1H NMR (399.76 MHz; acetone-d6;
4H), 3.25 (s, 3H), 3.50 (m, 2H), 3.56 (m, 4H); F NMR (376.05 MHz;
19
TMS): 1.02 (t, J = 7.2 Hz, 3H), 1.96 (m, 2H), 2.33 (m, 4H), 3.25 (s,
H), 3.50 (m, 2H), 3.72 (m, 4H). 19F NMR (376.05 MHz; acetone-d6;
CCl F): 56.4 (s, 1F), −80.1 (s, 3F), −117.9 (s, 2F). Anal. Found: C,
acetone-d ; CCl F): ı = −79.9 (s, 3F). Anal. Found: C, 31.43; H, 4.65;
6
3
3
N, 6.73. Calc. For C H F N O S : C, 31.28; H, 4.77; N, 6.63.
11 20 6 2 4 2
3
2
9.35; H, 4.19; N, 6.59. Calc. for C1
H
18
F N O S : C, 29.41; H, 4.44;
3. Results and discussion
.1. Preparation and characterization
0
6
2
4 2
N, 6.86.
3
−
The four new FPFSI -based ILs (Fig. 1) are hydrophobic, and
2
.3.4. N-Methyl-N-
propylpiperidinium(fluorosulfonyl)(pentafluoroethanesulfonyl)imide
easily prepared in high quality and yield by metathesis reaction
between the corresponding ammonium bromide and equivalent
KFPFSI in water. The structures and compositions of these new ILs
(
PI13[FPFSI])
Colorless liquid, yield 91%. 1H NMR (399.76 MHz; acetone-d6;
1
19
TMS): 1.03 (t, J = 7.2 Hz, 3H), 1.75 (m, 2H), 1.93 (m, 2H), 2.07 (m, 4H),
.25 (s, 3H), 3.50 (m, 2H), 3.56 ppm (m, 4H). 19F NMR (376.05 MHz;
acetone-d ; CCl F): 56.4 (s, 1F), −80.1 (s, 3F), −117.9 (s, 2F). Anal.
were confirmed by NMR ( H, and F), and elemental analysis (see
Section 2.3).
3
6
3
The physicochemical and electrochemical properties of these
−
Found: C, 31.55; H, 4.49; N, 6.56. Calc. For C11
H
20
F N O S : C,
new FPFSI -based ILs, including glass transition temperature (T ),
6
2
4
2
g
3
1.28; H, 4.77; N, 6.63.
crystallization temperature (T ), solid–solid transition temperature
c
(Ts–s), and melting point (Tm), thermal stability, density, viscosity,
ionic conductivity, and electrochemical stability were measured.
Some of the data are summarized in Table 1. For comparison,
the four corresponding ILs containing the isomeric but symmet-
2
.3.5. 1-Ethyl-3-methylimidazolium
bis(trifluoromethanesulfonyl)imide (EMI[TFSI])
Colorless liquid, yield 90%. 1H NMR (399.76 MHz; acetone-d6;
TMS): ı = 1.57 (t, J = 7.2 Hz, 3H), 4.06 (s, 3H), 4.41 (q, J = 7.2 Hz, 2H),
−
ric TFSI anion were prepared, and their corresponding properties
(Table 1) were also measured under the same conditions. The mea-
7
.70 (s, 1H), 7.76 (s, 1H), 9.0 (s, 1H); 19F NMR (376.05 MHz; acetone-
sured data for these four TFSI -based ILs in the present study are
−
d ; CCl F): ı = −79.9 (s, 3F). Anal. Found: C, 24.66; H, 2.56; N, 11.05.
in agreement well with those reported in literature (see Table 1)
[5–8,11,17,19,35]. For examples, for melting point ( C): EMI[TFSI]
6
3
◦
Calc. for C H F N O S : C, 24.55; H, 2.83; N, 10.74.
8
11
6
3
4 2
(
[
−10 in Table 1 vs. −12 [11]), N2222[TFSI] (105 in Table 1 vs. 109
5], 105 [7], 104 [35]), PY13[TFSI] (11 in Table 1 vs. 12 [6], 9 [17]),
◦
2
(
.3.6. Tetraethylammonium bis(trifluoromethanesulfonyl)imide
N2222[TFSI])
White solid, yield 91%. H NMR (399.76 MHz; acetone-d ; TMS):
PI13[TFSI] (12 in Table 1 vs. 12 [8,11]); for viscosity (cP) at 25 C:
EMI[TFSI] (34 in Table 1 vs. 33 [11], PY13[TFSI]) (60 in Table 1 vs.
63 [6], 61 [19]), PI13[TFSI] (149 in Table 1 vs. 151 [11,19]); for spe-
cific conductivity (mS cm ) at 25 C: EMI[TFSI] (8.8 in Table 1 vs.
8.7 [11], 8.3 [19]), PY13[TFSI] (3.8 in Table 1 vs. 3.6 [10], 3.9 [19],
1
6
1
9
−1
◦
ı = 1.37 (t, J = 7.2 Hz, 4 × 3H), 3.46 (q, J = 7.2 Hz, 4 × 2H); F NMR
(
2
376.05 MHz; acetone-d ; CCl F): ı = −79.9 (s, 3F). Anal. Found: C,
6
3
◦
9.24; H, 4.78; N, 6.79. Calc. for C1
H
20
F N O S : C, 29.27; H, 4.91;
2.7 at 20 C [17]), PI13[TFSI] (1.5 in Table 1 vs. 1.5 [8], 1.4 [11,19]).
0
6
2
4 2
N, 6.83.
The data from different research groups for the important proper-
Table 1
−
−
Physicochemical properties of ionic liquids based on (fluorosulfonyl)(pentafluoroethanesulfonyl)imide (FPFSI ) and bis(trifluoromethanesulfonyl)imide (TFSI ).
Tga ( C)
◦
Tcb ( C)
◦
Ts–sc ( C)
◦
Tmd ( C)
◦
Tde ( C)
◦
ꢁf (g cm−1
)
ꢀg (cP)
ꢂh (mS cm−1
)
H2O (ppm)
i
Entry
Ionic liquids
1
2
3
4
5
6
7
8
EMI[FPFSI]
N2222[FPFSI]
PY13[FPFSI]
PI13[FPFSI]
EMI[TFSI]
N2222[TFSI]
PY13[TFSI]
PI13[TFSI]
−101
−89
−99
−90
−91
−63
−46
−19
302
324
343
348
419
405
427
409
1.56
1.44
1.46
1.44
1.55
27
104
56
130
34
8.0
1.6
3.5
1.5
8.8
30
32
34
30
35
6
−58, −48
−10
105
11
8, 54
1.45
1.44
60
149
3.8
1.5
38
40
12
a
Glass transition temperature.
Crystallization temperature.
solid–solid transition temperature.
Melting point.
b
c
d
e
f
Decomposition temperature.
◦
Density at 25 C.
g
h
i
◦
Viscosity at 25 C.
Specific conductivity at 25 C.
◦
Water content.

7
148
K. Liu et al. / Electrochimica Acta 55 (2010) 7145–7151
Fig. 2. DSC traces of various ionic liquids.
−
ties of the reported TFSI -based ILs were well repeated under our
experimental conditions.
suggest that it is an effective approach to obtain low-melting ILs by
utilizing asymmetric anions. However, the less electrochemical sta-
bility of TSAC makes its ILs unfavourable for electrolyte uses for
−
Li (or Li-ion) batteries [15]. It is expected that the family of asym-
metric (fluorosulfonyl)(perfluoroalkanesulfonyl)imide anions with
good electrochemical stability (see Section 3.5) would be attrac-
tive to produce low-melting and electrochemically stable ILs for
electrolyte applications in future.
3
.2. Phase behavior and thermal stability
The phase behavior of the eight salts in Table 1 was investigated
by DSC, and their DSC traces are shown in Fig. 2. Generally, four
types of phase transition behavior were observed for all these ILs:
The thermal stability of these eight ILs in Table 1 was measured
by thermogravimetric analysis. Fig. 3 shows the thermogravimet-
(
1) only glass transition at a very low temperature without melt-
ing/crystallization (Entries 3 and 4), (2) only melting (Entries 7 and
), (3) a glass transition at a very low temperature, sequentially
followed by crystallization and melting (Entries 1, 2, and 5), and
4) solid–solid transitions (i.e. ionic plastic crystals [6,12]) before
melting (Entry 6).
As seen in Table 1, the FPFSI -based ILs show relatively
−
ric traces of these ILs. The FPFSI -based ILs are thermally stable to
8
◦
−
>
300 C. The stability of the FPFSI -based ILs is lower than that of
the TFSI -based ones (Table 1). The same trend was also observed
in their corresponding alkali salts [32]. Additionally, the lower ther-
−
(
−
◦
◦
low melting points (e.g. −19 C for EMI[FPFSI], and 6 C for
[FPFSI]). For the same cation, the melting points are lower
N
2222
−
−
for the FPFSI -based ILs than for the corresponding TFSI -based
◦
◦
ones, i.e. EMI[FPFSI] (−19 C) vs. EMI[TFSI] (−10 C), N2222[FPFSI]
◦
◦
(
6 C) vs. N
[TFSI] (105 C). Relatively low melting points were
2
222
−
also reported for the alkaline salts of FPFSI [32], and the
related FTFSI -based ILs [30]. This could be attributable to rel-
−
atively lower symmetry for these fluorosulfonimide anions. It
is anticipated that the low melting points for these ILs would
help to improve their low-temperature performances as elec-
◦
trolytes. The N2222[FPFSI] exhibit a low melting point at 6 C,
+
though the tetraethylammonium (N2222 ) cation with a rela-
tive small size and high symmetry, generally tends to form
high-melting salts with flexible but symmetric fluoroanions
−
−
(
[
(
e.g. TFSI ). Thus far, only two asymmetric fluoroanions, FTFSI
30] and 2,2,2-trifluoro-N-[(trifluoromethyl)sulfonyl]acetamide
−
−
[(CF SO )(CF CO)N] , TSAC ) [36], have been reported to form
3
2
3
+
low-melting salts with the small and high symmetry N
Tm 8.5 C for N
cation
2222
◦
◦
(
[FTFSI], and 21 C for N2222[TSAC]). These results
Fig. 3. TGA traces of various ionic liquids.
2
222

K. Liu et al. / Electrochimica Acta 55 (2010) 7145–7151
7149
Fig. 5. Temperature dependence of specific conductivity (ꢂ) of various ionic liquids.
Fig. 4. Temperature dependence of viscosity (ꢀ) of various ionic liquids.
−
or “aggregate” (sufficiently long-lived, and appear neutral) species
[38]. Since the neutral “ion-pair” or “aggregate” species occurring
in the neat ILs behave more like neutral molecular fluid, and are
inactive in conduction processes, thus the more associated the ILs
are, the more fluid but less conductive of the ionic liquid bulk.
Fig. 6 shows equivalent conductivity (ꢃ) vs. reciprocal of viscos-
mal stability was also reported for the FSI -based salts than for the
−
corresponding TFSI -based ones [28,29,37]. These results strongly
suggest that having a fluorosulfonyl group (FSO -) in the perfluori-
nated sulfonimide anion cause a decrease in thermal stability. These
2
could be ascribed to more liability of FSO -group toward pyrolysis,
2
as observed in their alkaline salts [29,32]. However, it is still well
acceptable for electrolyte applications for Li (or Li-ion) batteries.
−
1
ity (ꢀ ) in logarithm form for the seven room-temperature liquid
◦
salts in Table 1 (Entries 1–5, 7, and 8) at 25 C, where ꢃ = ꢂM/ꢁ, M,
ꢂ, and ꢁ are the formula weight, specific conductivity, and density
of the salts, respectively. As seen in Fig. 6, the plots of all these ILs
are well below the so-called “ideal” Walden product line, indicat-
ing that the ILs are not fully ionized. Walden product line is more
3
.3. Viscosity
−
All the FPFSI -based ILs show low viscosities (ꢀ), ranging
◦
from 27 to 127 cP at 25 C (Table 1). Their low viscosities could
be attributable to highly structure flexibility of S–N–S bond,
good charge distribution, and a medium ion size for the FPFSI
anion. The viscosities increase in the cation order: EMI < PY
<
−
close to the TFSI -based ILs (Nos. 5, 7, and 8 in Fig. 6) than to the
−
−
FPFSI -based ones (Nos. 1–4 in Fig. 6), indicating that the former are
+
+
more ionic than the latter. This is consistent with their conductivity
trend. It seems that better charge distribution and charge shield-
1
3
N2222+ < PI13+. This trend in viscosity is consistent well with those
−
−
−
ing in the symmetric TFSI plays a key role in creating more ion
for the corresponding ILs with TFSI and FSI anions [7,8,11,19].
availability (higher conductivity) in the ILs. The homologous and
Fig. 4 shows the temperature dependence of viscosity of the
−
−
isomeric FPFSI and TFSI anions are exactly comparable examples
that display the effect of ion shape on the ionic conduction [38].
seven room-temperature liquid salts in the temperature range of
2
◦
0–60 C. It is interesting to note that the viscosities are generally
−
−
lower for the FPFSI -based ILs than for the corresponding TFSI -
based ones in the measured temperature range (Table 1 and Fig. 4),
though these two anions are homologues with isomeric structure.
These are impressive examples to display the effect of shape of the
homologous and isomeric anion on the viscosity, which is rarely
observed in the reported literature. The lower viscosities for the
−
FPFSI -based ILs may be attributed to a slightly higher degree of
association or ion correlations in the ILs (see Section 3.4), thus rela-
tively larger quantities of neutral “ion-pair” or “aggregate” species
occurred [38], which makes the ionic liquid bulk reluctant to the
nature of molecular fluid.
3.4. Ionic conductivity
−
The specific conductivities (ꢂ) of the four FPFSI -based ILs lie
−1
◦
between 1.5 and 8.0 mS cm
at 25 C (Table 1), and decreased
in the order of EMI > PY13 > N2222+ ≈ PI . Fig. 5 shows the
+
+
+
13
temperature dependence of specific conductivities of the seven
room-temperature liquid salts in the temperature range from
◦ −
20 to 60 C. Unexpectedly, the ILs containing the FPFSI anion
−
generally show slightly lower specific conductivities than the corre-
sponding ones containing the isomeric TFSI anion in the measured
−
temperature region (see Fig. 5), though the viscosities are lower for
the former. This would be attributed to a slightly higher degree of
ion association or other correlations in the FPFSI -based ILs, which
Fig. 6. Equivalent conductivity (ꢃ) vs. reciprocal viscosity (ꢀ 1) at 25 ◦C in common
logarithm form for the seven room-temperature liquid salts (Table 1), and the num-
ber is consistent with Entry number in Table 1. The solid line is the “ideal” Walden
−
−
−
1
results in the formation of larger quantities of neutral “ion-pair”
product line fixed with 1 mol L aqueous KCl solutions.

7
150
K. Liu et al. / Electrochimica Acta 55 (2010) 7145–7151
Table 2
Cathodic and anodic limits (the values for Ecathodic and Eanodic vs. ferrocene
+
−2
(
Fc)/ferricinium (Fc ) were taken at the polarization current (i) of 1.0 mA cm
)
and electrochemical windows (EWs) for ionic liquids based on (fluorosul-
−
◦
fonyl)(pentafluoroethanesulfonyl)imide (FPFSI ) with various ammonium cations,
determined on a glassy carbon electrode at 25 C.
Ionic liquids
Ecathodic (V)
Eanodic (V)
EWs (V)
EMI[FPFSI]
N2222[FPFSI]
PY13[FPFSI]
PI13[FPFSI]
EMI[TFSI]
−2.32
−3.20
−3.02
−3.08
−2.47
−3.33
−3.42
2.22
2.76
2.98
2.82
2.07
2.56
2.60
4.54
5.96
5.98
5.90
4.54
5.89
6.02
PY13[TFSI]
PI13[TFSI]
Fig. 7. Linear sweep voltammogram of ionic liquids based on (a) (fluoro-
−
sulfonyl)(pentafluoroethanesulfonyl)imide ([FPFSI] ), and (b) bis(trifluoro-
−
methanesulfonyl)imide ([TFSI] ) anions. Working electrode: glassy carbon elec-
trode (surface area: 7.85 × 10− cm ); counter electrode: Pt wire; scan rate:
3
2
−
1
+
5
0 mV s ; potential (V) was referred to ferrocene (Fc)/ferricinium (Fc ) redox
couple in each ionic liquid.
3.5. Electrochemical stability
The electrochemical stability of the seven liquid salts in Table 1
was investigated by linear sweep voltammetry (LSV) on a glassy
carbon electrode under the same conditions. Fig. 7 shows LSV
curves of these ILs, where the potential (V) were referenced to inter-
+
nal ferrocene (Fc)/ferricinium (Fc ) redox couple. The values for the
cathodic (Ecathodic) and anodic (Eanodic) limits, and electrochemical
windows (EWs = Eanodic − E
) of these ILs, were summarized
cathodic
−
in Table 2. The measured data for the TFSI -based ILs are well
consistent with those reported by Matsumoto [19]. For examples,
the values for the Ecathodic and Eanodic (V vs. Fc/Fc+ at 1.0 mA cm ):
−2
EMI[TFSI] (−2.47 and 2.07 in Table 2 vs. −2.5 and 2.1 [19]), PI13[TFSI]
(
−3.42 and 2.60 in Table 2 vs. −3.4 and 2.5 [19]). As shown in Table 2
+
and Fig. 7, among the ILs studied, the ILs containing EMI cation
show less cathodic and anodic stability, due to lower electrochem-
ical stability of the imidazolium [8,11,15,19]. The electrochemical
windows of the FPFSI -based ILs are comparable with those of
the corresponding TFSI -based ones (see Table 2). However, it
−
−
Fig. 8. Cyclic voltammograms of various ionic liquids containing 0.3 mol kg 1 of
−
−
is notable that the FPFSI -based ILs are slightly resistant toward
LiTFSI: (a) PY13[FPFSI], (b) EMI[FPFSI], and (c) EMI[TFSI]. Working electrode: Ni elec-
2
reduction, but less resistant toward oxidation, compared with the
corresponding TFSI -based ones.
trode (surface area: 0.06 cm ); counter and reference electrodes: Li foil; scan rate:
−
5 mV s−1; measurement temperature: 25 ◦C.

K. Liu et al. / Electrochimica Acta 55 (2010) 7145–7151
7151
Fig. 8 shows the first three scans of cyclic voltammograms of
.3 mol kg of LiTFSI in the respective three ILs of (a) PY13[FPFSI],
b) EMI[FPFSI], and (c) EMI[TFSI] on fresh Ni electrode. Li deposi-
[3] N. Papageorgiou, Y. Athanassov, M. Armand, P. Bonhote, H. Pettersson, A. Azam,
M. Graetzel, J. Electrochem. Soc. 143 (1996) 3099.
−1
0
(
[
4] M. Ue, M. Takeda, A. Toriumi, A. Kominato, R. Hagiwara, Y. Ito, J. Electrochem.
Soc. 150 (2003) A499.
tion/stripping could be obtained not only in PY13[FPFSI] (Fig. 8a)
but also in EMI[FPFSI] (Fig. 8b), although EMI[FPFSI] has much
less cathodic stability (Fig. 7) for Li+ ion reduction, and is not
expected for allowing Li deposition/stripping [8,11,15]. This unique
behavior was only observed for the corresponding ILs containing
[5] J. Sun, M. Forsyth, D.R. MacFarlane, J. Phys. Chem. B 102 (1998) 8858.
[
6] D.R. MacFarlane, P. Meakin, J. Sun, N. Amini, M. Forsyth, J. Phys. Chem. B 103
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(
[
[8] H. Sakaebe, H. Matsumoto, Electrochem. Commun. 5 (2003) 594.
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[10] P.C. Howlett, D.R. MacFarlane, A.F. Hollenkamp, Electrochem. Solid-State Lett.
−
+
FSI anion [19]. However, no redox peak of Li ions, except for
7
(2004) A97.
+
irreversible reduction of the EMI cations, was observed in the
[11] H. Matsumoto, H. Sakaebe, K. Tatsumi, J. Power Sources 146 (2005) 45.
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Phys. Chem. B 110 (2006) 19593.
EMI[TFSI] as supporting electrolyte (Fig. 8c), also observed in lit-
−
erature [19]. These results suggest that (1) the FPFSI -based ILs
[
display unique electrochemical properties allowing them accept-
able and favourable for electrolyte use for Li (or Li-ion) batteries,
[14] H. Nakagawa, Y. Fujino, S. Kozono, Y. Katayama, T. Nukuda, H. Sakaebe, H.
Matsumoto, K. Tatsumi, J. Power Sources 174 (2007) 1021.
−
and (2) the FSO -group in the fluorosulfonimide anions (e.g. FSI
2
[
[
15] H. Sakaebe, H. Matsumoto, K. Tatsumi, Electrochim. Acta 53 (2007) 1048.
16] S. Seki, Y. Ohno, H. Miyashiro, Y. Kobayashi, A. Usami, Y. Mita, N. Terada, K.
Hayamizu, S. Tsuzuki, M. Watanabe, J. Electrochem. Soc. 155 (2008) A421.
17] G.B. Appetecchi, M. Montanino, D. Zane, M. Carewska, F. Alessandrini, S.
Passerini, Electrochim. Acta 54 (2009) 1325.
18] S. Ferrari, E. Quartarone, P. Mustarelli, A. Magistris, S. Protti, S. Lazzaroni, M.
Fagnoni, A. Albini, J. Power Sources 194 (2009) 45.
19] H. Matsumoto, H. Sakaebe, K. Tatsumi, M. Kikuta, E. Ishiko, M. Kono, J. Power
Sources 160 (2006) 1308.
−
and FPFSI ) would play a unique role in creating a favourable inter-
face between the ILs and electrode to allow Li deposition/stripping
process. Therefore, it is attractive to design new ILs based on (flu-
orosulfonyl)(perfluoroalkanesulfonyl)imide anions as electrolyte
materials for Li (or Li-ion) batteries in future.
[
[
[
[
[
[
4
. Conclusion
20] M. Ishikawa, T. Sugimoto, M. Kikuta, E. Ishiko, M. Kono, J. Power Sources 162
(
2006) 658.
21] T. Sugimoto, M. Kikuta, E. Ishiko, M. Kono, M. Ishikawa, J. Power Sources 183
2008) 436.
New hydrophobic ionic liquids, comprised of (fluorosul-
(
−
fonyl)(pentafluoroethanesulfonyl)imide (FPFSI ) anion and vari-
ous oniums, have been prepared and characterized. Compared
22] S. Seki, Y. Kobayashi, H. Miyashiro, Y. Ohno, Y. Mita, N. Terada, P. Charest, A.
Guerfi, K. Zaghib, J. Phys. Chem. C 11 (2008) 16708.
−
[23] G.B. Appetecchi, M. Montanino, A. Balducci, S.F. Lux, M. Winter, S. Passerini, J.
Power Sources 192 (2009) 599.
with the ILs with the isomeric but symmetric TFSI anion, these
ionic liquids show lower melting points, slightly low viscosities
and conductivities, comparable electrochemical windows. Li depo-
sition/stripping could be obtained not only in electrochemically
stable PY13[FPFSI] (PY13 = N-methyl-N-propylpyrrolidinium), but
also in less electrochemically stable EMI[FPFSI] (EMI = 1-ethyl-3-
methylimidazolium). All these promising properties may support
them as new ionic solvents or electrolytes for electrochemistry
applications, including Li (or Li-ion) batteries and electrochemical
capacitors.
[
24] S.F. Lux, M. Schmuck, G.B. Appetecchi, S. Passerini, M. Winter, A. Balducci, J.
Power Sources 192 (2009) 606.
[25] Y. Wang, K. Zaghib, A. Guerfi, F.F.C. Bazito, R.M. Torresi, J.R. Dahn, Electrochim.
Acta 52 (2007) 6346.
[
26] A. Guerfi, S. Duchesne, Y. Kobayashi, A. Vijh, K. Zaghib, J. Power Sources 175
2008) 866.
(
[27] Q. Zhou, W.A. Henderson, G.B. Appetecchi, M. Montanino, S. Passerini, J. Phys.
Chem. B 112 (2008) 13577.
[
28] H.-B. Han, J. Nie, K. Liu, W.-K. Li, W.-F. Feng, M. Armand, H. Matsumoto, Z.-B.
Zhou, Electrochim. Acta 55 (2010) 1221.
[29] K. Kubota, T. Nohira, T. Goto, R. Hagiwara, Electrochem. Commun. 10 (2008)
886.
1
[
30] H. Matsumoto, N. Terasawa, T. Umecky, S. Tsuzuki, H. Sakaebe, K. Asaka, K.
Tatsumi, Chem. Lett. 37 (2008) 1020.
Acknowledgements
[
[
31] T. Umecky, Y. Saito, H. Matsumoto, J. Phys. Chem. B 113 (2009) 8466.
32] H.-B. Han, Y.-X. Zhou, K. Liu, J. Nie, X.-J. Huang, M. Armand, Z.-B. Zhou, Chem.
Lett. 39 (2010) 472.
33] C Michot, M. Armand, J.Y. Sanchez, Y. Choquette, M. Gauthier, US 5916475
(1999).
We thank the National High Technology Research and Develop-
ment Program of China (No. 2007AA03Z246), the National Natural
Science Foundation of China (No. 50873041), and the Key Labo-
ratory of Organofluorine Chemistry, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences for the financial support.
[
[
[
34] K. Zaghib, P. Charest, A. Guerfi, J. Shim, M. Perrier, K. Striebel, J. Power Sources
146 (2005) 380.
35] W.A. Henderson, M. Herstedt, V.G. Young Jr., S. Passerini, H.C. De Long, P.C.
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