Liquid Electrolytes Based on Ionic Liquids for Lithium-Ion Batteries

Article abstract of DOI:10.1007/s10953-015-0408-z

Low-melting temperature ionic liquids (ILs), based on aliphatic quaternary ammonium N_1xxx and 1-ethyl-3-methylimidazolium cations with the imide anion, were prepared and characterized. The physicochemical and electrochemical properties of these

Full text of DOI:10.1007/s10953-015-0408-z

J Solution Chem
DOI 10.1007/s10953-015-0408-z
Liquid Electrolytes Based on Ionic Liquids for Lithium-Ion
Batteries
3
2
•
My Loan Phung Le1,2 Ngoc Anh Tran Hoang Phuong Khanh Ngo
•
•
Truong Giang Nguyen3 Van Man Tran2
•
Received: 6 April 2015 / Accepted: 12 September 2015
Ó Springer Science+Business Media New York 2015
Abstract Low-melting temperature ionic liquids (ILs), based on aliphatic quaternary
ammonium N1xxx and 1-ethyl-3-methylimidazolium cations with the imide anion, were
prepared and characterized. The physicochemical and electrochemical properties of these
ILs, including melting point, degradation temperature, viscosity, density, ionic conduc-
tivity and electrochemical stability, were determined. The fluorinated imidazolium cation
was also studied because of its better electrochemical stability compared to the non-
fluorinated form. Due to the high viscosity of aliphatic ammonium based ILs, addition of
an organic solvent to the ionic liquid was investigated and found to be without detriment to
the interesting properties of ILs (i.e., nonvolatility, non-flammability, high thermal sta-
bility, electrochemical stability). The cycling performance of the mixtures IL–ethylene
carbonate was characterized using LiMn2O4 based electrode materials in the voltage range
of 3.5–4.3 V.
Keywords Ionic liquid Á Electrochemical properties Á Lithium battery Á LiTFSI
Electronic supplementary material The online version of this article (doi:10.1007/s10953-015-0408-z)
contains supplementary material, which is available to authorized users.
&
1
My Loan Phung Le
lmlphung@hcmus.edu.vn
Applied Physical Chemistry Laboratory (APCLab), University of Science, VNU-HCM,
Ho Chi Minh, Vietnam
2
Department of Physical Chemistry, Faculty of Chemistry, University of Science, VNU-HCM,
Ho Chi Minh, Vietnam
3
Department of Organic Chemistry, Faculty of Chemistry, University of Science, VNU-HCM,
Ho Chi Minh, Vietnam
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J Solution Chem
1 Introduction
Organic solvents are widely used in commercial lithium ion batteries with electrolytes
having reasonable electrical conductivity and good compatibility with electrode materials.
Nevertheless, the reactivity and volatility of organic electrolytes are disadvantageous to
commercial applications [1]. In addition, they are all oxidized at potential over 4.5 V
versus Li?/Li, which can’t be used for high voltage electrode materials such as LiNi0.5
Mn1.5O4 which have working potential up to 5 V versus Li?/Li [2]. The use of ILs in
lithium ion batteries has been investigated for more than one decade [3] due to their
interesting properties, such as non-volatility, non-flammability, high thermal stability, wide
liquid range and wide electrochemical window. In addition, the choice of cation and anion
can be made to tailor the properties such as ionic conductivity, viscosity and electro-
chemical stability [4–6].
The most promising of ILs used in Li ion batteries are based on the anion bis(trifluo-
rormethanesulfonyl)imide, TFSI-, combined with different cations such as: quaternary
ammonium, imidazolium, pyrrolidinium, and piperidinium. Due to the high flexibility, low
geometric symmetry and good electrochemical stability of the TFSI- anion, these ILs
exhibit low-melting points and low viscosities. Imidazolium based ILs have low viscosities
which seems to make them promising electrolytes in lithium batteries; however, the
electrochemical stability seems to be insufficient for high voltage batteries (about 5 V vs.
Li?/Li) [7, 8]. The quaternary ammonium based ILs exhibit relatively better electro-
chemical stability toward lithium metal but the high viscosity is still a big challenge. As a
result, tailoring of the cation structure has been a recent key development in ionic liquid
application. Some different approaches were proposed, such as variation of the alkyl chain
length, addition of functional group (methoxy, nitrile,…) or using the –CF3 group to
replace the –CH3 group of alkyl chains [5, 6]. Furthermore, the high viscosity of ILs can be
also decreased by adding a small amount of polar organic solvent (e.g., ethylene carbonate,
dimethyl carbonate) without negative effects on dissociation of the ILs and electrochemical
stability [9, 10].
In this work, we studied the physicochemical and electrochemical properties of ILs
based on aliphatic quaternary ammonium N1xxx and 1-ethyl-3-methylimidazolium cations.
Replacing the –CH3 group with the –CF3 moiety was studied for theimidazolium cation
based ILs in order to compare with the results obtained with those of fluorinated quaternary
ammonium or cyclic ammonium cations [5, 6]. We also continue to further explore the
electrochemical properties and cycling behavior of the of IL–organic solvent mixtures
(e.g., ethylene carbonate) at different amounts of solvent (vol.%) according to our previous
results [11].
2 Experimental Section
2.1 Synthesis of Quaternary Ammonium Based ILs
Alkylammonium bis(trifluorormethanesulfonyl) imide ionic liquids were synthesized by
the metathesis reaction of freshly prepared halide salts of substituted ammonium with
different alkyl chains, as previously reported [5].
The ethylene carbonate (EC) (Aldrich) was stored in molecular sieves in glove box. The
mixtures of ILs–EC were prepared, with stirring, in the glovebox.
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J Solution Chem
2.2 Synthesis of Imidazolium Based ILs
The ionic liquid EMI?TFSI- was synthesized by a one step methathesis: 1-ethyl-3-
methylimidazoliumchloride EMI?Cl- (1.465 g, 0.01 mol) and lithium bis(trifluo-
romethanesulfonyl)imide (LiTFSI) (2.871 g, 0.01 mol) were dissolved in acetonitrile in
two separate vials. An anion-exchange reaction occurred after adding slowly (drop by
drop) LiTFSI solution in a 10 mL round-bottom flask containing the EMI?Cl- solution,
whereby the mixture was precipitated. Then, the reaction mixture was stirred at 500 rpm at
room temperature for 48 h. After removal of the solvent, the mixture was washed
repeatedly with water until the Cl- could not be detected by addition of AgNO3 solution.
The organic phase was collected in a vial and was passed at least twice through Celite
silica column with ethyl acetate to completely remove Cl-. After removal of the solvent,
the final product was dried under vacuum to give a yellowish liquid (2.347 g, 62 %).
The fluorinated imidazolium based IL, (2,2,2-trifluoroethyl)-3-methylimidazolium
bis(trifluoromethanesulfonyl)imide, was prepared through the tosylate derivative pathway
without solvent and hazardous catalysts [12].
Water contents of all the ILs, determined by Karl-Fisher titration, were below 20 ppm.
The ILs were stored in a glove box ([H2O] \ 3 ppm).
2.3 Measurements
The structure of intermediate and final products were confirmed by 1H-NMR and
13C-NMR spectra measured on a Bruker 500 spectrometer (500 MHz for 1H and 125 MHz
for 13C) in DMSO-d6 or acetone-d6, and mass spectra were recorded via GC–MS and LC–
HR-MS techniques. GC–MS analyses were performed on an Agilent GC System 7890A,
equipped with a mass selective detector Agilent 5973 and a capillary column HP-5MS
(30 m 9 0.25 mm 9 0.25 lm). LC-HRMS analyses were performed on a Bruker
micrOTOF-Q II.
DSC measurements were performed using a Mettler Toledo DSC1 Star. Samples of
10 mg were sealed in aluminium pans in a glovebox. Each sample was heated from -80 to
100 °C at a heating rate of 10 °CÁmin-1. The temperature calibration was carried out
following the standard procedure using pure metal weighed into the hermitically sealed
aluminium crucible. The precision of this calibration is 0.5 °C for a scanning rate of
10 °CÁmin-1
.
Thermogravimetric measurements were carried out with a TGA Q500 V20.10 Build.
This instrument possesses a relatively automated mass calibration procedure. Temperature
calibrations were performed by using the Curie points of standard metals. A few mil-
ligrams of the sample was heated from the room temperature up to 600 °C at 10 °CÁmin-1
under nitrogen flow. The precision of this calibration is 0.5 °C for a scanning rate of
10 °CÁmin-1
.
The ionic conductivities and viscosities of the ILs in the glove box were measured by
using an OAKTON CON 2700 Conductimeter and a Cannon Ostwald viscometer.
Cyclic voltammetry of pure ILs was carried out at the a scan rate of 1 mVÁs-1 on a
MGP2 Biologic instrument (France) using a three-electrode cell. The counter electrode was
a Pt wire and the working electrode was a Pt electrode with diameters of 1 mm and 25 lm,
respectively. The reference electrode was a Ag wire in 10 mmolÁL-1 AgNO3 in acetoni-
trile ? 0.1 molÁL-1 tetrabutylammonium perchlorate (TBAP). Preparation of electrolyte
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solutions was carried out in a glovebox to protect the ionic liquids from absorbing water.
The resulting potentials can be converted to the Li?/Li half cell by adding 3.548 V.
The oxidation stability of the lithium electrolyte containing pure ILs with LiTFSI or
mixtures of ILs-EC were also studied by cyclic voltammetry (CV). The lithium salt
concentration was 0.25 or 0.5 molÁL-1 in order to obtain the optimized ionic conductivity
and viscosity of the electrolyte.
All galvanostatic cycling were performed on the three-electrode Swagelok cell using
lithium metal foil (Aldrich, battery grade) as anode, glass microfiber (Whatman, GF/D) as
separator, and a positive electrode prepared by combining LiMn2O4 (synthesized in lab-
oratory), PTFE 60 wt% in water as binder (Aldrich) and carbon C65 (Timcal, Swiss). The
electrode paste was prepared by mixing and laminating to the thin film electrode with
150 lm thickness. The final electrode composition was 80 wt% active material, 15 wt%
conducting carbon, and 5 wt% PTFE binder. As electrolyte, 0.25 molÁL-1 LiTFSI dis-
solved in mixed solution EC–ILs was utilized.
3 Results and Discussion
3.1 Quaternary Ammonium Based ILs (QAILs)
The QAILs obtained and their properties are summarized in Table 1. Abbreviations for the
substituted ammoniums ion were selected by subscripting the number of carbons in each of
the four alkyl substituents. For aliphatic quaternary ILs, using a low symmetry cation of
large size cation combined with low symmetry and relative large sized anion gives gen-
erally low melting temperature (Tm). For the N112x series, Tm decreases with the alkyl chain
length. A similar tendency was observed for N122x and N133x. The N1125, which has lower
Table 1 Properties of quaternary ammonium imide ionic liquids
Ionic
liquids
Tm
(°C)
Td
(°C)
Density
(gÁmL-1
30 °C)
Viscosity Conductivity Molar
,
g (mPaÁs, r (mSÁcm-1
,
conductivity K
(mSÁcm2Ámol-1
35 °C)
35 °C)
35 °C)
,
N
1118TFSI
7.0
380.0
395.2
400.3
405.0
452.1
446.0
403.3
446.2
420.0
445.2
485.1
433.3
1.26
1.32
1.41
1.32
1.22
1.39
1.56
1.35
–
145
119
88
1.10
1.70
3.30
2.10
3.30
2.60
1.90
2.80
0.50
1.80
0.70
0.70
0.39
0.54
0.95
0.63
1.09
0.77
0.52
0.88
0.16
0.60
0.26
0.27
N1116TFSI
N1114TFSI
N1233TFSI
N1123TFSI
N1124TFSI
N1125TFSI
N1224TFSI
N1224fTFSI
N1225TFSI
N1334TFSI
N1335TFSI
32.1
8.3/17.0
15.0/29.0
-12.0
-10.0
-8.0
10.2
36.0
–
84
55
70
95
85
361
88
1.31
1.23
1.20
–
289
269
–
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J Solution Chem
symmetry but the same molecular weight as N1224, exhibited the low Tm of -8 °C,
compared to -10 °C. Similar results were reported for N1123TFSI and N1114TFSI [5].
All the ILs exhibited excellent thermal stability up to 485 °C (Table 1). The degradation
temperature (Td) decreases quickly with increasing number of carbons in the alkyl chain.
The Td of N1125TFSI is lower than for N1224TFSI due to the longer C5 alkyl chain.
The density of QAILs, d, measured at 30 °C, lies in the range of 1.22–1.50 gÁmL-1. It
was observed that the least symmetrcal cation, N112xTFSI, exhibited an increase in density
as the alkyl chain length increases. However, in the case of N122x or N133x, the density
tends to decrease weakly.
The ionic conductivities of the ILs are mainly governed by their viscosities and formula
weight (i.e., the molar concentration). The evolution of conductivity with temperature is
plotted in Fig. 1. The ionic conductivities decrease with the increasing alkyl chain length.
The highest conductivity was obtained for N1123TFSI. For compounds of the same
moecular weight, particularly N1125 and N1224TFSI, there are significant differences in
conductivity. When the temperature increases, the IL’s conductivity increases rapidly. As
compared to the conductivity of the commercial electrolyte LiPF6/EC:DMC (1:1), the
highest value of N1123TFSI is still smaller at 30 °C, 3.3 mSÁcm-1 versus 5.0 mSÁcm-1
.
In order to compare the conductivity of the ILs at the same molar concentration, the
molar conductivity was determined at 35 °C (Fig. 2). Except for the fluorinated ammo-
nium IL, the molar conductivity increases with the molar concentration. A quasi-linear
behavior is observed. This behavior may be explained by an increase in the viscosity with
the increase in the molar mass (decrease in the molar concentration) and an increase in ion
association, which may be related to the formation of ionic clusters, being more favorable
for the ionic liquids with greater alkyl chain length. Moreover, the increase in alkyl chain
length induces a decrease in the amount of dissociation. The N1224fTFSI exhibits the lowest
conductivity, which may be due to its high viscosity associated with its high Tg value.
Fig. 1 Ionic conductivity as a function of temperature for several ionic liquids
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Fig. 2 Molar conductivity as a function of molar concentration at 35 °C
To evaluate the impact of these two effects (viscosity and dissociation) on the ionic
conductivity, the viscosities of the ILs were measured between -10 and 70 °C. The
evolution of the viscosity is plotted versus T-1 (Fig. 3); the data obtained at 35 °C are
given in Table 1 whereas the remaining values are reported in Table 2.
Fig. 3 Viscosity of various ionic liquids versus temperature
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Table 2 Viscosity values of linear ionic liquids as a function of temperature
T (°C)
1000 K/
T
N1116TFSI
N1118TFSI
N1224TFSI
N1123TFSI
N1114TFSI
N1124fTFSI
–10.00
-5.00
0.00
3.8023
3.7313
3.6630
3.5971
3.5336
3.4130
3.3003
3.1949
3.0960
3.0030
2.9155
2571
1039
1220
800.0
545.0
385.9
204.5
119.6
80.0
930.7
615.3
424.1
301.4
160.7
91.7
374.3
252.9
177.8
134.9
82.2
730.4
507.2
356.8
262.1
147.8
88.0
4798.2
2872.8
1805.6
773.4
361.1
199.8
119.1
22.0
5.000
10.00
20.00
30.00
40.00
50.00
60.00
70.00
498
257
145
92
74.7
59.8
70.5
59.2
52.8
65
41.4
65.6
30.6
29.7
60.6
26.5
22.1
23.9
Units of viscosity: mPaÁs
The viscosity increases with an increase in the molar mass, as expected due to van der
Waals interactions. For a given molecular weight, the viscosity increases with the length of
the alkyl chain and as the symmetry of the cation increases. For example, N1116TFSI
exhibits a viscosity of 119 mPaÁs at 35 °C, as compared to 85 and 84 mPaÁs for N1233TFSI
and N1224TFSI, respectively. As expected, the ILs’ viscosities decrease with an increase in
temperature. This effect is attributed to the increase in Brownian motion of the constituent
ions of the ionic liquids (Fig. 3).
The electrochemical windows of pure ILs were determined by cyclic voltammetry on a
platinum microelectrode. The voltammograms of ILs are given in Fig. 2. The anodic and
cathodic limits (Eanodic and Ecathodic) versus Li?/Li were arbitrarily defined as the potential
at which the current density reached 3 times the background current. All the ILs present
good electrochemical stability for oxidation, with oxidation potentials higher than 5.5 V
versus Li?/Li. As reported by Phung et al. [5], the oxidation stability depends on the length
of the longest alkyl chain of the ILs. The shorter it is, the higher is the oxidation potential.
The values of oxidation stability were determined as: N1335TFSI (5.7 V vs. Li?/
Li) \ N1225TFSI (5.8 V) \ N1125TFSI * N1224TFSI (6.0 V) \ N1123TFSI * N1124TFSI
(6.2 V). Compared to the commercial electrolyte (1 molÁL-1 LiPF6/EC:DMC 1:1), the
Eanodic value of most stable ILs is similar, but the oxidation current is much smaller than
for the last one.
3.2 Mixtures of ILs–Organic Solvent
The most symmetrical of these ILs, N1123TFSI, exhibits higher viscosity in comparison to
the conventional electrolyte (55.7 vs. 3.10 mPaÁs, respectively). One approach to decrease
the viscosity of an IL is by using a small amount of organic solvent that isn’t detrimental to
the high oxidation stability of ILs. Phung et al. [11] studied the addition of carbonate
solvents to the aliphatic ammonium based ILs. The addition of a small amount of solvent,
i.e. 20 vol.%. causes a notable increase in the conductivities. In addition, the oxidation
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potential was reduced when a large amount of solvent was added. The potential was
reduced by about 100 mV at 25 vol.%. EC in N1123TFSI or in N1124TFSI (Figs. 4, 5).
The addition of LiTFSI to the mixed electrolyte ILs–EC normally induces a decrease in
conductivity that is then lower than observed in the corresponding pure ILs. The minium
concentration of LiTFSI in the mixed electrolyte was 0.25 molÁL-1 for battery testing. The
electrolyte ILs–EC ? 0.25 molÁL-1 LiTFSI exhibits similar oxidation stability as the ILs–
EC mixtures, particularly for N1123TFSI and N1124TFSI (Table 3).
The battery cycling test was performed using ILs ? x vol.% EC ? 0.25 molÁL-1
LiTFSI (x = 20, 25 %) at the standard rate of C/10. The results seem to be promising for
N1123TFSI ? 20 vol.% EC ? 0.25 molÁL-1 LiTFSI compared to the commercial elec-
trolyte 1 molÁL-1 LiPF6/EC–DMC. In Fig. 6, the discharge capacity of 87–100 mAhÁg-1
for N1123TFSI ? 20 vol.% EC ? 0.25 molÁL-1 LiTFSI has relatively good cycle effi-
ciency. In Fig. 7, the N1124TFSI ? 20 vol.% EC ? 0.25 molÁL-1 LiTFSI exhibits lower
discharge capacity of 65–75 mAhÁg-1 due to the high viscosity of the electrolyte solution
(g = 13.3 mPaÁs).
3.3 Imidazolium Cation Based ILs
As reported earlier, the imidazolium based ILs exhibit lower viscosity than those with an
aliphatic cation. The inconvenience of imidazolium based ILs is their insufficient elec-
trochemical stability with anode materials due to the C-2 hydrogen’s acidity. The oxidation
feature can be improved by using a -CF3 substituent in the cation structure. For example,
by using [CF3CH2MIm][TFSI] to replace the [CH3CH2MIm][TFSI]. The physicochemical
properties and conductivities of imidazolium based ILs are given in Table 4 in comparision
to the lowest symmetry aliphatic ammonium based ILs.
Fig. 4 Cyclic voltammetry of different amounts of EC mixed in N1123TFSI
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Fig. 5 Cyclic voltammetry of different amounts of EC mixed in N1124TFSI
Table 3 Oxidation potential
Electrolyte
E versus Li?/Li/V
limit of quaternay ammonium
based ILs and mixtures of
N1123TFSI
6.2
6.2
5.9
6.2
6.1
6.2
5.9
ILs–EC determined at
i = 10 mAÁcm2
N1123TFSI ? 20 vol.% EC
N1123TFSI ? 25 vol.%. EC
N1124TFSI
N1124TFSI ? 15 vol.% EC
N1124TFSI ? 20 vol.% EC
N1124TFSI ? 25 vol.%. EC
The fluorinated imidazolium exhibits a lower melting temperature, about -22 °C
instead of -17 °C for [CH3CH2MIm][TFSI]. The replacement of a -CH3 group by a -CF3
group induces a decrease in the IL’s melting point; however, an increase of viscosity was
observed. The viscosity of fluorinated imidazolium obtained is nearly that of N1123TFSI.
In Fig. 8 the oxidation stability of the fluorinated imidazolium is higher than that of the
non-fluorinated one. Thus, it can be concluded that the incorporation of -CF3 moieties into
the imidazolium structure significantly improve the IL’s stability towards oxidation. The
reduction wall of fluorinated imidazolium based ILs observed at 1.5 V versus Li?/Li is
associated with the reduction of the imide anion [13]. The fluorinated imidazolium exhibits
better oxidation stability, i.e. 5.7 V versus Li?/Li compared to 5.1 V versus Li?/Li for
EMITFSI (non-fluorinated imidazolium). Similar results were reported for fluorinated
quaternary ammonium cation based ILs and non-fluorinated ones [5]. These results are
consistent with thoeritical calculation of HOMO values for the linear optimized geometries
of the ILs. The incorporation of a fluorinated group induces a decrease in the HOMO
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Fig. 6 Galvanostatic cycling of N1123TFSI ? 20 vol.% EC ? 0.25 molÁL-1 LiTFSI at C/10 rate in the
potential range 3.5–4.2 V, at room temperature
Qs / mAh.g-1
80
70
60
50
40
30
20
10
0
120
100
80
60
40
20
0
0
10
20
30
40
4.2
4
Efficiency
Qs
3.8
3.6
3.4
3.2
3
Li || N1124 TFSI + 0.5M LiTFSI + 20%EC || LiMn O4
2
Li || N1124TFSI + 0.25M LiTFSI +20 %EC || LiMn2O4
0
5
10
15
20
25
30
35
40
0
0.04 0.08 0.12 0.16
x / F.mol-1
0.2
0.24 0.28 0.32
Cycle number
Fig. 7 Galvanostatic cycling of N1124TFSI ? vol.% EC ? 0.25 molÁL-1 LiTFSI at C/10 rate in the
potential range 3.5–4.2 V, at room temperature
Table 4 Properties of imidazolium based ILs in comparison to aliphatic quaternary ammonium based ILs
Ionic liquids
Tm (°C)
Tg
(°C)
Td
(°C)
Density
(gÁmL-1
30 °C)
Viscosity
g (mPaÁs,
35 °C)
Molar conductivity K
,
(mSÁcm-1Ámol-1
35 °C)
,
[CH3CH2MIm][TFSI] -17
-35
414
417
452
1.48
1.54
1.22
24.9
55.1
55.7
2.35
0.54
1.09
[CF3CH2MIm][TFSI] -22/-12
N1123TFSI
-12
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J/ mA.cm-2
LiPF6
14
12
10
8
CF3CH3MI TFSI
E vs
Li+/Li /V
5.5
EMI TFSI
N1124TFSI
Electrolyte
LiPF6/(EC:DMC 1:1)
EMI TFSI
N1123TFSI
5.1
5.7
6.2
6.2
6
CF3CH2MI TFSI
N1123TFSI
4
N1124TFSI
2
0
5.4
5.6
5.8
6
6.2
6.4
6.6
E vs Li+Li/ V
Fig. 8 Cyclic voltammetry of different ionic liquids based on fluorinated imidazolium and aliphatic
ammonium cations
values, and thus it can be concluded that the fluorinated ILs are electrochemically more
stable than the non-fluorinated one.
4 Conclusions
Aliphatic ammonium bis(trifluoromethane sulfonyl) imides have been prepared and
characterized. The highest conductivity was obtained for the least symmetric aliphatic
ammonium cation, N1123TFSI; however the high viscosity is still a big challenge for using
it as an electrolyte in lithium ion batteries. Using a small amount of ethylene carbonate
solvent (EC), i.e. 20 vol.%, induces a significant decrease in viscosity and thus an increase
in ionic conductivity. The galvanostatic cycling test showed the promising capacity value
obtained for N1123TFSI ? 20 vol.% EC ? 0.25 molÁL-1 LiTFSI. The addition of the -CF3
moiety replacing the -CH3 group of the imidazolium cation showed an improvement of
oxidation stability though it gives rise to a high viscosity for the ILs.
Acknowledgments We are grateful to NAFOSTED program (National Foundation for Science and
Technology Development of Vietnam) for financial support through contract No. 104.03-2012.46. The
author L.M.L. Phung would like to thank F. Alloin (LEPMI-CNRS) for the scientific support.
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