Electrochemical investigations of ionic liquids with vinylene carbonate for applications in rechargeable lithium ion batteries

Article abstract of DOI:10.1016/j.electacta.2010.03.019

Ionic liquids based on methylpropylpyrrolidinium (MPPY) and methylpropylpiperidinium (MPPI) cations and bis(trifluoromethanesulfonyl)imide (TFSI) anion have been synthesized and characterized by thermal analysis, cyclic voltammetry, impedance spectroscopy as well as galvanostatic charge/discharge tests. 10 wt% of vinylene carbonate (VC) was added to the electrolytes of 0.5 M LiTFSI/MPPY.TFSI and 0.5 M LiTFSI/MPPI.TFSI, which were evaluated in Li || natural graphite (NG) half cells at 25 °C and 50 °C under different current densities. At 25 °C, due to their intrinsic high viscosities, the charge/discharge capacities under the current density of 80 μA cm^-2 were much lower than those under the current density of 40 μA cm^-2. At 50 °C, with reduced viscosities, the charge/discharge capacities under both current densities were almost indistinguishable, which were also close to the typical values obtained using conventional carbonate electrolytes. In addition, the discharge capacities of the half cells were very stable with cycling, due to the effective formation of solid electrolyte interphase (SEI) on the graphite electrode. On the contrary, the charge/discharge capacities of the Li || LiCoO₂ cells using both ionic liquid electrolytes under the current density of 40 μA cm^-2 decreased continually with cycling, which were primarily due to the low oxidative stability of VC on the surface of LiCoO₂.

Full text of DOI:10.1016/j.electacta.2010.03.019

Electrochimica Acta 55 (2010) 4618–4626
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Electrochemical investigations of ionic liquids with vinylene carbonate for
applications in rechargeable lithium ion batteries
∗
Xiao-Guang Sun , Sheng Dai
Chemical Science Division, Oak Ridge National Laboratory, P.O. Box 2008, MS6201, Oak Ridge, TN 37831-6201, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Ionic liquids based on methylpropylpyrrolidinium (MPPY) and methylpropylpiperidinium (MPPI) cations
and bis(trifluoromethanesulfonyl)imide (TFSI) anion have been synthesized and characterized by thermal
analysis, cyclic voltammetry, impedance spectroscopy as well as galvanostatic charge/discharge tests.
Received 28 December 2009
Received in revised form 3 March 2010
Accepted 7 March 2010
1
0 wt% of vinylene carbonate (VC) was added to the electrolytes of 0.5 M LiTFSI/MPPY.TFSI and 0.5 M
Available online 15 March 2010
◦
◦
LiTFSI/MPPI.TFSI, which were evaluated in Li || natural graphite (NG) half cells at 25 C and 50 C under
◦
different current densities. At 25 C, due to their intrinsic high viscosities, the charge/discharge capac-
Keywords:
Ionic liquids
Vinylene carbonate
Pyrrolidinium
Piperidinium
Additive
−2
ities under the current density of 80 ␮A cm were much lower than those under the current density
−2
◦
of 40 ␮A cm . At 50 C, with reduced viscosities, the charge/discharge capacities under both current
densities were almost indistinguishable, which were also close to the typical values obtained using con-
ventional carbonate electrolytes. In addition, the discharge capacities of the half cells were very stable
with cycling, due to the effective formation of solid electrolyte interphase (SEI) on the graphite electrode.
Lithium ion batteries
On the contrary, the charge/discharge capacities of the Li || LiCoO cells using both ionic liquid electrolytes
2
−2
under the current density of 40 ␮A cm decreased continually with cycling, which were primarily due
to the low oxidative stability of VC on the surface of LiCoO2.
Published by Elsevier Ltd.
1
. Introduction
As a routinely studied example of ionic liquids, 1-ethyl-
3
-methylimidazolium
bis(trifluoromethanesulfonyl)imide
Lithium ion batteries have become indispensable components
(EMI.TFSI) has the advantages of low viscosity and high
ionic conductivity [12,17,18,23,25]. However, it also suf-
in portable electronic device, ranging from digital camera to laptop
computers, due to their high voltage and high energy density as
compared with other rechargeable battery systems. With the press-
ing worldwide environmental concerns, they have been actively
proposed for applications in electric vehicles (EVs), hybrid-electric
vehicles (HEVs) and plug-in hybrid-electric vehicles (PHEVs). How-
ever, this has been held back by the safety concerns due to the
fact that most components of these batteries are volatile liquid
organic electrolytes, which can easily lead to local overheating
and eventually fire or explosion in the event of short circuits or
abuse conditions. Ionic liquids, which are known for their non-
flammability, high electrochemical stability and negligible vapor
pressure, are good candidates for tackling the safety issues in
lithium ion batteries. Currently extensive work has been carried
out in synthesizing and testing of ionic liquids as new generation
of electrolytes for much safer lithium ion batteries in vehicle appli-
cations [1–27].
fers from
a
big disadvantage: that is, the relatively high
+
reduction potential of 1.0 V versus Li/Li , which is too pos-
itive to allow lithium deposition or intercalation and thus
not suitable for lithium ion batteries. Saturated quaternary
ammoniums are more resistant toward oxidation and reduc-
tion than imidazolium cation, therefore, their ionic liquids
with TFSI, have much larger electrochemical window than
the corresponding imidazolium compounds. However, they
have relatively poor capability in forming efficient solid elec-
trolyte interphase (SEI) on the graphite anode electrode, which
results in continual reaction and loss capacity with cycling.
Organic additives have been routinely used to tackle the SEI
problems in lithium ion batteries [14,19,21,25,28–31]. There-
fore, in this report we used vinylene carbonate (VC) as an
additive to the ionic liquids based on methylpropylpyrroli-
dinium bis(trifluoromethanesulfonyl)imide (MPPY.TFSI) and
methylpropylpiperidinium
bis(trifloromethanesulfonyl)imide
(MPPI.TFSI) and investigated their electrochemical performance
via cyclic voltammetry, charge–discharge test and impedance
spectroscopy.
∗ Corresponding author. Tel.: +1 865 241 8822; fax: +1 865 576 5235.
E-mail address: sunx@ornl.gov (X.-G. Sun).
0
013-4686/$ – see front matter. Published by Elsevier Ltd.
doi:10.1016/j.electacta.2010.03.019

X.-G. Sun, S. Dai / Electrochimica Acta 55 (2010) 4618–4626
4619
2
. Experimental
2.1. Synthesis
Lithium foil, N-methylpyrrolidine, N-methylpiperidine, 1-
bromopropane, vinylene carbonate, ethanol, carbon black,
polyvinylvinylidifluoride, N-methylpyrrolidinole (NMP), and
charcoal were obtained from Aldrich and were used as sup-
plied. Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) was
obtained from 3 M. Potato graphite and lithium cobolt oxide
(
LiCoO ) were obtained from Pred Materials. Copper foil was
2
obtained from Storm Copper Component Co. Deionized H O
2
was obtained via the use of a Millipore ion-exchange resin
deionizer.
N-methyl-N-propylpyrrolidinium
bromide
and
N-
methyl-N-propylpiperidinium bromide were obtained by
refluxing 1-bromopropane with N-methylpyrrolidine and
N-methylpiperidine in ethanol for
3
days, respectively. N-
methyl-N-propylpyrrolidinium
bis(trifluoromethanesulfonyl)
imide
(MPPY.TFSI)
and
N-methyl-N-propylpiperidinium
bis(trifluoromethanesulfonyl) imide (MPPI.TFSI) were obtained by
mixing the aqueous solutions of N-methyl-N-propylpyrrolidinium
bromide and N-methyl-N-propylpiperidinium bromide with
excess aqueous solution of LiTFSI as described in the literature [1].
Fig. 2. DSC trace of ionic liquids (a) MPPY.TFSI and (b) MPPI.TFSI, with and without
LiTFSI.
The organic phases were separated and washed with Millipore
water until the silver test showed negative. The ionic liquids
were dried by freeze drying for four days under vacuum at room
temperature and then stored over freshly activated 4A molecular
sieves in Argon filled glovebox. The water content was less than
5
ppm as detected by Karl-Fisher’s titration.
LiTFSI was dried at 140 C under high vacuum for 48 h. 0.5 M
◦
LiTFSI electrolyte solutions were prepared by dissolving calculated
amount of LiTFSI in the ionic liquids with heating inside of the
glovebox. VC was added to the ionic liquid electrolyte as an additive
based on the total weight of the electrolyte.
2.2. Characterization
The thermal properties of the ionic liquids and their salt solu-
tions were determined by using thermal gravimetric analysis
TGA TA 2950) and differential scanning calorimetry (DSC Ther-
(
mal Q1000). The TGA measurement was carried out at a scan rate
◦
of 10 C/min in nitrogen atmosphere. The glass transition (onset
point) and melting point (peak position) were obtained under a
◦
◦
scan rate of 10 C/min in the temperature range of −90–100 C.
The cell impedance spectra were measured via a Gamry Instru-
5
−3
ment in the frequency range from 3 × 10 Hz to 1 × 10 Hz with
Fig. 1. TGA trace of ionic liquids (a) MPPY.TFSI and (b) MPPI.TFSI, with and without
LiTFSI.
perturbation amplitude of 5 mV.

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Fig. 4. Cyclic voltammetry of ionic liquid electrolytes (a) 0.5 M LiTFSI/MPPY.TFSI and
−
1
(
b) 0.5 M LiTFSI/MPPI.TFSI on graphite electrode under the scan rate of 0.1 mV s
.
Fig. 3. Cyclic voltammetry of (a) 0.5 M LiTFSI/MPPY.TFSI (b) 0.5 M LiTFSI/MPPI.TFSI
−1
on Platinum electrode under the scan rate of 10 mV s
.
3
. Results and discussion
The electrochemical stability window was evaluated by cyclic
voltammetry (CV) using Pt as the working electrode and lithium
metal as both the counter and the reference electrodes on the
3.1. Thermal properties
−
1
Gamry Instrument with a scan rate of 10 mV s . When graphite
electrode was used as the working electrode, the CVs were per-
The thermal stabilities of the ionic liquids, with and without
◦
LiTFSI, were tested under nitrogen atmosphere between 30 C and
−1
formed under a scan rate of 0.1 mV s in the coin cell configuration
with lithium foil as both the counter and the reference elec-
trodes.
◦
◦
6
00 C at a heating rate of 10 C/min. As shown in Fig. 1, all the
◦
ionic liquids exhibit high thermal stabilities i.e. well above 450 C.
In addition, the decomposition temperatures of the ionic liquids,
with and without LiTFSI, are almost same.
Fig. 2 compares the DSC profiles of the ionic liquids, with and
without LiTFSI. In Fig. 2a the DSC trace of pure MPPY.TFSI shows
Graphite electrodes were obtained by casting a well homoge-
nized slurry of graphite (90 wt%) and PVdF (10 wt%) in NMP on a
copper foil using a doctor blade. LiCoO2 electrodes were obtained
by casting a well homogenized slurry of LiCoO2 (85 wt%), carbon
black (5 wt%) and PVdF (10 wt%) in NMP on an aluminum foil by
using a doctor blade. After the solvent was evaporated, the elec-
trodes were pressed under a hydrolytic pressure of 1 ton for 1 min
◦
a melting point of 13.5 C while that of 0.5 M LiTFSI/MPPY.TFSI
◦
solution shows a glass transition temperature of −79 C, a crys-
◦
◦
tallization temperature of −25.9 C and a melting point of 4.9 C.
In Fig. 2b the DSC trace of pure MPPI.TFSI shows a melting point
2
before cutting into discs with areas of 2.27 cm and further dried at
◦
of 15.4 C while that of 0.5 M LiTFSI/MPPI.TFSI solution shows a
◦
1
30 C for 24 h before transferring into a glovebox for cell assem-
◦
glass transition temperature of −70 C, a crystallization temper-
bling.
◦
◦
ature of −17.2 C and a melting point of 6.2 C. The melting points
of the pure ionic liquids reported here are higher than those
reported in the literature [13], which might be due to the dif-
ference in the residual water content and measuring condition
etc.
The Li || graphite and Li || LiCoO cells were tested on Macco 4000
2
cycling station under different current densities with cut-off volt-
age of 2–0.01 V for the former and 3.0–4.2 V for the latter. The cell
temperature was controlled by a Maccor environmental chamber.

X.-G. Sun, S. Dai / Electrochimica Acta 55 (2010) 4618–4626
4621
Fig. 6. First charge/discharge profile of half cells using ionic liquids (a) 0.5 M
Fig. 5. Cyclic voltammetry of ionic liquid electrolytes (a) 0.5 M LiTFSI/MPPY.TFSI
LiTFSI/MPPY.TFSI and (b) 0.5 M LiTFSI/MPPI.TFSI, with and without 10 wt% VC, that
+
10 wt% VC and (b) 0.5 M LiTFSI/MPPI.TFSI +10 wt% VC on graphite electrode under
−
2
−1
tested under the current density of 40 ␮A cm
.
the scan rate of 0.1 mV s
.
3
.2. Cyclic voltammetry
0 and 0.15 V is apparently due to the lithium intercalation into
graphite. The corresponding lithium de-intercalation appears at
0.3 V during the anodic scan. The additional anodic peaks at 1.0
and 1.7 V are the oxidation of the electro-adsorbed species formed
during the cathodic scan [11]. In the second cycle, such reduc-
tion and oxidation peaks decrease and the corresponding lithium
de-intercalation peak increases significantly. These observations
indicate that the surface of the graphite electrode has been passi-
vated by the decomposition products of 0.5 M LiTFSI/MPPY. TFSI.
The coulombic efficiencies of the first two cycles are 37.8% and
53.6%, respectively.
The electrochemical windows of both 0.5 M LiTFSI/MPPY.TFSI
and 0.5 M LiTFSI/MPPI.TFSI on platinum electrode exhibit an oxida-
tion voltage of 5.5 V versus Li/Li (not shown), which are consistent
with the reported data [3].
+
Fig. 3a and b shows the CVs of 0.5 M LiTFSI/MPPY.TFSI and
.5 M LiTFSI/MPPI.TFSI electrolytes on platinum working electrode,
0
respectively, which were obtained in the voltage range of 3 V to
−1
−
0.5 V at a scan rate of 10 mV s . The lithium deposition peaks
below 0 V in the cathodic scan and the lithium stripping peaks
below 0.5 V in the anodic scan were observed as major peaks for
both ionic liquid electrolytes, though distinct multiple Li-Pt alloy-
ing peaks were also observed in the CV of 0.5 M LiTFSI/MPPI.TFSI
Fig. 4b shows the CV of 0.5 M LiTFSI/MPPI.TFSI on the graphite
−
1
electrode under the scan rate of 0.1 mV s . It is noted that
both the reduction peak at 0.4 V and the oxidation peak at 1.1 V
are very big and almost do not change with cycling. It is also
noted that no lithium intercalation/de-intercalation peaks are
observed. These observations suggest that the decomposition of
0.5 M LiTFSI/MPPI.TFSI cannot passivate the graphite surface to
support reversible lithium intercalation/de-intercalation, which is
supported by the half cell cycling data shown later. The coulombic
efficiencies of the first two cycles are 46.2% and 63.0%, respectively.
Since vinylene carbonate (VC) has been shown very effective
in passivating the graphite electrode surface to allow reversible
lithium intercalation/de-intercalation [14,28–30], it was added as
(
Fig. 3b) [4,5]. Due to the SEI formation on the surface of the
electrode, the initial coulombic efficiencies for both ionic liquid
electrolytes are very low [6]. For example, the coulombic efficien-
cies of the first two cycles for 0.5 M LiTFSI/MPPY.TFSI are 41.6% and
8
6.0% while those of the first two cycles for 0.5 M LiTFSI/MPPI.TFSI
are 56.4% and 77.6%, respectively.
Fig. 4a shows the CV of 0.5 M LiTFSI/MPPY.TFSI on graphite
−
1
electrode obtained under the scan rate of 0.1 mV s . The big
peak around 0.4 V in the cathodic scan is due to the decomposi-
tion of the ionic liquid [7–10]. The small cathodic peak between

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Fig. 7. First charge/discharge profile of half cells using 0.5 M LiTFSI/MPPY.TFSI
◦
◦
+
10 wt% VC that tested under different current densities at (a) 25 C and (b) 50 C.
Fig. 8. First charge/discharge profile of half cells using 0.5 M LiTFSI/MPPI.TFSI
+
◦ ◦
10 wt% VC that tested under different current densities at (a) 25 C and (b) 50 C.
an additive to the two ionic liquid electrolytes. Fig. 5a and b
shows the CVs of 0.5 M LiTFSI/MPPY.TFSI +10 wt% VC and 0.5 M
LiTFSI/MPPI.TFSI +10 wt% VC on the composite graphite electrode,
respectively. The onset reduction potential at 1.4 V and the major
reduction peak at 0.8 V, which are well before the reduction of
pure ionic liquid electrolytes at 0.4 V (Fig. 4), are apparently due
to VC reductions [32–34]. As a result, the graphite electrodes are
successfully passivated, and reversible lithium intercalation/de-
intercalation peaks are observed for both ionic liquid electrolytes
tively, resulting in a coulombic efficiency of 25%. These discharge
capacities are consistent with their CV data on graphite electrode,
that is, pyrrolidinium based ionic liquid participates in the forma-
tion of SEI while piperidinium based one does not. However, with
1
0 wt% VC, the charge and discharge capacities of both ionic liquid
electrolytes are almost same, which are also approaching to the
typical values obtained with conventional carbonate electrolytes.
Fig. 7a and b shows the results of the half cells using 0.5 M
LiTFSI/MPPY.TFSI +10 wt% VC as electrolyte under different cur-
(
Fig. 5). The calculated coulombic efficiencies of the first two cycles
of 0.5 M LiTFSI/MPPY.TFSI +10 wt% VC are 54.8% and 86.7% while
those of the first two cycles of 0.5 M LiTFSI/MPPI.TFSI +10 wt% VC
are 43.9% and 82.4%, respectively.
◦
◦
◦
rent densities at 25 C and 50 C, respectively. At 25 C, when the
−2
−2
current density is increased from 40 ␮A cm to 80 ␮A cm , the
discharge capacity decreases from 312 mAh g
However, under the same current density of 40 ␮A cm , when the
temperature is increased from 25 C to 50 C, the discharge capac-
ity is increased from 312 mAh g to 335 mAh g . The increase in
test temperature also improves the rate capability of the ionic liq-
uids. For example at 50 C (Fig. 7b) when the current density is
increased from 40 ␮A cm to 80 ␮A cm , the discharge capacity
only decreases from 335 mAh g
−1
−1
.
to 247 mAh g
−
2
◦ ◦
3.3. Performance of Li || graphite half cells
−
1
−1
Fig. 6 shows the first charge–discharge cycles of the graphite
◦
electrodes in the two ionic liquid electrolytes, with and without
−
2
◦
−2 −2
VC, that tested under the current density of 40 mA cm at 25 C.
The charge (intercalation) and discharge (de-intercalation) capac-
ities of the half cell using 0.5 M LiTFSI/MPPY.TFSI are 258 and
−
1
−1
to 325 mAh g . For the elec-
trolyte of 0.5 M LiTFSI/MPPI.TFSI +10 wt% VC, due to its higher
viscosity than that of 0.5 M LiTFSI/MPPI.TFSI +10 wt% VC [17], it
is more sensitive to the increase in current density. As shown
−
1
1
5
0
43 mAh g , respectively, resulting in a coulombic efficiency of
5%. The charge and discharge capacities of the half cell using
.5 M LITFSI/MPPI.TFSI are 241 mAh g
−1
−1
◦
and 60 mAh g , respec-
in Fig. 8a, at 25 C when the current density is doubled from

X.-G. Sun, S. Dai / Electrochimica Acta 55 (2010) 4618–4626
4623
Fig. 9. Discharge capacities and coulombic efficiencies of the half cells using (a)
.5 M LITFSI/MPPY.TFSI +10 wt% VC and (b) 0.5 M LITFSI/MPPI.TFSI +10 wt% VC that
0
−2
tested under the current density of 80 ␮A cm at different temperatures.
Fig. 10. Cycling data of the full cell Li || LiCoO2 using (a) 0.5 M LiTFSI/MPPY.TFSI
+
10 wt% VC and (b) 0.5 M LiTFSI/MPPI.TFSI +10 wt% VC that tested under the current
−
−
2
−2
−1
4
3
0 ␮A cm to 80 ␮A cm , its discharge capacity decreases from
−2
density of 40 ␮A cm
.
1
07 mAh g to 146 mAh g , with the latter being less than half of
◦
the former. However, at 50 C with reduced viscosity, its rate capa-
bility is also improved i.e. as the current density is doubled from
3
.4. Performance of Li || LiCoO2 cells
− −2
2
4
0 ␮A cm to 80 ␮A cm , its discharge capacity only decreases
−1
−1
from 334 mAh g to 314 mAh g (Fig. 8b).
Considering the viscosity effect shown in the half cell tests,
Fig. 9a and b summarizes the cycling performance at different
temperatures for the cells using 0.5 M LiTFSI/MPPY.TFSI +10 wt%
VC and 0.5 M LiTFSI/MPPI.TFSI +10 wt% VC, respectively. The cells
were tested under the current density of 80 ␮A cm , except the
first two formation cycles, which were tested under the current
−2
the lower current density of 40 ␮A cm
LiCoO2 cell tests. Fig. 10a and b shows the cycling data using
0
+
ity initially increases and then gradually decreases with cycling.
The coulombic efficiency is below 97% for the cell using 0.5 M
LiTFSI/MPPY.TFSI +10 wt% VC and is below 94% for the cell using
was used for Li ||
.5 M LiTFSI/MPPY.TFSI +10 wt% VC and 0.5 M LiTFSI/MPPI.TFSI
10 wt% VC, respectively. Both cells show that the discharge capac-
−
2
−2
density of 40 ␮A cm . As mentioned in previous paragraph that
◦
due to the high viscosity at 25 C, the discharge capacity is around
−
1
2
50 mAh g for the cell using 0.5 M LiTFSI/MPPY.TFSI +10 wt% VC
0
.5 M LiTFSI/MPPI.TFSI +10 wt% VC. The gradual decrease of the
−1
while it is stabilized at about 125 mAh g for the cell using 0.5 M
LiTFSI/MPPI.TFSI +10 wt% VC. At 50 C the capacity is increased
to 325 mAh g for the cell using 0.5 M LiTFSI/MPPY.TFSI +10 wt%
VC while it is stabilized at 310 mAh g
LiTFSI/MPPI.TFSI +10 wt% VC. The coulombic efficiencies, except the
first two formation cycles, are all above 96%. It is interesting to note
for both systems that even though the capacities at 50 C are much
higher than those at 25 C, the coulombic efficiencies at 50 C are all
lower than those at 25 C. This observation might result from the
cathode capacity with cycling and the accompanying low coulom-
bic efficiency may be directly related to the problems at the
LiCoO2 || electrolyte interfaces, as it has been shown earlier that
with VC these two ionic liquid electrolytes could successfully sup-
port lithium intercalation/de-intercalation in the graphite anode
with high coulombic efficiency. Owning to the high reactivity of
lithium metal, it might also partially responsible for the gradual
capacity fading. To further investigate the origin of the capac-
ity fading, fresh Li || LiCoO2 cells were built using both 0.5 M
LiTFSI/MPPY.TFSI +10 wt% VC and 0.5 M LiTFSI/MPPI.TFSI +10 wt%
VC. First, the time dependence of the impedance spectra was
◦
−1
−1
for the cell using 0.5 M
◦
◦
◦
◦
thermal instability of VC, which decomposes at a relatively faster
rate at higher temperature, and thus lower coulombic efficiencies.

4
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Fig. 11. a.c. impedance spectra of the full cell Li | 0.5 M LiTFSI/MPPY.TFSI +10 wt%
VC | LiCoO2 stored as a function of time (a) before charging and (b) after charged to
Fig. 12. a.c. impedance spectra of the full cell Li | 0.5 M LiTFSI/MPPI.TFSI +10 wt%
VC | LiCoO stored as a function of time (a) before charging and (b) after charged to
2
−2
−2
4
.2 V under the current density of 40 ␮A cm
.
4.2 V under the current density of 40 ␮A cm
.
monitored for 24 h right after cell assembling. Both cells show
huge interfacial resistance (Figs. 11a and 12a), which increases
with storage time. Since before charging the LiCoO2 electrode is
in low oxidation state, the main contribution of the increase in
interfacial resistance should come from the reaction of lithium
electrode with the ionic liquid electrolytes. This is supported by
the reports that the interfacial resistance of the symmetric lithium
cells using ionic liquid electrolytes, with and without VC, increase
with storage time [7,35]. To check the stability of the electrolyte
on charged LiCoO2 electrode, the Li || LiCoO2 cell was charged
to 4.2 V and the cell impedance was also monitored for 24 h.
Figs. 11b and 12b show the impedance evolution in the Li ||
the resistance of lithium ion diffusion (R ) inside the oxide par-
d
ticles [39]. The parameters of RSEI, Rct and Rd from the impedance
spectra of Figs. 11b and 12b are obtained via fitting. Within the
storage time, for the cell using 0.5 M LiTFSI/MPPI.TFSI +10 wt% VC,
RSEI slowly increases from 7.3 ꢀ to 7.9 ꢀ while Rct increases from
26.9 ꢀ to 38.8 ꢀ and Rd increases from 22.8 ꢀ to 25.6 ꢀ· For the
cell using 0.5 M LiTFSI/MPPY.TFSI +10 wt% VC, R slowly increases
SEI
from 8.4 ꢀ to 9·1 ꢀ and Rct increases from 23.0 ꢀ to 30.6 ꢀ while
R
keeps steady around 33.3 ꢀ. It is observed for both cells that
d
the biggest increase in resistance during the storage time after
charged to 4.2 V comes from the charge transfer resistance Rct.
For example, Rct is increased by 44.2% for the cell using 0.5 M
LiTFSI/MPPI.TFSI +10 wt% VC and is increased by 33.0% for the cell
using 0.5 M LiTFSI/MPPY.TFSI +10 wt% VC. Since the origin of Rct
mainly comes from the cathode interface, it can be concluded
from the above observation that the VC-containing ionic liquid
LiCoO cells using 0.5 M LiTFSI/MPPY.TFSI +10 wt% VC and 0.5 M
2
LiTFSI/MPPI.TFSI +10 wt% VC, respectively. There are several clear
contrasts worthy of mention. First, the total impedance is much
smaller than that recorded before charging. Secondly, there are
three well defined semi-circles in the cell impedance. Thirdly, all
the impedance spectra change with storage time. It is generally
recognized that the semicircle at high frequency is related to the
resistance of the surface layer, i.e. the SEI film formed on the
surface of the electrodes (RSEI) and the semicircle at medium fre-
quencies is the Faradic charge transfer resistance (Rct) [30,36–38].
As to the semicircle at low frequencies, it should be related to
electrolytes are not stable at the surface of the charged LiCoO elec-
2
trode, which are continually oxidized, leading to the increase in
charge transfer resistance. The increase in Rct not only results in
capacity fading with cycling but also results in lower coulombic
efficiency.
It should be noted that the cycling data shown in Fig. 10 were
obtained under the cycling protocol of constant current charge

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4625
4. Conclusion
It has been shown that both 0.5 M LiTFSI/MPPY.TFSI +10 wt%
VC and 0.5 M LiTFSI/MPPI.TFSI +10 wt% VC ionic liquid electrolytes
◦
were compatible with graphite electrodes. At 25 C, the Li || graphite
half cells using the VC-containing ionic liquid electrolytes exhibited
good cycling stabilities and high coulombic efficiencies; however,
they also exhibited a poor rate capability, owing to their high vis-
◦
cosities. At 50 C with reduced viscosity, the cells showed a much
◦
improved rate capability. On the contrary, at 25 C the capaci-
ties of the Li || LiCoO2 cells using the VC-containing ionic liquid
electrolytes continually decreased with cycling, mainly due to the
instability of VC on the surface of LiCoO2 electrodes.
It is worth to mention that the VC-containing ionic liquid elec-
trolytes exhibit an oxidation limit of 4.9 V (comparing 5.5 V for the
pure ionic liquid electrolyte), which is same as those of carbonate
electrolytes. As shown in this study that these VC-containing ionic
liquid electrolytes are not compatible with 4.0 V LiCoO , let alone to
2
be compatible with 5.0 V cathodes such as LiNi0.5Mn1.5O . To main-
4
tain the advantage of high oxidation stabilities, it is much better to
design ionic liquid electrolytes that do not need additives to pas-
sivate graphite electrodes for applications in high voltage lithium
ion batteries. In this regard, the ionic liquids based on fluorosul-
fonylimide (FSI) anion or the mixtures of ionic liquids containing
FSI anions have been shown to possess such promising properties
[
18,24,40,41].
Acknowledgments
This work was conducted at the Oak Ridge National Labora-
tory and supported by the ORNL laboratory-directed research and
development (LDRD) grants of D10-036. This work was also partly
supported by the ORNL LDRDprogram under contract No. DE-AC05-
0
0OR22725 with UT-Battelle, LLC.
References
[
[
1] J. Sun, D.R. MacFarlane, M. Forsyth, Ionics 3 (1997) 356.
2] M. Armand, F. Endres, D.R. MacFarlane, H. Ohno, B. Scrosati, Nat. Mater. 8 (2009)
621.
Fig. 13. The effect of charge protocol CC–CV on (a) charge–discharge capacity and
[3] M. Galinski, A. Lewandowski, I. Stepniak, Electrochim. Acta 51 (2006) 5567.
[4] R. Wibowo, S.E.W. Jones, R.G. Compton, J. Phys. Chem. 113 (2009) 12293.
[5] H. Matsumoto, H. Sakaebe, K. Tatsumi, J. Power Sources 146 (2005) 45.
(
b) cycling performance of the full cell Li| 0.5 M LiTFSI/MPPY.TFSI +10 wt% VC |LiCoO2
−2
that tested under the current density of 40 ␮A cm
.
[
6] P.C. Howlett, D.R. MacFarlane, A.F. Hollenkamp, Electrochem. Solid-State Lett.
(2004) A97.
7
[
[
7] C. Sirisopanaporn, A. Fernicola, B. Scrosati, J. Power Sources 186 (2009) 490.
8] S.K. Martha, E. Markevich, V. Burgel, G. Salitra, E. Zinigrad, B. Markovsky, H.
Sclar, Z. Pramovich, O. Heik, D. Aurbach, I. Exnar, H. Buqa, T. Drezen, G. Semrau,
M.A. Schmidt, D. Kovacheva, N. Saliyski, J. Power Sources 189 (2009) 288.
(
CC) followed by constant current discharge. In order to elim-
inate the contribution of concentration polarization within the
cathode for the lower coulombic efficiencies observed in Fig. 10,
different cycling protocol, i.e. constant current–constant voltage
charge (CC–CV) followed by constant current discharge, was used.
As shown in Fig. 13a, when the cell was charged under con-
[9] E. Markevich, V. Baranchugov, G. Salitra, D. Aubach, M.A. Schmidt, J. Elec-
trochem. Soc. 155 (2008) A132.
10] V. Baranchugov, E. Markevich, G. Salitra, D. Aubach, G. Semrau, M.A. Schmidt,
J. Electrochem. Soc. 155 (2008) A217.
[
[11] E. Markevich, V. Baranchugov, D. Aubach, Electrochem. Commun. 8 (2006)
331.
1
[
[
12] H. Sakaebe, H. Matsumoto, K. Tatsumi, Electrochim. Acta 53 (2007) 1048.
13] H. Sakaebe, H. Matsumoto, Electrochem. Commun. 5 (2003) 594.
[14] T. Sato, T. Maruo, S. Marukane, K. Takagi, J. Power Sources 138 (2004) 253.
[15] H. Matsumoto, M. Yanagida, K. Tanimoto, M. Normura, Y. Kitagawa, Y. Miyazaki,
Chem. Lett. 922 (2000).
16] A. Fernicola, F. Croce, B. Scrosati, T. Watanabe, H. Ohno, J. Power Sources 174
(2007) 342.
[17] H. Sakaebe, H. Matsumoto, K. Tatsumi, J. Power Sources 146 (2005) 693.
−2
stant current density of 40 ␮A cm to 4.2 V, followed by discharge
under the same current density to 3.0 V, the charge/discharge
−
1
−1
and 112 mAh g , respectively. As
capacities were 130 mAh g
a contrast, when the cell was charged under the constant cur-
[
−2
rent density of 40 ␮A cm to 4.2 V, followed by constant voltage
charge (CC–CV) for 1 h, and then discharged under the same cur-
rent density to 3.0 V, the charge/discharge capacities increased
[
18] M. Ishikawa, T. Sugimoto, M. Kikuta, E. Ishiko, M. Kono, J. Power Sources 162
2006) 658.
19] J. Xu, J. Yang, Y. Nulu, J. Wang, Z. Zhang, J. Power Sources 160 (2006) 621.
(
−1
to 149 and 134 mAh g , respectively. The CC–CV charge proto-
col resulted in 20% increase in the discharge capacity. However,
the charge/discharge capacities shown in Fig. 13b still decreased
with cycling, which further confirmed that the main contribu-
tion for capacity fading and lower coulombic efficiencies in Li ||
[
[20] C.M. Lang, P.A. Kohl, J. Electrochem. Soc. 154 (2007) F106.
[
[
[
21] H. Zheng, B. Li, Y. Fu, T. Abe, Z. Ogumi, Electrochim. Acta 52 (2006) 1556.
22] J.H. Shin, P. Basak, J.B. Kerr, E.J. Cairns, Electrochim. Acta 54 (2008) 410.
23] J.Y. Mun, Y.S. Jung, T. Yim, H.Y. Lee, H.J. Kim, Y.G. Kim, S.M. Oh, J. Power Sources
194 (2009) 1068.
[
24] G.B. Appetecchi, M. Montanino, A. Balducci, S.F. Lux, J. Power Sources 192 (2009)
LiCoO cells using VC-containing ionic liquid electrolytes were the
2
599.
instability of VC on the surface of highly oxidative LiCoO2 elec-
trode.
[25] S.F. Lux, M. Schmuck, G.B. Appetecchi, S. Passerini, M. Winter, A. Balducci, J.
Power Sources 192 (2009) 606.

4
626
X.-G. Sun, S. Dai / Electrochimica Acta 55 (2010) 4618–4626
[
[
[
[
26] D.R. MacFarlane, P. Meakin, J. Sun, N. Amini, M. Forsyth, J. Phys. Chem. B 103
1999) 4164.
27] P.C. Howlett, D.R. MacFarlane, A.F. Hollenkamp, J. Power Sources 114 (2003)
77.
28] O. Matsuoka, A. Hiwara, T. Omi, M. Toriida, T. Hayashi, C. Tanaka, Y. Saito, T.
Ishida, H. Tan, S.S. Ono, S. Yamamoto, J. Power Sources 119–121 (2003) 368.
29] D. Aubach, K. Gamolsky, B. Markovsky, Y. Gofer, M. Schmidt, U. Heider, Elec-
trochim. Acta 47 (2002) 1423.
30] X.G. Sun, C.A. Angell, Electrochem. Commun. 11 (2009) 1418.
31] Y.B. Fu, C. Chen, C.C. Qiu, X.H. Ma, J. Appl. Electrochem. 39 (2009) 2597.
32] O. Hitoshi, S. Yuuichi, I. Atsuyoshi, Y. Sghoji, J. Electrochem. Soc. 151 (2004)
A1659.
[34] S.K. Jeong, M. Inaba, R. Mogi, Y. Iriyama, T. Abe, Z. Ogumi, Langmuir 17 (2001)
8281.
[35] A. Lewandowski, A. Swiderska-Mocek, I.A. Acznik, Electrochim. Acta 55 (2010)
1990.
[36] S.S. Zhang, K. Xu, T.R. Jow, J. Power Sources 115 (2003) 137.
[37] Y.C. Chang, J.H. Jong, G.T. Fey, J. Electrochem. Soc. 147 (2000)
2033.
[38] T.S. Ong, H. Yang, Electrochem. Solid-State Lett. 4 (2001) A194.
[39] E. Barsoukov, D.H. Kim, H.S. Lee, H. Lee, M. Yakovleva, Y. Gao, J.F. Engel, Solid
State Ionics 161 (2003) 19.
(
2
[
[
[
[40] S. Seki, Y. Kokayashi, H. Miyashiro, Y. Ohno, Y. Mita, N. Terada, P. charest, A.
Guerfi, K. Zaghib, J. Phys. Chem. C. 112 (2008) 16708.
[41] E. Paillard, Q. Zhou, W.A. Henderson, G.B. Appetecchi, M. Montanino, S.
Passerini, J. Electrochem. Soc. 156 (2009) A891.
[
33] X. Zhang, R. Kostecki, T.J. Richardson, J.K. Pugh, P.N. Ross Jr., J. Electrochem. Soc.
1
48 (2001) A1341.