Formulation of Ionic Liquid Electrolyte to Expand the Voltage
Window of Supercapacitors
Katherine L. Van Aken, Majid Beidaghi, Yury Gogotsi
Department of Materials Science and Engineering and A.J. Drexel Nanomaterials Institute, Drexel University, Philadelphia, USA
Side View
Backing
(PTFE plate)
Electrode
(OLC)
Reference
(silver wire)
-1.5
-1.0
-0.5
0.0
0.5
1.0
0
0
100
-100
200
-200
-1.5
Voltage vs. Ag/AgCl (V)
-1.0
0.5
1.0
300
200
-100
100
0
0
100
-100
200
-200
-1.0
-0.5
0.0
0.5
20 mVs-1
10 mVs-1
5 mVs-1
-300
-200
300
200
-100
100
0
0
100
-100
-300
1.5
1.0
-1.5
-200
-1.0
-0.5
0.0
0.5
1.0
Voltage vs. Ag/AgCl (V)
Voltage vs. Ag/AgCl (V)
80% EMI-BF4
100% EMI-BF4
-400
20 mVs-1
10 mVs-1
5 mVs-1
300
200
-100
100
0
0
100
-100
200
-200
300
-1.5
-300
-1.0
-0.5
0.0
0.5
1.0
Voltage vs. Ag/AgCl (V)
1.5
Current Density (mA/g)
-200
1.5
50% EMI-BF4
20 mVs-1
10 mVs-1
5 mVs-1
-200
0.0
Voltage vs. Ag/AgCl (V)
20% EMI-BF4
-300
-0.5
400
20 mVs-1
10 mVs-1
5 mVs-1
-300
-200
1.5
300
200
-100
100
0
0
100
-100
200
-200
300
-1.5
-1.0
-0.5
0.0
0.5
1.0
-300
1.5
-1
30
EMI-TFSI:BF4 (80:20)
20
EMI-BF4
EMI-TFSI
10
0
2000
4000
• Through the use of a three-electrode setup, each electrode’s performance can
be monitored separately, helping to study electrolyte stability on anode or
cathode.
• By mixing two ionic liquids with different anions, we were able to balance the
charge on the two electrodes and increase the stability window of the device.
• The increased symmetry between electrodes successfully increased the
operating window to 3.5 V, confirmed by capacity retention after 10,000 cycles.
• Designing Ionic liquid mixtures for supercapacitors can result in an increase in
operating potential window and thereby the energy density of the resulting
device.
6000
8000
10000
Cycle
30
20
10
EMI-BF4
EMI-TFSI
EMI-TFSI:BF4 (80:20)
0
2000
4000
6000
8000
10000
Cycle
25
Before
Cycling
After
20
Cycling
15
10
EMI-BF4
5
EMI-TFSI
0
EMI-TFSI:BF4 (80:20)
0
5
Voltage vs. Ag/AgCl (V)
Conclusions
• Samples were tested using
conventional sandwich cells.
• Anode and cathode potentials
were independently measured
using an Ag(AgCl) wire reference
placed between separator layers.
• CV, GC and EIS measurements
performed on a BioLogic VMP3
potentiostat/galvanostat.
Electrode Capacitance Fg
100
Resistance (cm )
-200
-100
• Cyclic voltammograms for
mixtures of different ratios are
shown left.
• Pure EMI-TFSI and EMI-BF4
ionic liquids as well as mixtures
with 10%, 20%, 50%, and 80%
EMI-BF4 are shown.
• Performance of the mixtures
varies but the most symmetric
CV curves are obtained with
the mixture of 20% EMI-BF4
• To confirm the performance
of the 20% mixtures, a lifetime
test of 10,000 cycles was
performed at a higher
potential window of 3.5 V
(shown right).
• While the pure ionic liquids
show a decrease in
capacitance and an increase in
resistance, the mixture with
20% EMI-BF4 maintains > 92%
of its original capacitance.
• EIS before and after cycling
at 3.5 V (bottom right)
confirms the electrolyte
decomposition of the two pure
ionic liquids and the stability of
the mixture even at this high
potential.
-Im[Z] ()
200
200
Results: Lifetime Tests
2
-100
-200
Current Density (mA/g)
100
Current Density (mA/g)
0
Current Density (mA/g)
Current Density (mA/g)
0
-300
Experimental Setup: 3-Electrode Sandwich Cell
Separator
100
300
-1.5
• Onion-like carbon was
chosen as electrode due to its
open and accessible surface.
• This provides a model system
without the effect of pore size
and kinetic limitations.
Current collector
(carbon-coated Al)
-100
20 mVs
10 mVs-1
5 mVs-1
200
Experimental Setup: Onion-Like Carbon
Top View
200
300
-1
Current Density (mA/g)
• ILs are solvent-free
electrolytes; many are
electrochemically stable up to 4
V , compared to 2.5-2.7 V for
organic electrolytes.4
• EMI-TFSI and EMI-BF4 were
tested alone and in mixtures;
they are only stable up to ~3 V.
-200
Current Density (mA/g)
Experimental Setup: Ionic Liquid Electrolytes
20 mVs
10 mVs-1
5 mVs-1
10% EMI-BF4
-300
Current Density (mA/g)
Experimental
300
-1
Current Density (mA/g)
Fixed Voltage Window =
Ecounter + Eworking
0% EMI-BF4
-300
Current Density (mA/g)
• The energy density of supercapacitors can
be increased by increasing the operating
voltage window of the device.
• By balancing the performance of the two
electrodes, this window can be increased.
• A variety of methods including charge
injection1, mass balancing2, and asymmetric
carbon selection3 have been studied.
• Mixing ionic liquid electrolytes provides a
simpler, more effective way of balancing the
electrode’s performance, thereby increasing
the operating potential.
Results
Results: Ionic Liquid Mixtures
Current Density (mA/g)
Introduction
Current Density (mA/g)
Reference Wire
10
15
20
25
Re[Z] ()
Acknowledgements
This work was supported as part of the Fluid Interface
Reactions, Structures and Transport (FIRST) Center, an Energy
Frontier Research Center funded by the U.S. Department of
Energy, Office of Science, Office of Basic Energy Sciences.
References
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(2010).
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