Recent Advances in Electrolytes for Electrochemical
Double Layer Capacitors
V. R. Koch
Covalent Associates, Inc.
10 State Street
Woburn, Massachusetts 01801
USA
koch@covalentassociates.com
1.0 INTRODUCTION
Over the past twenty-five years, advances in electrochemical double layer capacitor
(EDLC) technology have been largely driven by improvements to their non-aqueous
electrolytes. Such electrolytes typically employ quaternary ammonium salts solubilized
in oxidatively stable solvents such as acetonitrile (AN) and propylene carbonate (PC) (1).
Commercially available EDLC electrolytes provide an electrochemical window of greater
than 4V at highly polished analytical working electrodes, and approximately 3V in
commercially available EDLC devices, also known as “ultracapacitors”. The ~1V lower
electrochemical window provided by commercial devices is due to the need for high
surface area electrodes, which store practical amounts of charge. Such carbonaceous
electrodes incorporate surface redox sites, which in turn truncate the available
electrochemical window.
The ideal EDLC electrolyte possesses the following characteristics:
•
•
•
•
•
A wide (>4V) electrochemical window in EDLC hardware
A specific conductance in excess of 75 mS/cm at room temperature
A thermal stability approaching 300°C
A freezing point or glass transition temperature below -60°C
Low toxicity and acceptable cost
Of course, no such electrolyte presently exists. That said, a worldwide effort is underway
to expand the performance envelope of EDLC electrolytes comprising non-aqueous
solvents and salts that incorporate charge delocalized anions and cations. This paper
focuses on the formulation, characterization and performance of some new ionic liquidbased electrolytes designed for low and high temperature performance in ultracapacitors.
2.0 IONIC LIQUID MATERIALS
Covalent’s EDLC electrolyte technology is based on a family of salts known as
hydrophobic ionic liquids (ILs). These materials possess a unique set of physical,
chemical, and electrochemical properties that strongly favor their use as electrolytes in
1
EDLCs. Unlike ILs that incorporate reactive ions that decompose to form HF, the
Covalent salts are thermally and hydrolytically stable. In this paper we focus on the
formulation of task specific electrolytes derived from IL materials, which in and of
themselves are suitable EDLC electrolyte media tailored for high temperature
applications.
Unlike molecular solvents that are charge neutral, ILs may be described as room
temperature molten salts composed solely of polyatomic anions and cations. Covalent’s
IL technology is based on the judicious pairing of delocalized heterocyclic organic
cations and charge stabilized organic and inorganic anions shown in Figures 1 and 2 (25).
N+
N
N+
N
N
N
Imidazolium
Pyrazolium
N+
N+
S
O
Thiazolium
N+
Pyridazinium
Inorganic
Triazolium
BR4PR6-
N+
Oxazolium
Organic
_
RSO3(RSO2)2N(RSO2)3C-
Pyridinium
N
N
N+
N+
N+
Pyrimidnium
Pyrazinium
Figure 1: Cation Structures
R = halide, CF3, C2F5, and other
electronegative alkyl and aryl
substituents.
N
Figure 2: Anion Structures
Owing to the strong coulombic attraction between ions of opposite charge, ILs possess no
measurable vapor pressure in contrast to highly volatile EDLC electrolytes derived from
AN. A number of highly desirable properties of neat ILs as EDLC electrolytes follow:
•
•
•
•
•
Ion concentrations of from 4M to 6M
Wide liquid range (-90°C to 400°C)
Non-flammable with low toxicity
Non-corrosive to electrode and packaging components at elevated temperatures
Isothermal stabilities approaching 300°C with no measurable vapor pressure
While this set of advantages is unprecedented in light of prior art, one must always be
cognizant of EDLC electrolyte conductivity, which is dependent on ion mobility. Ion
mobility in electrolytes is, in turn, governed by IL viscosity. While the strong coulombic
attraction between ions of opposite charge is directly responsible for the manifold of
desirable properties listed above, we have found that IL viscosities are minimally two
2
orders of magnitude greater than those of most common molecular solvents. Thus, the
ionic conductivities of neat ILs, which typically range from 4 to 14 mS/cm at 22°C, are
patently insufficient for EDLC applications at room temperature and below but are
suitable for high temperature applications.
Of course, one can reduce the viscosity of an IL by simply blending it with a molecular
solvent. While such blends afford excellent room and low temperature conductivities,
non-flammability may be lost and issues of toxicity must again be addressed, particularly
in respect to the use of AN as a co-solvent.
We have solved these problems by designing one class of EDLC electrolytes that perform
well at room temperature and below, and another class of electrolytes formulated for high
temperature applications. We consider each in turn.
3.0 ROOM TEMPERATURE/LOW TEMPERATURE EDLC ELECTROLYTES
3.1 Blended Electrolytes
Because of its low viscosity (0.33 cP at 30°C), wide electrochemical window (> 4V), and
large dielectric constant (37.5), AN is the solvent of choice for use in EDLC electrolytes
where high device power is required. However, in addition to flammability a more
serious problem involves its acute toxicity. AN may readily be inhaled and absorbed
through the skin where it is converted by the liver to formic acid and HCN.
An alternative solvent for use in formulating EDLC electrolytes is methyl formate (MF).
MF has a viscosity similar to that of AN (0.33 cP at 25°C), but a lower dielectric constant
(8.5) and narrower electrochemical window (~ 3.2V). Although flammable, MF does not
possess AN’s acute toxicity.
Figure 3 shows an overlay of two linear sweep voltammograms for AN and MF, both 2M
in 1-ethyl-3-methylimidazolium hexafluorophosphate (EMIPF6) (I) obtained at a glassy
carbon working electrode. The overlay clearly reveals the advantage that AN has over
MF in respect to the electrochemical window.
N
N
I
3
PF6-
Current µA/cm2
2M EMIPF6/MF
2M EMIPF6/AN
Potential (V)
Figure 3: Linear sweep voltammograms obtained at a glassy C working electrode vs. a
Ag wire quasi-reference electrode.
3.2 Non-Flammable Blended Electrolytes
In an effort to develop safer electrolytes for EDLC applications, we have reformulated a
number of IL/molecular solvent blends with and without AN. The room temperature
conductivity, electrochemical window, and flammability of these electrolytes were
evaluated.
The electrochemical window was assessed by linear sweep voltammetry at a glassy
carbon working electrode. The cutoff voltages were taken at the very conservative
current density of 20 µA/cm2. Flammability was determined by a modified Underwriters
Laboratories test (UL 94 Standard). In this test a 10 cm length of glass fiber wick was
suspended horizontally in a fume hood. The middle 5 cm of wick was thoroughly soaked
with electrolyte, then exposed to the flame from a propane torch. Those electrolytes that
burst into flame were designated as “flammable”. Those that smoked but did not ignite
were designated as “non-flammable”.
Table 1 presents electrochemical data for several formulated AN-containing electrolytes
and compares them with their flammable analogs. “Im” refers to the
bis(trifluoromethylsulfonyl)imide anion [(CF3SO2)2N-], “TEA” refers to the
tetraethylammonium cation [Et4N+], and “NF” refers to non-flammable.
4
Table 1: AN-Based EDLC Electrolytes
Electrolyte
σ, mS/cm at 25 °C
UE, V
2M EMIPF6/AN
2M EMIPF6/AN-NF
65
58
4.16
4.06
2M EMIIm/AN
2M EMIIm/AN-NF
47
44
4.16
4.05
1.5M TEABF4/AN
1.5M TEABF4/AN-NF
55
44
4.27
4.25
Table 1 reveals that a small price in conductivity and UE is paid for the benefit of nonflammability. The room temperature conductivities of the NF electrolytes are somewhat
lower, and the electrochemical window contracts slightly. Nonetheless, the 2M
EMIPF6/AN electrolytes possess both a higher ion concentration and ionic conductivity
than the saturated quaternary ammonium salt/AN electrolyte, which offers an advantage
in UE. The thermally stable EMIIm-based electrolytes provide a wide electrochemical
window, albeit with lower conductivity due to the significantly larger ionic radius of Imcompared to PF6-.
In Table 2, we present data for a group of AN-free electrolytes. Here we do not pay a
penalty in UE on going from a flammable to a reformulated non-flammable electrolyte.
However, the ionic conductivity is lowered for all non-flammable formulations as seen
Table 2: AN-Free EDLC Electrolytes
σ, mS/cm at 25 °C
UE, V
2M EMIPF6/MF
2M EMIPF6/MF-NF
46
34
3.10
3.10
Covalent 031
Covalent 031-NF
39
36
3.25
3.25
Covalent 111
Covalent 111-NF
29
27
3.30
3.30
Covalent 211
Covalent 211-NF
22
19
3.60
3.60
Electrolyte
previously in the AN-based electrolytes. Three new AN-free blends designated as
Covalent 031, 111, and 211 manifest either good conductivity coupled with a truncated
electrochemical window, or a wide electrochemical window coupled with lower
5
conductivity. These new formulations would be particularly attractive for those device
applications that preclude the presence of AN.
3.3 Summary of Room Temperature/Low Temperature Electrolytes
We have demonstrated the utility of a number of non-flammable and AN-free electrolytes
designed to operate at room temperature and below. Such electrolytes possess various
advantages and disadvantages in respect to flammability and toxicity that must be
carefully weighed in choosing an electrolyte for a specific EDLC application.
4.0 HIGH TEMPERATURE EDLC ELECTROLYTES
Two different ionic liquid-based electrolytes designated Covalent 400 series and 500
series were used in this study. The electrolytes were evaluated in respect to ionic
conductivity and electrochemical window at temperatures up to 175°C. The long-term
compatibility of electrolyte with electrode materials was studied over a 1000 hr storage
period at 150°C. EDLC cycling and capacity retention experiments were accomplished
in unoptimized 2325 crimped coin cells. Electrode disks were punched from two
different materials: SpectraCarb 2220 woven carbon cloth (SpectraCorp), and a
carbonaceous electrode material bonded to an Al current collector (EC150X1) kindly
provided by W. L. Gore & Associates, Inc.
4.1 Electrolyte Properties as a Function of Temperature
Ionic conductivity: Figure 4 presents a plot of specific conductance vs. temperature. At
150°C, the highest cycling temperature used in this project, the 400 series electrolyte was
found to possess a conductivity of 60 mS/cm while that for the 500 series electrolyte was
36 mS/cm. These values are similar to those for room temperature acetonitrile-based
EDLC electrolyte formulations where acetonitrile’s viscosity is an order of magnitude
lower compared to those for ionic liquid materials.
6
120
Conductivity (mS/cm)
100
400
80
500
60
40
20
0
0
50
100
150
o
Temperature ( C)
200
250
Figure 4: Specific conductance of 400 and 500 series electrolytes vs. temperature.
Electrochemical window: Figure 5 presents an overlay of cyclic voltammograms
obtained at three different temperatures on electrolyte 400. The electrochemical window,
∆V, was observed to rapidly contract with increasing temperature in accordance with the
formalism of the Nearnst equation. At room temperature ∆V approaches 4V, but falls to
less than 1.5V at 200°C. The electrochemical window of an electrolyte would be
expected to further contract when high surface area electrodes incorporating unspecified
and reactive surface sites are substituted for an analytical working electrode employed in
CV studies.
0.20
o
25 C
o
o
100 C
2
Current (mA/cm )
200 C
0.00
-0.20
-2.50
-1.50
-0.50
0.50
1.50
2.50
E (V vs. Ag )
Figure 5: Cyclic voltammograms obtained at a glassy C working electrode on 400 series
electrolyte at various temperatures. The sweep rate was 20 mV/s.
7
Electrode/Electrolyte Compatibility: To assess the long-term stability of the Gore
electrode at elevated temperatures, we immersed electrode disks in electrolytes 400 and
500 under Ar in Teflon bottles. The bottles were stored in an oven at 150°C for 1000 hr.
Figure 6 presents an overlay of linear sweep voltammograms obtained in electrolyte 400
after storage, and these data are virtually identical to those obtained with electrolyte 500.
B
A
2
Current (mA/cm )
0.2
C
B
0
C
A
-0.2
-3
-2
-1
0
1
2
3
E (V vs. Ag)
Figure 6: Linear sweep voltammograms obtained at a glassy C working electrode on
electrolyte 400 at room temperature: A) before exposure to 150°C soak; B) after 1000 hr
soak at 150°C; C) after 1000 hr soak at 150°C with the Gore electrode. The sweep rate
was 20 mV/s.
The three linear sweep voltammograms reveal that the ionic liquid-based electrolytes are
thermally stable at 150°C over very long periods of time. With the exception of a small
cathodic peak observed for C at –2V, there were no deleterious interactions between the
Gore electrode and the 400 and 500 electrolytes. Indeed, the Gore Al current collectors
were found to be smooth and shiny, i.e., unchanged after the 1000 hr soak at 150°C
indicative of the lack of corrosive processes.
4.2 EDLC Cycling Results at Elevated Temperatures
Four sets of 2325 coin cells were constructed. These cells incorporated SpectraCarb and
the Gore electrode materials along with electrolytes 400 and 500. Duplicate cells were
cycled symmetrically at the 4C rate to 2.5V at 100°C, and to 1.75V at 150°C. Figures 7
and 8 each present a plot of capacity vs. cycle life for the first 500 cycles. Over the first
50 cycles at 100°C, cells with SpectraCarb electrode material were observed to fade
slightly while those with the Gore electrode material did not. At 150°C, irrespective of
the electrode material, the coin cells were observed to cycle in parallel.
8
0.7
0.6
Capacity (mAh)
100oC
Gore
0.5
SpectraCarb
0.4
0.3
0.2
0.1
0.0
0
100
200
300
400
500
Cycle number
Figure 7: Capacity vs. cycle number for electrolyte 400 at 100ºC.
0.5
150oC
Gore
Capacity (mAh)
0.4
0.3
SpectraCarb
0.2
0.1
0.0
0
100
200
300
400
500
Cycle number
Figure 8: Capacity vs. cycle number for electrolyte 400 at 150ºC.
4.3 EDLC Capacity Retention at 150ºC
The ability to retain charge on open circuit is an important parameter in capacitor
technology. Coin cells incorporating the Gore and SpectraCarb electrodes underwent a
number of charge/discharge cycles at 150ºC before being charged to 1.75V, and then
switched to open circuit. The voltage/time behavior on open circuit of the Gore
electrodes after 100 and 600 cycles was found to be virtually identical as shown in Figure
9. Upon discharge after the 2-hour soak at 150ºC, coin cells containing the Gore
9
electrodes retained 78% of their initial capacity while coin cells containing the
SpectraCarb electrodes retained 70% of their initial capacity.
2.00
150oC
Voltage (V)
1.50
1.00
0.50
after 100 cycles
after 600 cycles
0.00
0
20
40
60
80
100
120
140
Time (min)
Figure 9: Capacity retention on OCV for Gore electrodes and electrolyte 400 at 150ºC.
Table 3 summarizes the data collected to date on the 400 and 500 electrolytes. These
data are compared to those collected by Sato and coworkers who studied the behavior a
quaternary ammonium tetrafluoroborate ionic liquid electrolyte in EDLCs as a function
of temperature.
10
Table 3: Electrode/electrolyte performance data at various temperatures.
Electrolyte
R4NBF4
400b
500b
a
Electrode
material
T, C
Applied
potential, V
C, F/g
Coulombic
efficiency, %
pyrolyzed
phenol resin
22
100
150
2.50
2.50
2.50
25.4
23.7
17.5
98.5
78.5
33.7
Gore
SpectraCarb
100
100
2.50
2.50
20.4
27.5
99
99
Gore
SpectraCarb
150
150
1.75
1.75
18.4
26.2
99
98
Gore
SpectraCarb
100
100
2.50
2.50
19.8
26.2
99
99
Gore
SpectraCarb
150
150
1.75
1.75
17.6
23.1
99
99
Gore
SpectraCarb
175
175
1.50
1.50
15.9
21.5
99
99
o
a
T. Sato, et al., Electrochimica Acta, 49, 3603 (2004); cycling data collected at the C
rate.
b
Cycling data collected at the 4C rate.
Of particular interest are the coulombic efficiency values derived from the 400 and 500
electrolytes at elevated temperatures from both the Gore and SpectraCarb electrode
materials. These values (obtained at the 4C rate) greatly exceed those of Sato and
coworkers (obtained at the C rate). The SpectraCarb electrode material afforded higher
capacitance values compared to the Gore electrode material (presumably due to a more
open pore structure). However, practical EDLC devices require a wound construction
that is not readily accomplished with a woven carbon cloth electrode material. Thus,
Covalent’s ionic liquid-based electrolytes in concert with the Gore electrodes provide a
solution to those EDLC power sources required to operate at high temperatures.
The data shown in Table 3 highlight the tradeoff between the conflicting requirements of
high device power and long cycle life at elevated temperatures. Because of
thermodynamic limitations relating to electrolyte stability at high potentials and the
activation of the carbonaceous electrode surface, device engineers must choose between
high power densities with a truncated cycle life, or lower power densities with a longer
cycle life.
4.4 Summary of High Temperature Electrolytes
The careful formulation of ionic liquid electrolyte components enables high temperature
EDLC applications. Such electrolytes ensure:
11
•
•
•
•
•
Long-term thermal stability
High coulombic efficiency
Excellent capacity retention
Non-flammable electrolyte materials with low toxicity
Compatibility with “off the shelf” electrode materials and packaging components
5.0 CONCLUSIONS
No single electrolyte formulation will meet all of the industry’s performance
requirements. However, we have demonstrated that task specific electrolytes may be
designed for EDLC applications over various thermal envelopes. Covalent is presently
working with device manufacturers to bring such EDLC devices to the market.
6.0 REFERENCES
1.
2.
3.
4.
5.
M. Ue, et al., J. Electrochem. Soc., 144, 2684 (1997).
U.S. Patent 5,827,602
U.S. Patent 5,965,054
U.S. Patent 5,973,913
U.S. Patent 6,531,241
12