1 Stable Electrolyte for High Voltage Electrochemical Double-Layer Capacitors

7 
electrode cells using EIS (Figure 2). The low-frequency EIS response of an ideal capacitor is a 
vertical line parallel to the imaginary axis in the Nyquist plot.
20
Any deviation from vertical 
indicates the presence of a leakage current or undesirable side-reactions.
43
EIS spectra acquired 
near the positive voltage limit are shown in Figure 2a. Up to 3.9 V vs. Na/Na
+
the Nyquist plots 
show vertical lines at 90° with respect to the real axis. At 4.05 V the line is slightly skewed to 
89°, and at even higher voltages the EIS spectra transition to a semicircular arc. The electrolyte 
and electrode are stable up to 3.9 V vs. Na/Na
+
, but some side reaction begins at higher voltage. 
The most likely side-reactions at high voltage are either corrosion of the aluminum collector or 
oxidation of DME. Our EIS results cannot distinguish these two mechanisms. Results from 
voltammetry with glassy carbon (Figure 1) would suggest that corrosion is responsible for the 
narrower voltage window when aluminum current collectors are used. However, it is also 
possible that the oxidation of DME is catalyzed by the BP2000 carbon black compared to glassy 
carbon. Voltammetry of glyme-based electrolytes on platinum electrodes show the onset of 
oxidation around 4.0 V vs. Li/Li
+
, suggesting that oxidation of the DME solvent could limit the 
upper voltage on more active electrodes.
62, 63
The negative voltage limit extends almost to the sodium potential as evident by EIS spectra 
collected between 3.9 and 0.05 V vs. Na/Na
+ 
(Figure 2b). At low-frequencies there is no 
deviation from vertical lines over this entire voltage range, indicating no side-reactions occur 
within this window. Importantly, the electrodes are not passivated even within 50 mV vs. 
Na/Na
+
.
A minimum in the capacitance measured by EIS occurs near 3.0 V vs. Na/Na
+
(Figure 3a). This 
minimum, which also corresponds to the open circuit potential, reflects the potential of zero 
charge of the carbon electrode.
64
Moving away from the potential of zero charge,
the capacitance 
steadily increases, confirming the electrodes do not passivate over this voltage range. The same 
trend is also seen in the cyclic voltammogram (Figure 3b). The capacitance values measured by 
EIS range from 50 to 250 F/g, which is comparable to what is obtained with high surface area 
electrodes in ACN-based electrolyte with three-electrode cells.
32, 35

8 
To verify that no side-reactions occur between 0.05 and 3.9 V vs. Na/Na
+
leakage currents were 
also measured in the three-electrode cells (Figure 4). The carbon electrode was held at different 
potentials for 12 h while the decay in the current was monitored. The leakage currents are ≤ 1 
µA/cm
2
(< 20 µA/F) at both the low and high limits of the voltage window. Impedance spectra 
were collected before and after the leakage current measurements to verify that the low currents 
were not due to electrode passivation (data not shown). Together, the data obtained from the 
three-electrode cells indicate that the NaPF
6
/DME electrolyte has a voltage window >3.8 V.
The voltage window was also measured in full cells with two high-surface-area carbon 
electrodes and without a sodium metal reference electrode. The electrochemical window for a 
full cell EDLC is very sensitive to the electrode balance (the relative mass loading of each 
electrode).
65-67
Since the open circuit potential of each electrode is near 3.0 V vs. Na/Na
+
, the 
positive electrode can only be charged +0.9 V before reaching the positive voltage limit of this 
electrolyte. The negative electrode must charge almost -3.0 V to reach the negative limit of the 
electrolyte, which is essentially the sodium electrode potential. Therefore, the full voltage 
window of the electrolyte cannot be demonstrated in a symmetric cell, where each carbon 
electrode has the same loading. Since capacitance adds reciprocally for capacitors in series 
(Equation 1), the specific capacitance is maximized for symmetric cells where each electrode has 
the same capacitance. In asymmetric cells, the total capacitance is dominated by the electrode 
with the lower capacitance. From a practical perspective, therefore, the specific capacitance and 
specific energy would be highest if the open circuit potential of the carbon could be shifted to lie 
near the middle of the voltage window of the electrolyte (approximately 2 V vs. Na/Na
+
). Efforts 
are currently underway in our laboratory to accomplish this. Here, in order to demonstrate the 
voltage window in full cells, we show results for cells built with different mass loadings for the 
positive and negative electrodes. The mass loading of the positive electrode is approximately a 
factor of five greater than the negative electrode. 
 
The voltage window for the full cells was first checked by EIS (Figure 5). Up to 3.8 V the 
Nyquist plots exhibit vertical tails, indicating the cell behaves as an ideal capacitor with no 
measurable side-reactions (Figure 5a). At 4.0 V the Nyquist plot is no longer vertical, 
demonstrating that the voltage window has been exceeded. The same behavior is also seen in the 
Bode plots of the phase angle (Figure 5b) where the phase approaches -90° at low frequency up 

9 
to 3.8 V, but begins to deviate from ideal behavior at 4.0 V. The capacitance values extracted 
from the EIS data are shown in Figure 5c. The specific capacitance for the full cell with DME-
based electrolyte is comparable to what can typically be achieved with ACN-based electrolytes 
in similar two-electrode cells.
32, 33
The long-term performance of the full cells was tested using a combination of charge-discharge 
cycling and voltage holds (float tests).
68
One duty cycle of the testing protocol is illustrated in 
Figure 6a. For comparison, cells using a standard ACN-based electrolyte (1 m TEABF
4
in ACN) 
were subjected to the same tests. The cells built with the ACN electrolyte used two carbon 
electrodes with the same loading for both positive and negative electrodes. The voltage window 
of the ACN electrolyte is more symmetric with respect to the point of zero charge of carbon 
electrodes (approximately 3.0 V vs. Na/Na
+
),
35
and EDLCs made with ACN-based electrolyte 
typically use electrodes with identical loading. As expected, the ACN-based electrolyte was very 
stable when charged to 3.0 V. Figure 6b shows EIS spectra collected after the cell with ACN-
based electrolyte was held at 3.0 V for different periods. The EIS spectra remain unchanged after 
up to 50 hours of floating at 3.0 V. An identical cell with ACN electrolyte charged to 3.5 V 
degrades rapidly (Figure 6c). The resistance increases significantly and the capacitance 
decreases. By contrast, a cell with the DME electrolyte charged to 3.5 V is much more stable 
(Figure 6d).
The EIS data from Figure 6 were used to determine the effective series resistance (ESR) and 
specific capacitance as a function of the total time held at the maximum voltage (float time) 
(Figure 7). The ESR is measured by extrapolating the low frequency impedance (vertical line) to 
the real axis, and the capacitance is also calculated from the low frequency impedance. The ESR 
of cells with the DME electrolyte is initially higher than with ACN electrolyte, which is expected 
based on the lower conductivity of the DME electrolyte (Figure 7a). However, no resistance 
increase is measured after 50 hours of floating at 3.5 V with the DME electrolyte. In contrast, the 
cell with ACN shows a rapid increase in resistance after 20 hours at 3.5 V. At first the specific 
capacitance of cells with the DME electrolyte is lower than with ACN, but the specific 
capacitance of the DME cell increases during the float test at 3.5 V (Figure 7b). This is in sharp 
contrast to the ACN cell, which shows a steep drop in specific capacitance after 20 h at 3.5 V. 

10 
The origins of the capacitance rise for the DME cell are not known, but could derive simply from 
better wetting of the electrolyte with time. These results clearly show that the DME electrolyte 
enables cells to operate at higher voltage with similar specific capacitance compared to cells built 
with ACN-based electrolytes. 
Full cells with the DME electrolyte were also tested by charge-discharge cycling at constant 
current between 0 and 3.5 V without voltage holds. EIS data were also collected after every 100 
cycles after the cell was fully discharged (0 V cell voltage). Representative voltage profiles are 
shown in Figure 8 up to 3000 cycles. The cycling stability was also evaluated from EIS and 
coulombic efficiency (Figure 9). The capacitance measured by EIS (at 0 V cell voltage) increases 
with cycling (Figure 9a), consistent with the results of the float tests (Figure 7b). However, the 
capacity measured by charge-discharge cycling steadily decreases with cycle number (Figure 8). 
The capacity fade (Figure 8) may reflect an increase in the leakage current due to electrolyte 
impurities or other side-reactions that occur at high voltage, but do not occur when the cell is 
fully discharged to 0 V.
69
The results highlight the need to use multiple measurement techniques 
to fully characterize the performance of EDLCs.
One concern is that sodium ions may intercalate into the negative electrode.
35, 44, 45
If small 
amounts of sodium intercalate, the process is highly reversible. The coulombic efficiency 
exceeds 99% after 20 cycles and 99.9% after 500 cycles (Figure 9b). Raman spectroscopy was 
also performed on electrodes after 3000 charge-discharge cycles to investigate if the carbon 
structure changed due to intercalation or other side reactions (Figure 10).
70, 71
No changes were 
observed in the D and G bands at either the positive or negative electrode, confirming the 
electrode stability.
While increasing the voltage window of the electrolyte is one approach to improve EDLC energy 
density, high electrolyte conductivity is required for high power performance. Acetonitrile-based 
electrolytes have conductivities > 50 mS/cm, which are unmatched by any other non-aqueous 
electrolyte.
16, 17
Jow and Shacklette measured the conductivity of NaPF
6
in DME and found the 
maximum conductivity is 13 mS/cm for a 1.5 molar solution.
72
We verified that the conductivity 
of the 1 molal solution used in this study is 12 mS/cm at 24 °C. The viscosity of the same 

11 
solution is 1.4±1 cP at 20 °C. The conductivity is similar to what can be achieved with 
carbonate-based electrolytes used in lithium batteries and some commercial capacitors.
17, 27, 73
The conductivity of the DME-based electrolyte is superior to many ionic liquids under 
consideration for EDLC applications.
30
While DME-based electrolytes are unlikely to compete 
with ACN electrolytes for applications that demand very high power, they could provide a 
significant improvement in energy density over today’s commercial EDLCs. DME electrolytes 
could enable EDLCs with high energy density and moderate power, potentially expanding the 
EDLC market.

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