1 Stable Electrolyte for High Voltage Electrochemical Double-Layer Capacitors

16 
Figure 2. EIS (20 kHz – 10 mHz) of high-surface-area carbon electrodes in 1 m NaPF
6
/DME 
electrolyte at different voltages. EIS data were measured in three-electrode cells with sodium 
counter and reference electrodes. The carbon electrode was held at each voltage step for 2 h prior 
to measurement. (a) Nyquist plots of the impedance response between 3.75 and 4.35 V vs. 
Na/Na
+
. (b) Nyquist plots of the impedance response between 0.05 and 3.9 V vs. Na/Na
+
.

17 
Figure 3. (a) Capacitance (normalized to F/g of carbon) extracted from the EIS response of a 
high-surface-area carbon electrode in 1 m NaPF
6
/DME electrolyte at different voltages. A subset 
of the EIS spectra are shown in Figure 2b. (b) Cyclic voltammogram of a high-surface-area 
carbon electrode in 1 m NaPF
6
/DME electrolyte taken at a scan rate of 2 mV/s between 0.05 and 
3.9 V vs. Na/Na
+
. Both EIS and CV data were measured in three-electrode cells with sodium 
counter and reference electrodes. 

18 
Figure 4. Leakage current of high-surface-area carbon electrode in 1 m NaPF
6
/DME electrolyte 
at different voltages. Leakage currents were measured in three-electrode cells with sodium 
counter and reference electrodes.

19 
Figure 5. EIS (20 kHz – 100 mHz) of a full cell (two-electrode button cell) with 1 m 
NaPF
6
/DME electrolyte at cell voltages between 0 and 4.0 V. The cell was held at each voltage 
step for 2 h prior to measurement. (a) Nyquist plot. (b) Bode plot of the phase angle. (c) 
Capacitance extracted from the EIS results shown in panel a. The capacitance is normalized to 
F/g of carbon for each individual electrode. 

20 
Figure 6. (a) Schematic of one duty cycle of the protocol used to test full cells (two-electrode 
button cells). Cells were cycled four times at 1 mA/cm
2
followed by a two hour hold at the 
maximum voltage. Cells were then fully discharged prior to EIS analysis (20 kHz – 100 mHz). 
EIS results for up to 25 consecutive duty cycles are shown corresponding to a total float time of 
50 h at the maximum voltage. (b) Nyquist plots for a full cell with 1 m TEABF
4
/ACN electrolyte 
after 10-50 hours of floating at 3.0 V. (c) Nyquist plots for a full cell with 1 m TEABF
4
/ACN 
electrolyte after 10-40 hours of floating at 3.5 V. (d) Nyquist plots for a full cell with 1 m 
NaPF
6
/DME electrolyte after 10-50 hours of floating at 3.5 V. 

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