1 Stable Electrolyte for High Voltage Electrochemical Double-Layer Capacitors

5 
(CMC, Sigma Aldrich, molecular weight 700,000). BP2000 and CMC were ultrasonicated in 
water to produce a homogenous suspension. Electrodes were deposited by spray-coating onto 
aluminum foil. Electrode thickness ranged from 25 to 130 µm with loadings from 0.4 to 2.2 
mg/cm
2
. 
2.3 Electrochemical testing: 
Two-electrode button cells (316L stainless steel, size CR2032, Hohsen Corp. Japan) were 
prepared using high-surface-area carbon electrodes and a polymer separator (Celgard 2325). 
Two-electrode cells with DME-based electrolyte were fabricated using electrodes with different 
loadings. The positive electrode in the DME cells had a loading of 2.2 mg/cm
2
and the negative 
electrode had a loading of 0.4 mg/cm
2
. Two-electrode cells with ACN-based electrolyte were 
fabricated using electrodes with the same loading of 2.2 mg/cm
2
each. The capacity of full cells 
(in mAh/g) is normalized to the total mass of carbon in the capacitor including both electrodes. 
In contrast, the capacitance is normalized to F/g of carbon for each individual electrode. The 
normalization was calculated by treating the capacitance of the full cell (C
cell
) as the series 
combination of the positive and negative electrodes (C
pos
and C
neg
): 
1
𝐶
𝑐𝑒𝑙𝑙
=
1
𝐶
𝑝𝑜𝑠
+
1
𝐶
𝑛𝑒𝑔
(Equation 1) 
We make the approximation that the capacitance is proportional to the mass loading. For 
symmetric cells where the positive and negative electrodes have the same loading, C
pos
= C
neg 
and C
cell
= ½ C
pos
= ½ C
neg. 
For asymmetric cells where the positive and negative electrodes have 
different loadings C
pos
≠ C
neg
. For the cells made with DME-based electrolyte C
pos 
= 5.5∙C
neg
and
C
cell
= 0.15∙C
pos
= 0.85∙C
neg
.
Some measurements were performed using a three-electrode cell (EL-Cell GmbH) with a high-
surface-area carbon electrode for the working electrode and sodium metal for the reference and 
counter electrodes. One polymer separator and one glass fiber separator were used in these three-
electrode cells to provide sufficient electrode separation to accommodate the reference electrode. 
Some cyclic voltammetry measurements were performed with glassy carbon as the working 
electrode in three-electrode T-cells. T-cells were constructed using polypropylene tees. The 
glassy carbon disc was freshly polished with alumina (Buehler MicroPolish) prior to use. Sodium 
metal was used as counter and reference electrodes in T-cells. Potentials measured with three-

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electrode cells are referenced to the Na/Na
+
potential (E
Na/Na+
≈ +0.13 V vs. E
Li/Li+
). Unless stated 
otherwise, 1 m NaPF
6
in DME was used as the electrolyte. Cyclic voltammetry (CV), 
electrochemical impedance spectroscopy (EIS), galvanostatic charge-discharge cycling, float 
tests, and leakage current measurements were acquired using Bio-Logic instruments (VSP and 
MPG2). All tests were conducted at room temperature.
2.4 Raman spectroscopy: 
Raman spectra were collected on high-surface-area carbon electrodes before and after charge-
discharge cycling. Cycled electrodes were rinsed with pure DME and dried under vacuum prior 
to analysis. All sample preparation was carried out in an argon glove box, and electrodes were 
sealed under glass in a special cell to prevent air exposure during Raman analysis. Raman spectra 
were acquired with an Alpha 300 confocal Raman microscope (WITec, GmbH) using a solid-
state 532 nm excitation laser, a 20x objective, and a 600 grooves per millimeter grating. The 
laser spot size was approximately 1 µm, and the laser power was attenuated to 1 mW.

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