1 Stable Electrolyte for High Voltage Electrochemical Double-Layer Capacitors

1 
Stable Electrolyte for High Voltage Electrochemical Double-Layer Capacitors 
Rose E. Ruther
a,z
, Che-Nan Sun
b
, Adam Holliday
a
, Shiwang Cheng
a
, Frank M. Delnick
a
, Thomas 
A. Zawodzinski Jr.
a,c
, and Jagjit Nanda
a,c,z 
a
Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA 
b
Electrosynthesis Company, Lancaster, New York 14086, USA 
c
Department of Chemical and Biomolecular Engineering, University of Tennessee, Knoxville, 
Tennessee 37996, USA 
z
Corresponding Authors: 
E-mail address: rutherre@ornl.gov. Telephone:(865)946-1578 
E-mail address: nandaj@ornl.gov. Telephone:(865)241-8361

This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of 
Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the 
United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published 
form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will 
provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan 
(http://energy.gov/downloads/doe-public-access-plan). 

2 
Abstract 
 
A simple electrolyte consisting of NaPF
6
salt in 1,2-dimethoxyethane (DME) can extend the 
voltage window of electric double-layer capacitors (EDLCs) to >3.5 V. DME does not passivate 
carbon electrodes at very negative potentials (near Na/Na
+
), extending the practical voltage 
window by about 1.0 V compared to standard, non-aqueous electrolytes based on acetonitrile. 
The voltage window is demonstrated in two- and three-electrode cells using a combination of 
electrochemical impedance spectroscopy (EIS), charge-discharge cycling, and measurements of 
leakage current. DME-based electrolytes cannot match the high conductivity of acetonitrile 
solutions, but they can satisfy applications that demand high energy density at moderate power. 
The conductivity of NaPF
6
in DME is comparable to commercial lithium-ion battery electrolytes 
and superior to most ionic liquids. Factors that limit the voltage window and EDLC energy 
density are discussed, and strategies to further boost energy density are proposed.