Virtually all state-of-the-art, non-aqueous electrochemical capacitors use the same electrolyte: tetraethylammonium tetrafluoroborate (TEABF₄) in acetonitrile (ACN). ACN-based electrolytes have very high conductivity (>50 mS/cm), which minimizes resistive losses and enables capacitors to operate at very high power. However, these electrolytes are limited by a practical voltage window around only 2.5 – 3.0 V, beyond which the capacitor lifetime is significantly shortened. Since the energy stored in a capacitor increases quadratically with voltage, extending this electrochemical window could significantly boost energy density. In this contribution, we explore the possibility of using ether-based electrolytes to increase the voltage window of double-layer capacitors. Ethers such as dimethoxyethane (DME) do not passivate carbon electrodes at very negative potentials (near Na/Na⁺), extending the practical voltage window by about 1.0 V compared to standard, ACN-based electrolytes. A simple electrolyte of NaPF₆ salt in DME enables a voltage window > 3.5 V in full cells with conventional, high-surface-area carbon electrodes. Electrolyte stability is evaluated in both two and three-electrode cells with EIS, charge-discharge cycling, float tests, and measurements of leakage current. The conductivity of NaPF₆in DME (> 10 mS/cm) is comparable to commercial lithium-ion battery electrolytes and superior to most ionic liquids. Factors that limit the voltage window and energy density will be discussed, and strategies to further boost energy density will be proposed.

Acknowledgement

This work was sponsored by the Oak Ridge National Laboratory Technology Innovation Program using technology transfer license royalties. Oak Ridge National Laboratory is managed by UT-Battelle, LLC, for the U. S. Department of Energy under Contract No. DE-AC05-00OR22725.