An effort to extend the low-temperature operational limit of supercapacitors is currently underway. At present, commercially available non-aqueous supercapacitors are rated for a minimum operating temperature of –40 °C. A capability to operate at lower temperatures would be desirable for delivering power to systems that must operate in outer space or in the Polar Regions on Earth.
Supercapacitors (also known as double-layer or electrochemical capacitors) offer a high power density (>1,000 W/kg) and moderate energy density (about 5 to 10 Wh/kg) technology for storing energy and delivering power. This combination of properties enables delivery of large currents for pulsed applications, or alternatively, smaller currents for low duty cycle applications. The mechanism of storage of electric charge in a supercapacitor — at the electrical double-layer formed at a solid-electrode/liquid-electrolyte interface — differs from that of a primary or secondary electrochemical cell (i.e., a battery) in such a manner as to impart a long cycle life (typically >106 charge/discharge cycles).
Commercially available non-aqueous supercapacitors are limited in operation to temperatures ≈–40 °C due to the relatively high melting point of the solvent used. Typical electrolytes in commercially available supercapacitors consist of a tetraethylammonium tetrafluoroborate (TEATFB) salt dissolved in one or more solvent(s) that can include acetonitrile [AN (which freezes at –45.7 °C)] and/or propylene carbonate [PC (which freezes at –49 °C)]. Moreover, the viscosities of these solvents increase at lower temperatures, with a consequent increases in the equivalent series resistance (ESR) of the supercapacitor cell. This increase in ESR limits the power that can be delivered by the supercapacitor cell at low temperatures.
Therefore, efforts to enable operation at lower temperatures have focused on tailoring electrolytes with lower melting points (to extend the temperature range) and higher ionic conductivities and lower viscosities (to minimize increases in ESR). The approach followed thus far has targeted the use of co-solvents to depress the melting point of TEATFB/AN electrolytes while maintaining sufficient solubility of TEATFB at low temperature. These electrolytes are used in conjunction with appropriate electrode materials capable of exhibiting a suitable energy density.
The electrolytes investigated comprise the basic AN/TEATFB formulation in combination with various proportions of co-solvents that include methyl formate, methyl acetate, ethyl acetate, and 1,3-dioxolane (DX) (see table). Coin cells for evaluating the performance of these electrolytes have been fabricated using electrodes made of a commercially available high-surface-area porous carbon-based material. The electrodes were electrically isolated from each other by use of a polyethylene- based separator material. From measurements on these cells, it was concluded that the use of suitable co-solvents can enable retention, at temperatures <–45 °C, of room-temperature capacitance values. For one electrolyte formulation (comprised a 3:1 AN/DX blend), it was found that dc charging and discharging at a temperature as low as –75 °C is possible, albeit with capacitance reduced to about half its room-temperature value. By tailoring the nature of the co-solvent and the concentration of the salt used, the ESR can be minimized as well (see figure).
This work was done by Erik J. Brandon, William C. West and Marshall C. Smart of Caltech for NASA’s Jet Propulsion Laboratory.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:
Innovative Technology Assets Management
Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109-8099
Refer to NPO-44386, volume and number of this NASA Tech Briefs issue, and the page number.