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This Defense Intelligence Reference Document from the Defense Intelligence Agency, dated 1 November 2010, was one of a series of advanced technology reports produced in FY 2010 under the Advanced Aerospace Weapons System Applications (AAWSA) Program. It reviews ultracapacitors as energy and power storage devices. It covers how they work, their materials, their commercial and military uses, and new developments such as carbon nanotubes, thin films and magnetic capacitors. It concludes that the U.S. government must understand and adopt advanced ultracapacitors.
“The Advance”13 pages
UNCLASSIFIED/ ,'F811. 8FFl!lllit 1?1!11!! t!IIU:!Y Chapter 2: Materials Technology The standard electrochemical capacitor is made of two electrodes, an electrolyte, and a separator, packaged in either a metal container or a laminated pouch. Ultracapacitors are often packaged in a fashion similar to a typical battery configuration. The separator is an ultrathin material that allows ion transport but is electrically insulating. There are several polymer separators on the market that have the desired porosity needed to create fast ion transport. The electrodes are traditionally the same material in an ultracapacitor-this is in stark contrast to a battery. Electrodes are typically carbon; this is advantageous in making a cost-effective energy/power-storage device. Activated carbon has a very high surface area, vital to storing energy in the electrical double layer between the electrode and the electrolyte. This thin double layer is responsible for the impressive high-power-storage capabilities of the ultracapacitor. The electrolyte's resistivity and dielectric constant play a vital role in behavior of the double layer; consequently, the choice of electrolyte impacts performance. The earliest electrochemical capacitors were introduced 30+ years ago; they were symmetric designs (two identical electrodes) in aqueous electrolyte. While these electrolytes have excellent ionic conductivity, the operating cell voltage was limited to ~1.2 V/cell and these had a nominal cell rating of ~0.9 volts. In the second generation of electrochemical capacitors, the use of organic electrolyte led to an increase of the rated cell voltage from about 0.9 V/cell to 2.3-2.7 V/cell. Today, ultracapacitors using an organic electrolyte are the most popular. 14 ELECTROLYTES Initial ultracaps used aqueous electrolytes, saturated with Al2(S04)3, or 30% concentrations of H2S04 or KOH. While these electrolytes have excellent ionic conductivity, the operating voltage is limited to 1.2 volts using carbon electrodes. Improvements to ultracapacitor performance were made by transitioning from aqueous electrolytes to an organic medium. Organic electrolytes have lower ionic conductivity, but the higher breakdown potential increases the nominal cell voltage. An increase in cell voltage up to as high as 3 volts has been realized by use of an organic electrolyte. These electrolytes are typically an ammonium salt dissolved in an organic solvent, such as propylene carbonate or acetonitrile. Table 3 highlights the comparison between the various electrolytes used in ultracapacitors. Ionic liquids are beginning to become more common in ultracapacitors, as they allow for an increase in cell voltage to as high as 4 volts. These ionic liquids have a higher resistivity, especially at lower temperatures. The tradeoff in resistivity for voltage may be beneficial for some high-power applications. Table 3: Properties of Various Electrolytes Used in Ultracapacitors15 Electrolyte Density Resistivity Cell fom/cm3) fOhm-cm) Voltaoe KOH 1.29 1.9 1.0 Sulfuric acid 1.2 1.35 1.0 Pronvlene carbonate 1.2 52 2.5-3.0 Acetonitrile 0.78 18 2.5-3.0 Ionic liquid 1.3-1.5 125 (25°() 4.0 28 (100°() 3.25 8 UNCLASSIFIED/ ,'1'81l t!ll'Fl!lllit W!ii a,11o>J
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Report, from the dia collection. The PDF is mirrored here; the original link is above. 34 pages are in the text index: search them above, or from the library's search.