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EPRI Proprietary Licensed Material Figure 12 Panasonic 2000 F, 2.3 V (six-inch ruler also shown) Saft Saft’s electrochemical capacitor program compliments their large lithium ion battery products. Saft manufactures their large capacitors in France. The product is available in two variations: high energy and high power. The products are of type II design, have cylindrical geometry with a terminal on each end, and are available with capacitance values up to 3200 F. The manufacturing is at the stage of advanced prototype. These capacitor products may be purchased from the Saft’s Cockeysville, Maryland office. Figure 13 Saft 3200 F, 2.5 V (six-inch ruler also shown) Current Technology Developments Development thrusts in the year 2002 generally relate to capacitor design, manufacturing cost reductions, and electrode materials development. Capacitor researchers apparently see performance or other advantages of the asymmetric design and are making it popular. Numerous research papers have been presented on this concept since it was first described in 1997. Patents are appearing with various descriptions of type III and IV materials and construction. For example, of the ~45 papers presented at the 2002 Spring meeting of the Electrochemical Society, 18 were related to asymmetric electrochemical capacitors. There were few if any presented on this topic at previous meetings. Electrochemical Society meetings represent a forum where professionals often first present major developments and new technology directions. Another current development thrust relates to technical issues surrounding capacitor thermal management. Here interest originates from the need to create large, high-voltage Electrochemical Capacitors 32
EPRI Proprietary Licensed Material energy storage systems capable of rapid cycling. Such systems require uniform voltage among the many capacitor cells in series-connected strings for reliable operation. This motivates increased emphasis on cell temperature uniformity and efficient heat removal from cells. Although charge/discharge efficiency is generally high for capacitors, they nevertheless dissipate energy, which can cause excessive internal temperature rise without appropriate heat removal techniques. An important issue related to the creation of reliable high-voltage strings of cells is cell uniformity. So reducing manufacturing variability is certainly important. Improving control of the production process is an ongoing effort for many companies according to recent reports. Still another issue in capacitor design relates to product cost reduction. For example, Maxwell has reported on their cost-reduction program. They are developing spiral-wound cell construction capability using particulate-carbon electrode materials pasted on current collectors. This is in contrast with the carbon cloth used with a manual, accordion-fold design. Development thrusts in electrode materials include examining the performance of various activated carbons to find lower-cost materials. Some new carbon materials are being implemented. Several companies are attempting to find replacements for activated carbon cloth material, which is much more expensive than the particulate carbon, especially particulate materials that have a natural origin. Other electrode materials that have been investigated include metal-oxides of ignoble elements, ones having good performance without associated high-costs typically found in the platinum group metals. There has been some development activity using nano-structured materials, both for carbons in symmetric double layer capacitors, and in the pseudocapacitor electrode of an asymmetric capacitor. The third major development thrust has been with the electrolyte. Work has been reported on using polymer electrolytes for both aqueous and non-aqueous designs. Also, there has been some effort to find replacement materials for the acetonitrile-based electrolytes used in many type II products. The performance of these electrolytes is very good but its use creates concerns because of toxicity and safety issues. The thrusts for the asymmetric capacitor activity have expanded from the nickel oxyhydroxide/carbon system to other systems including a lead oxide/carbon system and a MnO 2 /carbon system. Reports of device performance using these other material systems are most encouraging. A major advantage of these systems is low materials cost. Yet another system that has been described in several papers recently is a lithium-titanate electrode in combination with a carbon electrode and an organic electrolyte. This design offers higher voltage than can be obtained in present symmetric organic electrolyte capacitors, and it is referred to as a type IV electrochemical capacitor. Yet another design that has been described in the literature is a graphite/carbon capacitor. This type IV capacitor relies on charge intercalation in the graphite of one electrode and double layer charge storage on activated carbon in the other electrode. The electrolyte for this system is an organic solvent with a lithium salt. This particular system has an operating voltage approaching 4 V. None of these advanced devices are commercially available at this time. Electrochemical Capacitors 33
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