Materials Research for High Energy Density Electrochemical Capacitor
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1100-JJ06-02
Materials Research for High Energy Density Electrochemical Capacitor Andrew F. Burke Institute of Transportation Studies, University of California-Davis, One Shields Ave., Davis, CA, 95616
Abstract
In April 2007, the Office of Basic Energy Science, United States Department of Energy organized and conducted a Basic Energy Sciences Workshop for Electrical Energy Storage at which basic research needs for capacitive energy storage were considered in detail. This paper is intended to highlight the materials research findings/needs of the workshop and to relate them to the development of high energy density capacitors that can have an energy density approaching that of lead acid batteries, a power density greater than that of lithium ion batteries, and cycle life approaching that of carbon/carbon double-layer capacitors. Capacitors inherently have long cycle life and high power capability so the key issue is how to increase their energy density with a minimum sacrifice of their inherent cycle life and power advantages. This requires the development of electrode charge storage materials with an effective high specific capacitance (F/g) and high electronic conductivity. The most promising electrode materials appear to be optimized activated carbons, graphitic carbons, nanotube carbons, and metal oxides. Cells can be assembled that utilize one of these materials in the one electrode and another of the materials in the other electrode. Such hybrid cells can operate at 3-4V using organic electrolytes and potentially can have energy densities of 15-25 Wh/kg. Initial research is also underway on solid-state, high energy density devices utilizing high dielectric materials (K>15000) which would operate at very high cell voltage. If such dielectric materials can be developed, these devices may have energy densities approaching those of lithium batteries.
Introduction Early work [1] in the 1970s was done to develop electrochemical capacitor devices for low power applications such as backup for uninterruptible power supplies and memory in computers. That work recognized the potential of using high surface area carbons to achieve capacitance in devices much greater than achievable using thin film dielectric materials. In the United States, more recent work [2,3] in 1990 on high power electrochemical capacitors (supercapacitors or ultracapacitors) was started and supported by the United States Department of Energy as part of their electric and hybrid vehicle programs. From the outset of that work, the twin goals were to achieve an energy density of 5 Wh/kg for high power density (at least 500 W/kg) constant power discharges. The life cycle goal was 500,000 deep discharge cycles. These goals have now been met and the present research is directed to the development of electrochemical capacitors that far exceed the early goals with an energy density approaching that of lead-acid batteries and power density much higher than lithium-ion batteries. In April 2007, the Office of Basic Energy Science, United States Department o
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