Ultracapacitors for Electric and Hybrid Vehicles - Performance Requirements, Status of the Technology, and R&D Needs

The development of ultrcapacitors for electric and hybrid vehicle applications has been underway since the early 1990s. In this report, those development activities are reviewed and the status of the technology, as of mid-1995, is assessed. It was found that work is being done in the United States, Canada, Europe, and Japan to develop ultracapacitors using various types of carbons, doped conducting polymers, and metal oxides as electrode materials and aqueous, organic, and solid polymer electrolytes. The present state-of-the-art for ultracapacitor cells is 1.5 Wh/kg and 1 kW/kg for devices using an aqueous electrolyte and 5-7 Wh/kg and 2 kW/kg for devices using an organic electrolyte. Prototype multicell units of ultracapacitors having the above characteristics are being developed by capacitor/battery companies in the United States, Europe, and Japan and should be ready for marketing within 1-2 years. Projections of the future ultracapacitor technology indicate that the development of units with an energy density of 10-15 Wh/kg and a power density of 3-4kW/kg can be expected in the near-term (within 5 years) and even higher energy and power densities in the longer term (5-10 years). Carbon electrode and organic electrolytes (3V cells) appear to offer the best opportunities for achieving high energy density and affordable cost for vehicle applications. Key areas of research and development for increasing the energy density of ultracapacitors are identified. These include the development of carbon materials for electrodes having a specific capacitance of 75-100F/cm3 and organic electrolytes with resistivities of lower than 30 ohm-cm. Possible synergisms between ultracapacitor, lithium-ion battery, and fule cell development are indicated as a means of decreasing the time required to commercialize advanced ultracapacitors.