Technical Status of High-Power Electrochemical Capacitors
presented at the International Seminar on Primary and Secondary Battery Technologies and Applications, February 28, 1994
John R. Miller

Introduction

Electrochemical capacitors (ECs) are gaining acceptance in the electronics industry as system designers become familiar with their attributes and benefits. In CMOS memory backup applications, for instance, a one-Farad EC having a volume of only one-half cubic inch can replace nickel cadmium or lithium batteries and provide backup power for months. And in electric vehicle applications, large ECs can load-level the power on the battery system and thereby increase battery life and extend vehicle range.

The high volumetric capacitance density of an EC (10 to 100 times greater than conventional capaci tors) derives from using porous electrodes to create a large effective plate area and from storing energy in the diffuse double layer. This double layer, created naturally at a solid-electrolyte interface when voltage is imposed, has a thickness of only 1 nrn, thus forming an extremely small effective plate separation. In some ECs, stored energy is substantially augmented by so-called pseudocapacitance effects, occurring again at the solid-electrolyte interface.

Although the energy storage capability of the double layer was recognized more than 100 years ago, it took the development of low-current-draw volatile computer memories to create a market for ECs.

ECs do not approach the energy density of batteries. Nevertheless, they are extremely attractive power sources. Compared with batteries, they require no maintenance, offer much higher cycle-life, require a very simple charging circuit, experience no memory effect, and are generally much safer. Physical rather than chemical energy storage is the key reason for their safe operation and extraordinarily high cycle-life.

Readily available EC products are limited in size and power perforrnance, due primarily to their targeted memory backup use. They are manufactured by several companies, primarily Japanese, and have capacitance values of up to a few Farads, an equivalent series resistance (ESR) of one to fifty ohms, and a working voltage of 3 to 11 V.

Until recently, ECs suitable for high-power applications have been unavailable. But interest in automotive starting, lighting, and ignition(SLI) applications, as well as in electric vehicle (EV) load-leveling, has stimulated product development activities for such high-power devices. These efforts have beenfocused primarily on development of ECs with improved frequency response. The goal is to develop prod ucts that can be efficiently charged and then discharged in the time specified for these high-rate applications.

The following sections discuss properties that are important for an EC in a high-power application. Particular emphasis is given to the EV application. Ongoing R&D activities are reported. Critical issues relating to the development of viable high-power EC products are highlighted.