Fibers for Textile - Based Electrical Energy Storage

This research will demonstrate feasibility for manufacturing fiber-based ceramic capacitors suitable for electrical energy storage within multifunctional textile products. Capacitive energy storage is well matched to applications requiring high specific power, long cycle life, and high reliability. We are using a simple chemical scheme for preparing fiber-based capacitors by coating electrically-conductive fibers with an ultrathin layer of a conformal, pinhole-free dielectric film, followed by coating with a second electrically-conductive layer to create the capacitor. Application of a potential difference between the two conductors creates an electric field across the dielectric film which accomplishes capacitive energy storage. Energy storage in capacitors scales with the square of the electric field across the dielectric therefore ultrathin dielectric films that are pinhole-free and resist dielectric breakdown when large electric fields are applied across them are desired. In one approach, films of appropriate high-κ ceramics including tantalum, titanium, lanthanum, and hafnium oxides will be formed on electricallyconductive fibers of suitable materials including aluminum, carbon, and electronically conductive polymers such as polyaniline. In a second approach, ultrathin polymer dielectric films will be formed on the fibers by a combination of spinning and layer-bylayer polyelectrolyte deposition. In both cases, the resulting dielectric-film-coated fibers are subsequently coated with a second electrically conductive outer layer to create the desired capacitor fiber. Prospects for integration of the resulting fibers into fiber bundles, yarns and woven fabrics to be used in creating multifunctional energy-storing textile products are being explored. NTC Project M06-CL07 Page 2 of 9 National Textile Center Annual Report: November 2006 INTRODUCTION Capacitors are useful energy storage devices in applications where high specific power, long cycle life, and high reliability are especially important. In contrast with storage batteries, there are no charge-transfer or chemical reactions in a capacitor, only charge separation across a dielectric material. Capacitors are therefore not subject to the kinetic bottlenecks and failure-causing dimensional changes that are commonplace in storage batteries. Solid-state ceramic capacitors are especially desirable due to their fast response time, e.g. sub-second vs. tens to hundreds of seconds relative to redox and electrochemical double-layer capacitors. Also, solid-state capacitors contain no liquid electrolyte that could potentially leak out and cause device failure. This latter point is especially important in multifunctional materials for which such a failure could compromise other materials properties related to function. Materials properties needed to achieve high specific capacitance in a solid-state capacitor include the following: a thin, pinhole-free layer of a material with a high dielectric constant which can support a high degree of charge separation in a large electric field without failing, e.g. by dielectric breakdown, electron emission, or charge motion; a large surface area within the capacitor volume; and low overall mass. Most present solid-state capacitors achieve these properties using polycrystalline inorganic ceramic materials arranged in layers on the order of a few micrometers thick in a planar stacked geometry with alternating ceramic and current-collector electrode layers. This approach is well suited for fabricating capacitors for microelectronic circuits and for creating discrete Fiber coating