Increasing Energy Density of Electrochemical Capacitors By Advanced Nanomanufacturing and Ionic Liquid Electrolyte
Authors: Skinner Anna P , Igwe Rachna , Cox Philip , Hill Justin J 
Published: December 2020

The growing number of energy consuming devices is driving an ever increasing demand for high efficiency, high capacity, and portable energy storage devices. In addition to higher levels of energy storage, there is a growing need for the capability to produce sustained high power pulses. For many applications, the energy storage devices must also provide a high cycle life to provide the sustained operation life, along with increased capability of providing high power over ever increasing pulse durations. Moreover, these energy storage devices are in demand over a wide range of sizes from chip-scale power storage to large-scale, kilovolt-level power delivery. Rechargeable batteries, in particular lithium ion, are well known and have been extensively developed for their high energy density, but their major drawback remains in their limited power density and cycle-life capability. Electrostatic capacitors, on the other hand, provide a high power density, and research has shown they have great potential for meeting the power and cycle life demands for the new energy storage device demands. However, for conventional electrostatic capacitors, the energy density and thus pulse power duration capability is very limited. Alternatively, electrochemically-based double-layer capacitors (EDLCs) or ultracapacitors, provide a pathway to an energy storage device that shows significant promise for delivering the high power density and high cycle life that is achievable with electrostatic capacitors, while providing a path to much greater energy storage and thus sustained long duration pulsed power delivery. Through the surface charge adsorption-based mechanism of energy storage utilized in EDLCs, it is possible to get high frequency response times while providing the ability to supply power for much longer periods of time than can be achieved with conventional capacitors. Despite this major benefit, currently available commercial EDLC devices still provide energy densities that are much lower than that of batteries. To increase the number of potential applications for EDLCs and displace batteries, the energy density must be increased. Thus the main focus of our ultracapacitor research, presented here, is increasing the energy density of ultracapacitors through the use of highly ordered nanostructured electrode materials, which are combined with a high voltage window ionic liquid-based electrolytes to increase the power and energy density.

In EDLCs, the capacitance and corresponding energy storage capability are directly proportional to the electrode’s solution accessible surface area. To maximize this area, we have developed electrode structures with a tunable, highly controlled porosity that provides very high, electrolyte-accessible, surface area. We’ve developed a robust and scalable templated approach to allow the fabrication of high surface area carbon nanotube (CNT)-based electrodes with highly aligned and vertically-oriented CNTs. This approach provides an areal CNT density that is much greater than typical carpet-grown CNTs. Furthermore, through our templated fabrication approach, we can tune the electrode pore size, thus allowing us to optimize it for the specific electrolyte. Our approach, combining tuning of both the electrode and electrolyte, ensures maximum charge adsorption for energy storage, while maintaining excellent solution access for fast charge transport during charging and discharging, thereby ensuring power capability.

Ionic liquids are becoming popular electrolytes for a range of applications because of their large voltage window, and chemical properties that can be tailored for the specific application. The use of ionic liquid electrolytes allows operation at higher cell voltages than the conventional aqueous or organic solution-based capacitors. Larger operation voltage further enhances the power and energy density in our capacitors. Additionally, since ionic liquids are nonflammable and have a negligible vapor pressure, they make the ideal electrolyte choice in higher temperature applications or for micro- and on-chip devices, where the removal of heat is difficult because of the confined space operating conditions. Through the use of designer electrolytes based on pure and mixed ionic liquids, and combining and optimizing the interaction of the electrolyte with our templated CNT electrodes, we are able to produce an energy storage technology which combines the optimal balance and benefits of high energy density and high-power density. This talk will detail our electrode development and device optimization efforts for our large format capacitors.

[DISTRIBUTION STATEMENT A. Approved for public release; Distribution is unlimited 412TW-PA-20122]