Designing supercapacitors via ion counting

The negative impact of continued consumption of fossil fuels on the environment, global warming and climate change is well known. This has necessitated the development of alternative sustainable and renewable energy technologies. Among them is the transition towards distributed energy systems and electric vehicles. Electrochemical energy storage (EES) technologies, such as lithium-ion batteries, play a pivotal role in the development and application of these technologies.

Supercapacitors have attracted considerable attention as energy storage devices owing to their high cyclability, high power density, and long lifetime. To this end, supercapacitors have been considered promising candidates for hybrid and electric vehicles and other peak power applications. Presently, improving the performance of supercapacitors has primarily focused on developing innovative and high-performance supercapacitor materials, such as electrolytes, activated carbon and separators.

Supercapacitor systems are mostly developed based on different types of electrolytes: organic, aqueous, solid state and ionic liquid. The electrolyte properties, such as the electrical and ionic conductivities, directly influence the overall performance of supercapacitors. Despite the extensive research on the electrolytes and active materials of supercapacitors, there are limited reports on effective strategies for maximizing the performance of supercapacitors based on material optimization. Moreover, high-performance materials do not necessarily translate to high-performance devices. This calls for a holistic optimization of various processes and material performance metrics at each stage and their subsequent integration in the final device assembly. The optimal performance also requires a well-balanced amount of electrolyte and active materials.

Herein, Corning Incorporated researchers in the United States: Dr. Shrisudersan Jayaraman, Travis Rawson and Marina Belyustina developed a new method to establish the crucial and sufficient conditions for building supercapacitors with optimized performance. This methodology was based on a theoretical framework coupled with experiments. While the experiments were used to determine the optimal supercapacitor performance conditions, the theoretical framework was employed to guide the determination of design rules based on counting the ions present in the electrolyte and those adsorbed on the carbon electrode. D-cell sized supercapacitors were fabricated with Type A and Type B carbon electrode systems. Their research work is currently published in the peer-reviewed journal, Energy and Environmental Science.

The research team established that for any application with well-defined energy and power density targets, optimizing the device capacitance, device volume, equivalent series resistance (ESR) and rated voltage could achieve the best device performance. These variables were highly dependent on the device, process and material characteristics. The volume of the selected carbon electrolyte-electrode system and the electrolyte concentration were used to control the ionic resistance of the ESR. The two high level constraints that help achieving optimal device performance identified in this study eventually cascade backward to define the material-level characteristics and performance targets.

Type A electrode system with a specific capacitance of 75 F cm^-3, exhibited a requirement of excess electrolyte volume of at least 3 mL and a lower bound for the conductivity of 28 mS cm^-1. While Type B electrode system displayed a superior Ragone performance than Type A, it did not meet the targeted  aging behavior. Thus, Type A emerged as the best overall candidate for improved device performance. Furthermore, eliminating excess materials that added no value, like electrolyte salt, could result in weight reduction, improved performance and cost savings.

In a nutshell, the authors demonstrated the importance of determining the threshold value of conductivity at charged state and excess electrolyte volume in maximizing the performance of any carbon electrode-electrolyte system. Determining the two most crucial design rules resulted in applying constraints on different materials and device design variables. This facilitated the optimization of the individual processes and materials during the fabrication of the device. In a statement to Advances in Engineering, Dr. Jayaraman explained that this will be a valuable tool for material scientists who seek to develop novel supercapacitor materials. The outlined strategy will also aid device manufacturers as they pursue commercializing high-performance supercapacitors. This approach was a lynchpin of their developmental efforts, and they successfully employed this strategy in their technology development. They were able to tie research, development and engineering together during device fabrication based on such solid understanding, with significant financial impact. This approach also provided valuable feedback to their scientists working on various aspects of the device, namely carbon synthesis, electrode development and electrolyte development, and their engineers working on the electrode fabrication, electrolyte synthesis and device assembly. He further said that the presented methodology could be extended to achieve optimal design of other energy storage systems.