Chapter 4 - Electrolyte materials for supercapacitors

Abstract

Electrolyte is one of the components and the most influential one in dictating the performance of any electrochemical energy device. It is necessary to enhance the energy densities of the state of the art electrochemical supercapacitors (ES) known to date to an extent of ~10 Wh kg−1 for EDLCs and > 50 Wh kg−1 for both pseudocapacitors and hybrid capacitors, in order to qualify them to meet the demands of high energy density applications, compared to other electrochemical devices, such as batteries and fuel cells. Extensive R&D efforts have been bestowed to enhance the energy density of ESs, and to widen the scope of their application. As the energy density (E) of ESs is proportional to the capacitance (C) and the square of the voltage (V), that is, E = 1/2 CV2, increasing either/or both of the capacitance and the cell voltage is a direct means to increase the energy density. This can be accomplished by employing advanced electrode materials with high capacitance, electrolytes (electrolyte salt + solvent) with wide potential windows, and integrated systems/advanced cell architectures with a new and optimized structure. Various developmental attempts that are made to date can be enlisted as follows: (1) increasing the specific capacitance of carbon-based electrodes by incorporating novel carbon structures with high effective specific surface area and high packing densities; (2) developing pseudocapacitors based on pseudocapacitive materials, such as electroactive transition-metal oxides and conducting polymers with high specific capacitance which contribute to pseudocapacitance; (3) enhancing the cell voltage through the development and application of new electrolytes and (4) exploring ESs based on new concepts and novel structures, such as the hybrid or asymmetric capacitors (e.g., lithium-ion capacitors, LICs). It has to be pointed out that all of these developments are closely related to each other. Although it is relatively simple to fabricate the individual components, such as electrode materials, electrolytes, and structures of the ESs, it is challenging to identify and develop an appropriate material to promote the synergistic effect in addition to its compatibility with other cell components. For instance, the design and preparation of the porous carbon electrode materials with high specific surface area should also consider the matching between the pore structure and size of the electrolyte ions in order to fabricate a high capacitive electrode. During the development of advanced electrolytes for any electrochemical device, it is essential to consider their possible interaction and compatibility with the electrode materials and other electrode components.