Investigation of strategies for improving the energy density of symmetric electrical double-layer capacitors

With the booming development of electrical double-layer capacitors (denoted as EDLCs) as a solution to the energy depletion problem caused by traditional fossil fuels, improving the energy densities of EDLCs has become the primary goal in the design of high-performance capacitors. To overcome the limitations of electrode materials, especially in terms of energy density, different strategies have been used. In this review, we discuss the recent progress in the modification of electrode structures and the selection of suitable electrolytes. First, we focus on summarizing the design methods for electrodes that have been reported in recent years, including various carbonization-activation techniques to modulate pore distribution, graphitized-structure designing to construct conductivity network, and heteroatom doping to modify carbon network for higher electron affinity and better wettability. These methods have successfully been used to achieve porous carbons with excellent energy density. Second, different types of commonly used electrolytes are also introduced in detail to pursue the matching between porous carbon electrodes and electrolytes. Finally, the further directions and remaining challenges for obtaining efficient EDLCs are discussed to provide insights for future development.