Advanced materials for sodium-ion capacitors: Progress and perspectives

The development of electrochemical energy storage devices with high energy and power densities, long cycle life, and low cost is of great significance in energy storage fields. Sodium-ion capacitors (SICs) bridge the gap between batteries and supercapacitors. They combine the reactions of high-energy battery-type anodes and high-power capacitor-type cathodes, offering a potential solution to the limitations of both battery and supercapacitor technologies. However, in contrast to lithium-ion analogues that have been successfully commercialized, research on SICs is still in its infancy and requires significant attention to enable their use in practical applications. Consequently, the rational design of materials for SICs is still required in order to meet the increasing demands for SICs with superior energy and power performance and low cost. In recent years, a number of materials have been investigated to developing SICs that offer the aforementioned advantages, including superior electrochemical performance, low cost, good stability, and environmental friendliness. Herein, after a brief introduction to the principles of SICs, the recent developments on materials for SICs are summarized, including capacitor-type cathode, battery-type anode, and electrolytes, especially focusing on material design strategies as well as the relationship between structure and corresponding electrochemical performances. Furthermore, the regulatory aspects relating to the structure and composition of electrode materials for dual-carbon SICs are introduced. Finally, the challenges and opportunities for future developments in electrode and electrolyte materials for SICs are proposed, with the aim of guiding the scientific community in their future studies.