Recent progress and future prospects of sodium-ion capacitors

To satisfy the requirements for various electric systems and energy storage devices with both high energy density and power density as well as long lifespan, sodium-ion capacitors (SICs) consisting of battery anode and supercapacitor cathode, have attracted much attention due to the abundant resources and low cost of sodium source. SICs bridge the gap between the batteries and the supercapacitors, which can be used as competitive candidates for large-scale energy storage. In this review, the battery-type anode materials and the capacitor-type cathode materials are classified and introduced in detail. The advantages of various electrolytes including organic electrolytes, aqueous electrolytes and ion liquid electrolytes are also discussed sequentially. In addition, from the perspective of practical value, the presentations of the SICs at the current situation and the potential application in urban rail are displayed. Finally, the challenge, future research and prospects towards the SICs are put forward.